A neutral, brand-aware UK guide to understanding boiler fault codes — what they mean, when a single reset is appropriate, and when to isolate the boiler and call a Gas Safe registered engineer. Includes illustrative common codes for seven major UK domestic boiler brands.
This guide is written for UK homeowners, landlords, and tenants who see a fault code or lockout indicator on their boiler and want to understand what it means before calling an engineer. It does not replace manufacturer instructions or a Gas Safe registered engineer’s diagnosis. This guide is editorially independent. It includes a disclosed reference to the VerifiedPlumbers London directory in section 23.
Methodology: this guide is based on UK Building Regulations (Approved Document J for combustion appliances and CO alarms), the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 as amended 2022, HSE Gas Safety guidance, the Gas Safety (Installation and Use) Regulations 1998, Gas Safe Register technical guidance, and manufacturer support documentation from Worcester Bosch, Vaillant, Ideal Heating, Baxi, Glow-worm, Vokèra, and Potterton. Manufacturer references are illustrative of factual product information only and are not used as authority sources for regulatory claims.
📍 Regulatory scope. Gas Safe registration applies UK-wide. CO alarm legal requirements detailed in this guide are England-specific (Approved Document J and the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 as amended 2022). Scotland, Wales, and Northern Ireland have their own separately specified requirements.
⚠️ Safety first
If you smell gas, hear hissing, or suspect a gas leak — do not touch the boiler or any electrical switches. Do not use flames, electrical appliances, or smoke. If the smell is strong, leave the property and call the gas emergency service from outside (free, 24/7):
- Natural gas — Great Britain (England, Wales, Scotland): 0800 111 999
- Natural gas — Northern Ireland: 0800 002 001
- LPG (liquefied petroleum gas): the contact number printed on the bulk storage vessel or meter. For LPG cylinder supplies, see the supplier’s literature or local directory. On caravan sites and boats, the site owner or boat operator may also have gas safety responsibilities.
Otherwise, open windows and doors and shut off the gas at the meter emergency control valve if you know where it is and it is safe to do so, then call the appropriate emergency number above.
If a carbon monoxide alarm sounds, or anyone in the property has symptoms of CO exposure (headache, dizziness, nausea, breathlessness, loss of consciousness), evacuate the property immediately and call the gas emergency service for your area (above). For non-emergency CO safety advice, the HSE Gas Safety Advice Line is 0800 300 363 (Mon–Fri office hours).⁶⁰ Seek medical advice. Do not re-enter the property until the gas emergency service has confirmed it is safe. Once you have re-entered, do not use the appliance again until it has been checked by a Gas Safe registered engineer.⁶¹
Under the Gas Safety (Installation and Use) Regulations 1998 (enforced by HSE), gas work within the scope of those Regulations must be carried out by a Gas Safe registered engineer competent for that specific category of gas work.⁵ ¹⁵ This guide does not provide DIY repair instructions for any internal boiler component, gas-side fault, flue fault, condensate fault, or persistent lockout.
Contents
- TL;DR — What to do when you see a fault code
- What is a boiler fault code?
- How to read your fault code
- The four fault categories
- The single-reset rule
- When to isolate the boiler and call a Gas Safe registered engineer
- Common codes — Worcester Bosch
- Common codes — Vaillant
- Common codes — Ideal Heating
- Common codes — Baxi
- Common codes — Glow-worm
- Common codes — Vokèra
- Common codes — Potterton
- Frozen condensate pipe in winter
- Low water pressure faults
- Persistent lockouts and recurring codes
- Carbon monoxide and combustion faults
- Boiler under warranty — what to do first
- Costs of common boiler repairs in 2026 — commercial estimate only
- Landlord and tenant responsibilities
- What we can’t advise without a site visit
- FAQs
- How to find a Gas Safe registered engineer
- Sources
1. TL;DR — What to do when you see a fault code
If your boiler displays a fault code or has gone into lockout (often shown by a flashing or steady red indicator):
- Note the exact code. Letters and numbers. Photograph it if you can.
- Check your boiler’s user guide (or the manufacturer’s online support page — links throughout this guide).
- If the manufacturer guidance for your specific model permits, attempt a single reset following the boiler’s reset procedure.
- If the code returns immediately after reset, or returns within hours, do not keep resetting. Isolate the boiler and call a Gas Safe registered engineer.
- For any code involving gas supply, flame, ignition, flue, combustion, or persistent lockout, do not attempt resets repeatedly. Isolate the boiler and call a Gas Safe registered engineer.
If you smell gas, suspect a gas leak, or a CO alarm sounds — leave the property and call from outside: 0800 111 999 (natural gas, Great Britain), 0800 002 001 (natural gas, Northern Ireland), or for LPG, the contact number on your bulk storage vessel or meter.
Find your fault code fast — jump to your boiler brand: Worcester Bosch (EA, D5, C6) | Vaillant (F22, F28, F75) | Ideal Heating (F1, L2) | Baxi (E119, E133) | Glow-worm (F22, F28) | Vokèra (A01, A04) | Potterton (E119, E133)
2. What is a boiler fault code?
A boiler fault code is a diagnostic message displayed by the boiler when it detects a problem and stops normal operation as a safety measure. The boiler enters what’s commonly called a lockout — a deliberate shutdown that prevents unsafe operation until the underlying fault is investigated and (if appropriate) cleared.
Modern condensing gas boilers are fitted with multiple sensors monitoring gas pressure, ignition, flame detection, water pressure, flow temperature, return temperature, fan speed, flue gases, condensate flow, and primary heat exchanger temperature. If any of these sensors detects a value outside the safe operating range, the boiler control board displays a code and shuts down the burner.
Different manufacturers use different code formats:
- Worcester Bosch — alphanumeric (EA, D5, C6) or numeric with suffixes (227, 1017)
- Vaillant — F-codes (F22, F28, F75)
- Ideal Heating — F, L, or C codes (F1, L2, FA)
- Baxi — E-codes (E119, E133, E168)
- Glow-worm — F-codes (F22, F28, F75) — Glow-worm is a Vaillant Group brand and uses a similar F-code format on many models
- Vokèra — A-codes (A01, A04, A09)
- Potterton — E-codes (E133, E168) — Potterton is a Baxi Heating brand and uses a similar E-code format on many models
A fault code is a message, not a diagnosis on its own. The same code can indicate different underlying issues depending on the model, age and history of the boiler, the surrounding system, and the conditions at the time of fault.
3. How to read your fault code
Most modern boilers show fault codes on a digital display on the boiler fascia. Older boilers may use a series of LED indicators that flash in patterns to indicate a fault.
Where to find the code
- Look at the boiler display panel. Codes typically appear when the boiler is locked out and not heating.
- Some codes alternate — for example, a Baxi E133 code displays as
E1then33, alternating. Take note of the full sequence. - On older boilers without a digital display, look for a flashing red lockout indicator. The number of flashes (or flash pattern) indicates the fault category — see your boiler’s user guide for the specific pattern.
Where to find what the code means
Always check the manufacturer’s official support page first, as code meanings vary between models even within the same brand:
- Worcester Bosch — troubleshooting tool⁵²
- Vaillant — boiler fault codes list⁵³
- Ideal Heating — boiler fault codes FAQ⁵⁴
- Baxi — error codes page⁵⁵
- Glow-worm — fault codes guide⁵⁶
- Vokèra — customer support (codes are typically published in the boiler’s user manual)
- Potterton — error codes page⁵⁷
If the manufacturer’s published guidance for the specific code states that user action is appropriate (such as a single reset following a clear, brief power interruption), follow it carefully and only once. For anything beyond that — including persistent codes, returning codes, or any code involving gas, flame, flue, or combustion — call a Gas Safe registered engineer.
4. The four fault categories
Across UK domestic boiler brands, fault codes generally fall into four categories. Understanding the category your code belongs to is more useful than memorising specific codes — it tells you the level of risk and the urgency of the response.
Category 1: Water-side faults
Faults relating to the heating system water — water pressure, flow rate, circulation, water temperature sensors. Examples: low system pressure, no circulation, flow temperature high.
These are generally not gas-side faults. However, the underlying cause (a leak, a stuck pump, a failed sensor) should be investigated by a Gas Safe registered engineer for any work on the boiler or its associated gas systems. Recurring water-side faults can have safety implications if they lead to overheating.
Category 2: Gas, flame and ignition faults
Faults relating to the gas supply, ignition system, flame detection, or burner operation. Examples: ignition failure, flame loss during operation, no flame detected after multiple ignition attempts.
These are gas-side faults and are not appropriate for any DIY action beyond a single reset following manufacturer guidance. Persistent gas, flame, or ignition codes indicate a fault that must be diagnosed by a Gas Safe registered engineer. Continued resetting can be unsafe and can cause further damage.
Category 3: Flue, condensate and combustion faults
Faults relating to the flue, condensate discharge pipe, fan, or combustion air supply. Examples: blocked flue, frozen condensate pipe, fan speed fault, flue gas temperature high.
Flue and combustion faults can be linked to incomplete combustion, which can produce carbon monoxide. These faults must be investigated by a Gas Safe registered engineer. The exception — for a clearly identifiable frozen external condensate pipe in winter — is covered in section 14.
Category 4: Electronic, sensor and component faults
Faults relating to the boiler’s PCB (control board), sensors, electrical wiring, fan motor, or pump. Examples: flow sensor fault, PCB fault, fan tacho signal fault, communication fault.
These faults usually indicate a component requiring replacement or diagnosis. They are not user-serviceable. A Gas Safe registered engineer with the appropriate competence should diagnose and replace the component.
5. The single-reset rule
Modern boilers have a reset button or reset position on the mode selector. After certain transient faults (a brief power interruption, a momentary low-pressure event, a single failed ignition), a single reset attempt can clear the lockout and the boiler will resume normal operation.
This is the approach this guide takes; it is editorial safety policy, not a regulatory rule, and is commonly reflected in manufacturer guidance from multiple UK brands.
When a single reset is appropriate
A single reset attempt is appropriate when:
- The fault appears to have a clear, brief external cause (such as a power cut)
- The manufacturer’s published guidance for the specific code and your specific model indicates user reset is acceptable
- The boiler has not displayed the same code repeatedly in recent days
Follow your boiler’s reset procedure as described in its user guide. The procedure varies by manufacturer and model — typically holding a reset button for around 5 seconds, or turning the mode selector to a reset position.
Why one attempt only
If the fault returns immediately after reset, or returns within hours, this indicates an underlying problem that resetting will not fix. Continuing to reset:
- Will not resolve the underlying fault
- May cause additional component wear (particularly to the ignition system)
- Can mask a developing safety issue
- May affect manufacturer warranty terms (check your specific warranty documentation)
Manufacturer guidance from multiple UK brands explicitly cautions against repeated resets after a code returns. If the code returns, the next step is to isolate the boiler and call a Gas Safe registered engineer.
What “isolate the boiler” means
For the purposes of this guide:
- Turn the boiler off at its mode selector or main switch
- If accessible and safe to do so, turn off the boiler’s electrical supply at the spur switch (usually located near the boiler)
- Do not attempt to remove any panels, access any internal components, or interfere with gas, flue, or condensate connections
This is what you do while waiting for a Gas Safe registered engineer to attend.
⚠️ Important: this is electrical isolation only. If you smell gas, suspect a gas leak, or a CO alarm sounds, electrical isolation is not sufficient — follow the emergency gas procedure in the Safety first section above instead, including evacuation and the appropriate emergency number for your fuel type and region.
6. When to isolate the boiler and call a Gas Safe registered engineer
Call a Gas Safe registered engineer immediately and do not attempt further resets if any of the following apply:
- The fault code involves gas supply, flame, ignition, or combustion (most letter-prefixed codes such as EA, F28, A01, E133, L2, depending on brand)
- The fault code involves flue, condensate, or fan faults that aren’t a clearly identifiable frozen external condensate pipe in winter
- The fault code involves overheat or safety thermostat activation (E110, E130, F.20 family, A02, E9, depending on brand)
- The same code returns within minutes or hours of reset
- The boiler has produced multiple different fault codes in a short period
- You see, smell, or hear anything unusual — leaks, soot marks, hissing, knocking, banging
- A carbon monoxide alarm has sounded recently in the property
- Anyone in the property has symptoms consistent with CO exposure
- The boiler is more than 12–15 years old (industry-typical service life range, non-regulatory) and faults are increasingly frequent (consider replacement — see our guide: boiler repair vs replacement UK (2026))
Under the Gas Safety (Installation and Use) Regulations 1998 (enforced by HSE), gas work within the scope of those Regulations must be carried out by a Gas Safe registered engineer competent for that specific category of gas work.⁵ ¹⁵ Check the engineer (and their business listing) on the Gas Safe Register before booking, and check the engineer’s Gas Safe ID card before any gas work begins — the back of the card lists the categories of gas work the engineer is qualified for.
7. Common codes — Worcester Bosch
⚠️ Read this before using any brand code table below.
Code tables checked against manufacturer pages and manuals in April 2026. Fault code meanings vary by exact model and firmware version. The tables in sections 7–13 are manufacturer product information only — not safety, legal, or diagnostic advice. For safety-critical action, rely on Gas Safe and HSE guidance and a Gas Safe registered engineer. Always verify against your specific model’s user manual, and call a Gas Safe registered engineer for any persistent fault, gas-side fault, flue-side fault, or anything you are not certain about.
Worcester Bosch is a major UK boiler brand owned by the Bosch Group. Codes vary across the Greenstar i, Greenstar Si, CDi Classic, CDi Compact, and Greenstar 8000 ranges.
Note for all brand sections that follow: code meanings published below are illustrative manufacturer-published examples only. Code meanings can be model-dependent — always verify against your specific model’s user manual. Inline “(model-dependent)” tags on specific codes flag where meanings vary substantially between models within the same brand.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| EA / EA 227 (model-dependent) | No flame detected after multiple ignition attempts | Gas / flame | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer |
| D5 (model-dependent) | Condensate fault or low pressure | Flue / water | Verify against user guide; if frozen condensate suspected in winter, see section 14; otherwise isolate and call Gas Safe registered engineer |
| C6 | Fan speed fault | Component / flue | Isolate; call Gas Safe registered engineer |
| E9 | Safety temperature limiter tripped (overheat) | Overheat | Isolate; call Gas Safe registered engineer |
| Fd | Reset button pressed in error / flame detected after gas shut-off | Operational / safety | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer |
| EA 229 | No flame detected — frequently linked to frozen condensate in cold weather | Gas / flue | See section 14; if not condensate, isolate and call Gas Safe registered engineer |
These are illustrative codes for the most common Worcester Bosch fault scenarios. Older Greenstar models use LED flash patterns rather than displayed codes — refer to the boiler’s user guide for the specific flash sequence. For the full code reference for your specific model, see the Worcester Bosch troubleshooting tool⁵² (verified April 2026).
8. Common codes — Vaillant
Vaillant is a major UK boiler brand. Codes apply primarily to the ecoTEC plus, ecoTEC pro, ecoTEC sustain and ecoFIT sustain ranges.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| F22 | Low water pressure / system water shortage | Water | Verify against user guide; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| F28 (model-dependent) | Ignition failure (3 unsuccessful attempts) | Gas / flame | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer |
| F29 | Flame loss during operation | Gas / flame | Isolate; call Gas Safe registered engineer |
| F61 / F62 | Gas valve fault | Gas | Isolate; call Gas Safe registered engineer |
| F75 | Pump pressure or water shortage fault | Water / circulation | Isolate; call Gas Safe registered engineer |
| F83 | Flow/return temperature differential fault | Water / circulation | Isolate; call Gas Safe registered engineer |
For the full code reference, see the Vaillant boiler fault codes list⁵³ (verified April 2026).
9. Common codes — Ideal Heating
Ideal Heating is a UK manufacturer producing the Logic, Logic+, Logic Max, Vogue and Mexico ranges. Codes vary by range.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| F1 | Low system water pressure | Water | Verify against user guide; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| F2 | Flame loss / no gas detected | Gas / flame | Do not attempt to diagnose gas supply issues; isolate and call Gas Safe registered engineer |
| F3 | Fan fault | Component / flue | Isolate; call Gas Safe registered engineer |
| F4 | Flow thermistor fault | Sensor | Isolate; call Gas Safe registered engineer |
| L2 (model-dependent) | Ignition lockout — often linked to frozen condensate in winter | Gas / flue | See section 14; otherwise isolate and call Gas Safe registered engineer |
| FA | Flow and return pipes reversed (rare) | Water / installation | Isolate; call Gas Safe registered engineer (installation issue) |
For the full code reference, see the Ideal Heating fault codes FAQ⁵⁴ (verified April 2026).
10. Common codes — Baxi
Baxi is a major UK manufacturer producing the Platinum, EcoBlue, Duo-tec and 800 ranges. Codes are E-prefixed.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| E119 | Low system water pressure | Water | Verify against user guide; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| E133 (model-dependent) | Gas supply interrupted, ignition failure, flame failure, or blocked condensate | Gas / flame / flue | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer (or see section 14 if frozen condensate suspected in winter) |
| E110 / E130 | Overheat — primary water or flue system | Overheat | Isolate; call Gas Safe registered engineer |
| E125 | Circulation fault (primary circuit) | Water / circulation | Isolate; call Gas Safe registered engineer |
| E160 / E168 | Fan fault | Component / flue | Isolate; call Gas Safe registered engineer |
| E20 / E28 / E50 | Sensor faults (CH NTC / flue NTC / DHW NTC) | Sensor | Isolate; call Gas Safe registered engineer |
For the full code reference, see the Baxi error codes page⁵⁵ (verified April 2026).
11. Common codes — Glow-worm
Glow-worm is a Vaillant Group brand producing the Energy, Betacom, Ultracom, Easicom and Compact ranges. Glow-worm uses a similar F-code format to Vaillant on many modern models, though specific code meanings can still vary by exact model and firmware — always verify against your model’s user manual.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| F22 (or F9 on older models) | Low water pressure | Water | Verify against user guide; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| F28 (or F1 on older models) (model-dependent) | Ignition failure | Gas / flame | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer (or see section 14 if frozen condensate suspected) |
| F29 | Flame loss during operation | Gas / flame | Isolate; call Gas Safe registered engineer |
| F61–F68 | Main board (PCB) faults | Component | Isolate; call Gas Safe registered engineer |
| F75 | Pump pressure fault | Water / circulation | Isolate; call Gas Safe registered engineer |
| F83 | Flow/return differential fault | Water / circulation | Isolate; call Gas Safe registered engineer |
For the full code reference, see the Glow-worm boiler fault codes guide⁵⁶ (verified April 2026).
12. Common codes — Vokèra
Vokèra is a UK boiler brand. Codes are A-prefixed and apply across the Vision, Mynute, Easi-Heat and Unica ranges.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| A01 | Ignition failure / flame not sensed | Gas / flame | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer |
| A02 | Limit thermostat fault (overheat) | Overheat | Isolate; call Gas Safe registered engineer |
| A03 | Fan tacho signal fault | Component / flue | Isolate; call Gas Safe registered engineer |
| A04 | Insufficient system water pressure | Water | Verify against user manual; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| A06 | DHW thermistor fault | Sensor | Isolate; call Gas Safe registered engineer (boiler may still produce some hot water) |
| A09 | Flue NTC fault or service reminder | Flue / service | Call engineer (service reminder may not require immediate action) |
Vokèra fault codes are typically published in the boiler’s user manual rather than on a single dedicated webpage. For your specific model, refer to the user manual or the Vokèra customer support page (verified April 2026).
13. Common codes — Potterton
Potterton is a Baxi Heating brand producing the Promax, Gold, Suprima and Ultra ranges. Potterton uses a similar E-code format to Baxi on many modern models, though specific code meanings can still vary by exact model and firmware — always verify against your model’s user manual.
| Code | Common meaning | Category | First step |
|---|---|---|---|
| E119 | Low system water pressure | Water | Verify against user guide; do not repeatedly top up the system; if pressure has dropped or code returns, isolate and call Gas Safe registered engineer (see section 15) |
| E133 (model-dependent) | Gas supply interrupted, ignition failure, or blocked condensate | Gas / flame / flue | Single reset only if manufacturer guidance permits; if returns, isolate and call Gas Safe registered engineer (or see section 14 if frozen condensate suspected in winter) |
| E110 / E130 | Overheat | Overheat | Isolate; call Gas Safe registered engineer |
| E125 | Circulation fault | Water / circulation | Isolate; call Gas Safe registered engineer |
| E168 | Fan fault | Component / flue | Isolate; call Gas Safe registered engineer |
For the full code reference, see the Potterton error codes page⁵⁷ (verified April 2026).
14. Frozen condensate pipe in winter
A frozen condensate pipe is a common cause of boiler lockouts during cold weather across UK boiler brands. Modern condensing boilers produce condensate (acidic water vapour from flue gases) that drains through a plastic pipe to an outside drain or soil stack. If the pipe runs externally, in an unheated garage, or through an unheated loft, freezing temperatures can block it with ice. The boiler detects the blockage and locks out as a safety measure.
Brand-specific codes commonly linked to frozen condensate include:
- Worcester Bosch: EA 229, D5
- Baxi / Potterton: E133, E28
- Ideal Heating: L2
- Vaillant / Glow-worm: sometimes shown as F28 with cold-weather correlation
If a frozen condensate pipe is suspected, check the manufacturer’s published guidance for your specific model — manufacturer guidance may include thawing steps, but follow only the official instructions published for your model. Only visually confirm an external pipe if safely accessible; do not attempt any intervention unless explicitly permitted by the manufacturer’s guidance for your model. Do not remove boiler covers, disconnect pipework, clear internal traps, or pour boiling water onto plastic pipework — boiling water can crack the pipe. If you are not completely certain the external pipe is safely accessible, or you are not confident performing what the manufacturer’s guidance describes, do not attempt any action — call a Gas Safe registered engineer.
If the lockout code does not clear after the pipe has thawed (or after waiting for a sustained rise in outside temperature), or if the pipe re-freezes repeatedly, isolate the boiler and call a Gas Safe registered engineer. The fault may be unrelated to the condensate pipe even if the symptoms appeared in cold weather. A persistently freezing condensate pipe may need permanent re-routing — for example, to an internal drain — which is work for a Gas Safe registered engineer.
15. Low water pressure faults
Low system water pressure is another common cause of boiler lockouts. The system water pressure (shown on the boiler’s pressure gauge — usually a small dial or digital reading) needs to be in a manufacturer-specified range (commonly around 1.0–1.5 bar; check your model manual) for the boiler to operate.
Brand-specific codes for low water pressure include:
- Worcester Bosch: D5 (model-dependent), CE
- Vaillant / Glow-worm: F22
- Ideal Heating: F1
- Baxi / Potterton: E119
- Vokèra: A04
A low-pressure fault is generally a water-side fault rather than a gas-side fault. However, the underlying cause matters: pressure that drops over days or weeks indicates a leak in the heating system, a failed expansion vessel, or a faulty pressure relief valve.
If the boiler shows a low-pressure fault, check the manufacturer’s user guide to understand the code, but do not repeatedly top up the system. If pressure has dropped or the code returns, isolate the boiler and call a Gas Safe registered engineer. Recurring repressurisation is not a fix — the underlying fault should be diagnosed and addressed properly.
16. Persistent lockouts and recurring codes
A single fault code that clears after one reset and does not return is most likely a transient event — a brief power interruption, a one-off ignition glitch, or a momentary sensor reading.
A code that returns within minutes, hours, or days is not transient. It indicates an underlying fault that resetting will not resolve. Common reasons for persistent or recurring codes include:
- A failing component (gas valve, ignition electrode, fan, sensor, PCB)
- A developing leak in the heating system
- A blockage in the condensate pipe, flue, or pump
- A gas supply issue at the meter or governor
- An installation issue (incorrect commissioning, wrong gas pressure, undersized supply)
Continuing to reset a persistently locking-out boiler:
- Will not fix the underlying fault
- Risks accelerating wear on the ignition system
- Can mask a safety issue (particularly with gas, flame, or flue codes)
- May affect manufacturer warranty terms — check your specific warranty documentation
The right response to persistent or recurring codes is to stop resetting, isolate the boiler, and call a Gas Safe registered engineer.
17. Carbon monoxide and combustion faults
Carbon monoxide (CO) is a colourless, odourless gas produced by incomplete combustion of any fuel, including natural gas. It is highly toxic and can be fatal at low concentrations over time.
Symptoms of CO exposure include headache, dizziness, nausea, breathlessness, confusion, and loss of consciousness. Symptoms can be mistaken for flu or food poisoning, particularly because they often improve when the affected person leaves the property.
A correctly installed and serviced gas boiler should not produce CO at unsafe levels. However, faults that can cause incomplete combustion or CO leakage include:
- A blocked or damaged flue
- A cracked heat exchanger
- An incorrectly adjusted gas-air mixture
- A failing combustion seal
- Inadequate ventilation in the boiler’s location
Some boiler fault codes — particularly those involving the flue, fan, combustion air, or flame stability — can be early warnings that combustion is not operating correctly. These codes must be investigated by a Gas Safe registered engineer.
CO alarms — UK requirements (England)
The detail below applies in England. Two distinct frameworks apply — Building Regulations guidance (which applies on installation or replacement of a fixed combustion appliance) and a separate landlord statutory duty (which applies continuously to rented properties):
Building Regulations guidance — England. Approved Document J (as amended October 2022) gives statutory guidance that a CO alarm should be fitted where a new or replacement fixed combustion appliance burning gas (excluding gas cookers), oil, or solid fuel is installed in a dwelling.⁵⁸
Landlord statutory duty — England. For privately and socially rented properties in England, the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 (as amended 2022) require landlords to install a CO alarm in every room used as living accommodation that contains a fixed combustion appliance (excluding gas cookers).⁵⁹ ⁶²
Scotland, Wales, and Northern Ireland have their own separately specified CO alarm requirements. Even where not strictly required, fitting a working CO alarm in the same room as the boiler is strongly recommended by HSE.⁶¹
What to do if a CO alarm sounds
If a CO alarm sounds:
- Switch off the appliance and turn off the gas supply at the meter if safe to do so and you know how
- Open doors and windows for ventilation
- Evacuate the property
- Call the gas emergency service for your area from outside: 0800 111 999 (natural gas, Great Britain), 0800 002 001 (natural gas, Northern Ireland), or for LPG, the contact number on your bulk storage vessel or meter
- Seek medical advice — symptoms of CO exposure may not be immediate
- Do not re-enter the property until the gas emergency service has confirmed it is safe
- Once you have re-entered, do not use the appliance again until it has been checked by a Gas Safe registered engineer⁶¹
- For non-emergency CO safety questions, the HSE Gas Safety Advice Line is 0800 300 363 (Mon–Fri office hours)⁶⁰
Replace CO alarm batteries (or the entire alarm if it has a sealed lifetime battery) as instructed by the manufacturer, and replace the entire alarm at the manufacturer’s recommended end-of-life date (typically every 5–10 years depending on model).
18. Boiler under warranty — what to do first
If your boiler is under manufacturer warranty, the manufacturer’s customer support team is your first call. Calling a third-party engineer for a warranty repair may complicate the warranty claim or invalidate it depending on the warranty terms.
Warranty length varies — entry-level boilers are typically supplied with shorter warranties, while premium and accredited-installer routes can extend warranty length significantly. The above is indicative; specific warranty length and terms depend on the boiler model and any installer accreditation — check your warranty documentation.
Manufacturer warranties typically require:
- The boiler to be installed by a registered installer (often manufacturer-accredited)
- Annual servicing by a Gas Safe registered engineer (some manufacturers require manufacturer-accredited)
- The Benchmark commissioning checklist to be completed
- Warranty registration within 30 days of installation
- Genuine manufacturer parts to be used in any repairs
Manufacturer customer support pages (verify current contact details on the manufacturer’s official website):
- Worcester Bosch — Customer Support
- Vaillant — Contact us
- Ideal Heating — Customer Support
- Baxi — Customer Support
- Glow-worm — Contact us
- Vokèra — Customer Support
- Potterton — Customer Support
If the boiler is out of warranty, you can call a Gas Safe registered engineer of your choice. Always check the engineer’s Gas Safe registration before booking — see section 23.
19. Costs of common boiler repairs in 2026 — commercial estimate only
💷 Commercial estimate box. The price ranges below are editorial market estimates based on independent observation of UK installer pricing in early 2026, not regulated rates, audited data, or guaranteed prices, and not sourced from official regulatory data.
Methodology: ranges are derived from observed UK installer pricing and quote portals in early 2026. We do not publish a sample size or vendor list because the dataset is informal market observation, not audited research. Service-life and warranty figures elsewhere in this guide carry the same caveat.
Indicative UK/London market ranges. London prices may be higher than national averages. Prices vary significantly by property, boiler model, age, and engineer; always obtain a quote before agreeing to repair work.
| Repair / job | Indicative range |
|---|---|
| Diagnostic call-out (engineer attendance + diagnosis, no parts) | £80 – £180 |
| Annual boiler service by a Gas Safe registered engineer (no faults found) | £80 – £140 |
| Frozen condensate pipe — engineer attendance and thaw | £100 – £200 |
| Repressurisation visit (system filling, leak check) | £100 – £200 |
| Gas valve replacement (parts + labour) | £350 – £600+ |
| Ignition electrode / lead replacement | £150 – £280 |
| Flame sensor replacement | £150 – £280 |
| PCB (control board) replacement | £350 – £700+ |
| Fan replacement | £300 – £550+ |
| Pump replacement | £300 – £550+ |
| Heat exchanger replacement (combi) | £500 – £1,200+ |
| Condensate pipe re-routing (to internal drain) | £200 – £500+ |
| Power flush (whole system) | £400 – £750 |
For repairs estimated above approximately one-third the cost of a new boiler — and particularly for boilers over 12–15 years old — replacement is often more economic than repair. See our guide: boiler repair vs replacement UK (2026) for the decision framework.
If the boiler is still under manufacturer warranty, repair costs (parts and labour) may be covered subject to warranty terms. See section 18.
20. Landlord and tenant responsibilities
For privately rented properties, Section 11 of the Landlord and Tenant Act 1985 places repair duties on the landlord for installations including space heating and water heating, for tenancies covered by Section 11.¹³
Per HSE, landlords have a separate legal duty under the Gas Safety (Installation and Use) Regulations 1998 to arrange an annual gas safety check by a Gas Safe registered engineer for any gas appliances (permanent or portable) and gas flues the landlord owns and provides for tenants. Landlord gas safety record duties: existing tenants must be given a copy of the record within 28 days of the check; new tenants must be given a copy before they occupy the property; records must be kept for at least 2 years.¹⁸
In England, landlords also have a duty under the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 (as amended 2022) to install a CO alarm in every room used as living accommodation that contains a fixed combustion appliance (excluding gas cookers).⁵⁹ ⁶² Scotland, Wales, and Northern Ireland have their own separately specified CO alarm requirements for rented properties.
If you are a tenant and your boiler shows a fault code
- Note the code and any other observations (smell, sound, leaks, what happened just before).
- Contact your landlord or letting agent as soon as possible. The landlord is responsible for arranging the repair under Section 11.
- Do not commission third-party repair work unless your landlord explicitly authorises it. Tenants who arrange unauthorised repair work may not be reimbursed and may complicate the landlord’s repair pathway.
- If you smell gas, suspect a CO leak, or a CO alarm sounds — leave the property immediately and call the gas emergency service for your area: 0800 111 999 (natural gas, Great Britain), 0800 002 001 (natural gas, Northern Ireland), or for LPG the contact number on your bulk storage vessel or meter. Then notify your landlord.
If you are a council or housing-association tenant
Council and housing-association tenants normally cannot arrange their own boiler repair work — the council or housing association has its own repair contractor pathway. Always go through the council or housing association repair line first. Contact details are usually on your tenancy agreement, the council/HA website, or your most recent rent statement.
For repairs that the council or housing association is slow to action, tenants have rights under Section 11 and the Homes (Fitness for Human Habitation) Act 2018 — but the first step is still to log the repair through the official channel.
21. What we can’t advise without a site visit
This guide explains the general framework for understanding boiler fault codes. It cannot substitute for a Gas Safe registered engineer’s site visit, which is the only way to:
- Confirm the underlying cause of any fault code in your specific boiler
- Test gas pressure at the boiler inlet
- Test combustion (CO/CO₂ ratio, flue gas temperature) using calibrated equipment
- Inspect internal components (heat exchanger, gas valve, ignition electrode, sensors)
- Check the flue route, terminal, and condensate discharge route
- Confirm the boiler is operating within its design parameters
- Determine whether a repair or replacement is the better choice
This guide also cannot tell you:
- Whether your specific fault is safe to ignore until a service appointment (it generally isn’t, for gas-side codes)
- Which specific component has failed in your boiler
- The exact cost of repair for your boiler
- Whether your boiler model is approaching end-of-life
Always book a Gas Safe registered engineer for any persistent fault code, any gas-side or flue-side fault, and any code where you are not certain about the right response.
22. FAQs
No. Repeated resets do not fix the underlying fault and may cause additional damage.
If a code returns after one reset, isolate the boiler and call a Gas Safe registered engineer.
Yes, especially if the code relates to gas supply, flame, ignition, flue or combustion.
Some faults allow limited operation, but a Gas Safe registered engineer should diagnose the issue.
No. Recurring pressure loss usually indicates a leak, failed expansion vessel or pressure relief issue.
Repeated topping up is not a fix. Isolate the boiler and call a Gas Safe registered engineer.
If the code does not return, the fault was likely temporary and the boiler may operate normally.
If you noticed any unusual smell, sound or behaviour, book a Gas Safe registered engineer to inspect.
Carbon monoxide is odourless. Treat every alarm as genuine until proven otherwise.
Switch off the appliance and turn off the gas supply at the meter if safe to do so and you know how, evacuate, and call from outside: 0800 111 999 (Great Britain), 0800 002 001 (Northern Ireland), or your LPG supplier number. Do not re-enter until declared safe.
A fault code indicates a problem affecting operation.
A service reminder indicates scheduled maintenance is due, but the boiler continues operating normally.
It depends on repair cost and overall condition.
If a repair exceeds roughly one-third of a replacement cost, replacement is often more economical. Older boilers may also have limited parts availability. See our guide: boiler repair vs replacement UK (2026).
Gas work must be carried out by a Gas Safe registered engineer competent for that category of work.
A general plumber without Gas Safe registration is not legally permitted to carry out gas work.
It depends on the warranty terms.
Many warranties require repairs through the manufacturer’s support team. Always check terms first and contact the manufacturer before arranging third-party work.
Requirements differ between owner-occupied and rented homes in England.
Rented properties must have CO alarms in rooms with fixed combustion appliances. Owner-occupied homes are guided by Building Regulations. Even where not required, fitting a CO alarm is strongly recommended.
23. How to find a Gas Safe registered engineer
Gas work within the scope of the Gas Safety (Installation and Use) Regulations 1998 must be carried out by a Gas Safe registered engineer competent for that specific category of gas work.⁵ ¹⁵
The official register — the only verification source
The Gas Safe Register is the UK’s only official register of gas engineers legally permitted to undertake gas work. Use it to:
- Find a Gas Safe registered business or engineer in your area
- Check whether a specific business or engineer is registered
- Verify the categories of gas work each engineer is qualified for
You can also call the Gas Safe Register consumer helpline on 0800 408 5500 to verify a business or engineer by phone.
Always check the engineer (and their business listing) on the Gas Safe Register before booking, and check the engineer’s Gas Safe ID card before any gas work begins. The back of the card lists the categories of gas work the engineer is qualified for, including whether they are qualified for domestic boiler work. If the categories on the back of the card do not cover the work you need, do not allow gas work to proceed.
About the VerifiedPlumbers London directory
VerifiedPlumbers is a commercial directory of London plumbers and gas engineers. This directory is not endorsed by any regulator. Always verify the business and engineer directly on the Gas Safe Register before booking and before work starts. The Gas Safe Register is the only authoritative verification source.
For the directory, see VerifiedPlumbers — London.
Related decision guides
For deciding whether to repair or replace an existing boiler, see our guide: boiler repair vs replacement UK (2026) — the diagnostic-first approach is normally cheaper than replacement. For choosing between combi and system boilers if you do replace, see Combi vs System Boiler UK: Which Is Better in 2026?.
Get 2–3 quotes from Gas Safe registered engineers before agreeing to any non-emergency repair or replacement work.
Last reviewed
Last reviewed: April 2026. Reviewed against UK Building Regulations (Approved Document J), the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 as amended 2022, HSE Gas Safety guidance, the Gas Safety (Installation and Use) Regulations 1998, Gas Safe Register technical guidance, and current manufacturer support documentation from Worcester Bosch, Vaillant, Ideal Heating, Baxi, Glow-worm, Vokèra, and Potterton.
This guide is editorially independent. It includes a disclosed reference to the VerifiedPlumbers London directory in section 23. Manufacturer references are illustrative of factual product information only and do not constitute endorsement.
24. Sources
Regulatory and authoritative sources
⁵ Gas Safe Register — Check An Engineer (consumer helpline 0800 408 5500). https://www.gassaferegister.co.uk/find-an-engineer-or-check-the-register/check-an-engineer/
¹³ Landlord and Tenant Act 1985, Section 11 — legislation.gov.uk. https://www.legislation.gov.uk/ukpga/1985/70/section/11
¹⁵ HSE — Check an engineer – are they Gas Safe registered? https://www.hse.gov.uk/gas/gas-safe-register-check.htm
¹⁸ HSE — Gas safety records. https://www.hse.gov.uk/gas/landlords/gassaferecord.htm
⁵⁸ GOV.UK — Approved Document J: Combustion appliances and fuel storage systems (as amended October 2022). https://www.gov.uk/government/publications/combustion-appliances-and-fuel-storage-systems-approved-document-j
⁵⁹ Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 — legislation.gov.uk. https://www.legislation.gov.uk/ukdsi/2022/9780348234978
⁶⁰ HSE — Gas safety contacts (HSE Gas Safety Advice Line: 0800 300 363). https://www.hse.gov.uk/gas/domestic/contacts.htm
⁶¹ HSE — Gas safety: home owners. https://www.hse.gov.uk/gas/domestic/faqownerocc.htm
⁶² GOV.UK — Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022: guidance for landlords and tenants. https://www.gov.uk/government/publications/smoke-and-carbon-monoxide-alarms-explanatory-booklet-for-landlords/the-smoke-and-carbon-monoxide-alarm-england-regulations-2015-qa-booklet-for-the-private-rented-sector-landlords-and-tenants
HSE — Gas safety checks: who needs them? https://www.hse.gov.uk/gas/landlords/safetycheckswho.htm
Gas Safe Register — Contact us (gas emergency numbers and consumer helpline). https://www.gassaferegister.co.uk/contact-us/
Gas Safety (Installation and Use) Regulations 1998 — legislation.gov.uk. https://www.legislation.gov.uk/uksi/1998/2451/contents/made
Homes (Fitness for Human Habitation) Act 2018 — legislation.gov.uk. https://www.legislation.gov.uk/ukpga/2018/34
National Gas Emergency Service (Great Britain) — 0800 111 999 (free, 24/7). https://www.nationalgas.com/safety-and-emergencies/gas-emergencies
Northern Ireland Gas Emergency Service — 0800 002 001 (free, 24/7). https://www.nidirect.gov.uk/articles/energy-power-supply-and-utilities
Manufacturer references — illustrative only, not regulatory sources
These are cited for factual product-information examples (such as published fault code meanings) and are not used as authority sources for regulatory claims:
⁵² Worcester Bosch — Troubleshooting tool. https://www.worcester-bosch.co.uk/support/troubleshooting-tool
⁵³ Vaillant — Boiler fault codes list. https://www.vaillant.co.uk/service/boiler-fault-codes/
⁵⁴ Ideal Heating — Boiler fault codes FAQ. https://idealheating.com/support/faqs/boiler-fault-codes
⁵⁵ Baxi — Boiler error codes. https://www.baxi.co.uk/professional/services/technical-support/error-codes
⁵⁶ Glow-worm — Boiler fault codes guide. https://www.glow-worm.co.uk/homeowner/advice-knowledge/boiler-fault-codes-and-what-to-do-if-they-occur-2802216.html
Vokèra — Customer support. https://www.vokera.co.uk/owner-help/
⁵⁷ Potterton — Error codes. https://www.potterton.co.uk/customer-support/error-codes