Expert Reference Guide
Boiler Fault Codes Explained
Look up any error code for Worcester Bosch, Vaillant, Ideal, Baxi, Glow-worm, and more — with causes, DIY checks, and when to call an engineer.
How to Use This Guide
When your boiler displays a fault code, it is telling you something specific is wrong. This guide covers the most common error codes for all major UK boiler brands, explaining what each code means, what typically causes it, and whether you can safely address it yourself or need a Gas Safe registered engineer.
Safety first: Never attempt to repair gas components yourself. Only qualified Gas Safe registered engineers may legally work on gas appliances in the UK. If you can smell gas, turn off your supply at the meter and call the National Gas Emergency Service on 0800 111 999 immediately.
Use the search bar below to find your specific code, or scroll to your boiler brand. If you need a London engineer, our Gas Safe registered team can often attend the same day — send us your fault code on WhatsApp.
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Worcester Bosch Fault Codes
Applies to: Greenstar 2000, 4000, 8000, CDi, CDi Compact, i, Si, Ri, Life
Worcester Bosch codes appear as alphanumeric sequences on the display. Newer 8000 and Life series use a more descriptive display; older models show short codes.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| E9 / EA | Safety Thermostat / Ignition Lockout | The boiler overheated or failed to ignite. Can indicate poor circulation, pump failure, or no gas supply. | Check pressure (1–1.5 bar). Reset after boiler cools. Ensure gas supply is on. | Engineer |
| H07 / 1L6 | Low Water Pressure | System pressure has dropped below 0.5 bar, commonly due to a minor leak or natural pressure loss. | Repressurise via the filling loop to 1.0–1.5 bar — refer to your boiler manual for the filling loop location. | DIY Check |
| EA 229 / E28 | Ignition Failure — No Flame Detected | The boiler could not light the burner. Gas supply interrupted, faulty igniter, dirty flame sensor, or gas valve fault. | Check other gas appliances work. Press reset once. If the code returns, call a Gas Safe engineer. | Engineer |
| E5 | Overheat Hard Lockout | Persistent overheating has caused a hard lockout — often a blocked heat exchanger or pump failure. | Wait 30 minutes, then hold the reset button for 3 seconds. If the code returns, do not keep resetting — call an engineer. | Engineer |
| C6 | Fan Speed Too High | The fan is running above its expected speed, indicating a fan, PCB, or flue restriction problem. | No safe DIY fix for this fault. Call a Gas Safe registered engineer. | Engineer |
| C1 | Fan Speed Too Low | The fan is not reaching the required speed before ignition. Fan motor failure or blocked flue. | Check the external flue outlet is unobstructed. Call an engineer for fan inspection or replacement. | Engineer |
| 4L6 | Pump / Circulation Fault | Insufficient water circulation detected. The pump may be seized, air-locked, or the system filter is blocked. | Check pressure and that all heating valves are open. Bleed radiators. If the pump is noisy or not running, call an engineer. | Engineer |
| 6L6 | Return Temperature Sensor Fault | The return pipe NTC sensor has failed or developed a wiring problem. | No DIY fix available. Requires component replacement by a Gas Safe engineer. | Engineer |
| A1 | CH Flow NTC Sensor Fault | The central heating flow temperature sensor is reading incorrectly or has failed. | No DIY repair available. Requires engineer diagnosis and likely sensor replacement. | Engineer |
| D1 | Flue Gas Temperature Exceeded | Flue gas temperature is too high, suggesting a blocked or restricted flue or a heat exchanger problem. | Check the external flue terminal for debris, ice, or bird nests. If clear, call an engineer. | Engineer |
| F4 | False Flame Signal Detected | A flame signal is being detected when there should be none — typically a gas valve or flame sensor fault. | Switch off the boiler at the mains immediately. This is a safety fault — call a Gas Safe engineer. | Engineer |
| 2L6 | Overheat / High Limit Fault | High temperature detected at the heat exchanger. Restricted water flow or faulty thermistor. | Check pressure and ensure all heating valves are open. Press reset. If it recurs, call an engineer. | Engineer |
Vaillant Fault Codes
Applies to: ecoTEC Plus, ecoTEC Pro, ecoFIT Pure, ecoTEC Exclusive, turboTEC Plus
Vaillant displays F.XX fault codes and S.XX status codes. F codes are active faults requiring attention; S codes are informational. Both appear on the front panel digital display.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| F.22 | Low Water Pressure | Pressure has dropped below 0.5 bar — most commonly a slow leak or natural pressure loss over time. | Top up via the filling loop to 1.0–2.0 bar (1.2 bar is ideal). Check for visible leaks around radiators or pipes. | DIY Check |
| F.75 | No Pressure Change on Start-Up | The boiler started but detected no pressure change — usually a faulty pump or a failed pressure sensor. | Check water pressure first. If correct, an engineer is needed to inspect the pump and pressure sensor. | Engineer |
| F.28 | Ignition Failure — No Gas Detected | The boiler could not ignite because no gas was detected. Gas supply interrupted, faulty gas valve, or ignition fault. | Check other gas appliances. Reset by holding the reset button for 3 seconds. If persistent, call an engineer. | Engineer |
| F.29 | Flame Failure After Lighting | The boiler lit but the flame went out unexpectedly. Dirty flame sensor, gas fluctuation, or a draught problem. | Press reset. Repeated flame loss requires professional diagnosis. | Engineer |
| F.20 | Safety Temperature Limiter Tripped | The overheat safety device has cut out the boiler. Often a circulation fault, pump issue, or blocked heat exchanger. | Let the boiler cool for 30 minutes. Check pressure and reset. Do not keep resetting — call an engineer if recurring. | Engineer |
| F.24 | Flow Temperature Rising Too Quickly | The flow temperature is climbing abnormally fast, suggesting restricted water circulation through the boiler. | Open all radiator valves. Check accessible system filters. Call an engineer if the issue persists. | Engineer |
| F.00 / F.10 | Flow NTC Temperature Sensor Fault | F.00 = sensor short circuit; F.10 = open circuit. The flow temperature sensor has failed or has a wiring fault. | No DIY fix available. Requires sensor replacement by a Gas Safe registered engineer. | Engineer |
| F.01 / F.11 | Return NTC Temperature Sensor Fault | F.01 = short circuit; F.11 = open circuit on the return temperature sensor. Sensor failure or wiring issue. | No DIY fix. Requires engineer inspection and likely sensor replacement. | Engineer |
| F.25 | Flue Gas Overheating | Flue gas temperature has exceeded safe limits — usually caused by a blocked or restricted flue, or a heat exchanger problem. | Check the external flue outlet for blockages such as ice, debris, or bird nests. If clear, call an engineer. | Engineer |
| F.32 | Fan Speed Fault | The fan is not maintaining the correct operating speed. Fan motor failure, PCB fault, or a blocked flue. | Check the flue outlet is clear. No DIY repair — requires a Gas Safe engineer. | Engineer |
| F.49 | eBus Communication Fault | A communication error between the boiler and a connected room thermostat or programmer on the eBus wiring. | Check all cable connections to your thermostat or controller. Restart both devices. If unresolved, call an engineer. | DIY Check |
| F.73 / F.74 | Water Pressure Sensor Fault / Overpressure | F.73 = pressure sensor short circuit; F.74 = system pressure too high (over 3 bar). Sensor fault or genuine overpressure. | If pressure reads over 3 bar, bleed a radiator to reduce it. For F.73, an engineer is needed. | Engineer |
| F.61 / F.62 | Gas Valve Fault | F.61 = gas valve control fault; F.62 = gas valve sealing fault. These are serious faults requiring immediate attention. | Switch off the boiler at the mains. Do not reset repeatedly. Call a Gas Safe engineer. | Engineer |
Ideal Boilers Fault Codes
Applies to: Logic+, Logic Max, Vogue Max, Logic Heat, Logic Combi, Mexico, Isar
Ideal uses F codes for live faults and L codes for lockouts that require a manual reset. L codes will not clear until the underlying fault is resolved and the reset button is pressed.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| F1 | Ignition Lockout | The boiler failed to ignite after multiple attempts. Gas supply issue, faulty igniter, or dirty flame sensor. | Check gas supply. Press reset. If the code appears 3 or more times, call a Gas Safe engineer. | Engineer |
| F2 | Low Water Pressure | Pressure has fallen below 0.5 bar due to a slow leak or natural pressure loss. | Repressurise via the filling loop to 1–1.5 bar. Consult your boiler manual for the filling loop location. | DIY Check |
| F3 | Overheat Lockout | The overheat safety device has tripped — usually a blocked filter, pump failure, or restricted flow rate. | Let the boiler cool. Check pressure and that all heating zone valves are open. Reset. Repeated trips need an engineer. | Engineer |
| F4 | Heat Exchanger NTC Sensor Fault | The heat exchanger temperature sensor has failed or has a wiring fault. | No DIY fix. Requires an engineer to test and replace the sensor. | Engineer |
| F5 | CH Flow NTC Sensor Fault | The central heating flow temperature sensor is faulty or has disconnected from the PCB. | No DIY fix. Requires a Gas Safe engineer. | Engineer |
| F6 | Return NTC Sensor Fault | The return pipe temperature sensor has failed or has a wiring issue. | No DIY fix. Requires engineer inspection. | Engineer |
| F7 | Flue Safety Stat Fault | The flue safety thermostat has detected overheating in the flue — blocked flue or faulty stat. | Check the external flue terminal for blockages. Call a Gas Safe engineer if clear. | Engineer |
| F8 | Pump Fault | The pump is not functioning correctly — may be seized, air-locked, or the PCB is not receiving correct feedback. | Check system pressure. Bleeding the pump access point may help temporarily. Engineer required for pump replacement. | Engineer |
| F9 | Fan Fault | The fan is not reaching operating speed or has stopped. Fan motor failure or flue restriction. | Check the flue outlet is unobstructed. No DIY repair — call an engineer. | Engineer |
| L1 | Low Pressure Lockout | A manual-reset lockout caused by persistent low pressure. The boiler will not restart automatically. | Repressurise the system to 1–1.5 bar, then press and hold the reset button for 10 seconds. | DIY Check |
| L2 | Flue Blockage Lockout | The boiler detected a blocked or restricted flue and locked out for safety. | Check the external flue terminal for blockages (ice in winter, bird nests in summer). If clear, call an engineer. | Engineer |
Baxi Fault Codes
Applies to: Duo-tec, Platinum, Main Eco, Megaflo, 100, 200, 400, 600, 800 series
Baxi boilers show E-codes for current faults and three-digit numbers in the fault log history. Some models also use amber and red LED indicators alongside the digital display.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| E1 / E131 / 133 | Ignition Lockout | Failed ignition after multiple attempts. Faulty ignition lead, electrode, no gas supply, or dirty flame sensor. | Check gas supply is on. Press reset. If it locks out again, call a Gas Safe engineer. | Engineer |
| E2 / 119 | Flame Loss During Operation | The boiler lit successfully but the flame was lost mid-cycle. Gas supply issue or draught problem. | Press reset. Persistent flame loss requires a Gas Safe engineer to investigate. | Engineer |
| E3 / 100 / 125 | Overheat Safety Stat Tripped | The boiler overheated. Circulation fault, blocked filter, or pump issue. | Let the boiler cool for 30 minutes, check pressure, then press reset. Recurring trips need an engineer. | Engineer |
| E4 / 160 | Fan Fault | The fan is not running at the correct speed. Fan motor failure or blocked flue. | Check the external flue terminal for obvious blockages. Requires engineer for fan replacement. | Engineer |
| E5 / 168 | Low Water Pressure | System pressure has dropped below the safe minimum level. | Repressurise using the filling loop to 1–1.5 bar. Check for visible leaks around the system. | DIY Check |
| 118 | Gas Pressure / Gas Valve Fault | The boiler is not receiving adequate gas pressure, or the gas valve has developed a fault. | Check that other gas appliances work. Call a Gas Safe engineer — never attempt gas valve work yourself. | Engineer |
| 128 | Flue Fault / Thermofuse Tripped | The flue overheat thermostat has tripped or blown — usually caused by a blocked flue or combustion problem. | Check the external flue terminal for blockages. If clear, an engineer is needed to replace the thermofuse. | Engineer |
Found Your Fault Code?
If your boiler needs an engineer, our Gas Safe registered team covers all London boroughs — with same-day availability most days.
Glow-worm Fault Codes
Applies to: Flexicom, Ultracom, Energy, Betacom, Easicom, Clearly Flexicom
Glow-worm and Vaillant are part of the same group and share a similar fault code system. Codes are displayed as F.XX on the front panel digital display.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| F.1 / F.2 | Ignition Lockout | The boiler could not light the burner after the full ignition sequence. Gas supply issue, faulty igniter, or flame sensor problem. | Check other gas appliances are working. Press reset. Repeated lockouts need a Gas Safe engineer. | Engineer |
| F.22 | Low Water Pressure | System pressure is below the safe minimum — often a slow leak or normal pressure loss. | Top up via the filling loop to 1.0–1.5 bar. Refer to your boiler manual. | DIY Check |
| F.28 | No Gas Detected / Ignition Failure | No gas was detected during the ignition sequence. Gas supply interrupted or gas valve fault. | Check other gas appliances work. Reset. If persistent, call a Gas Safe engineer. | Engineer |
| F.29 | Flame Failure After Lighting | The boiler lit but the flame went out. Dirty or faulty flame sensor, gas fluctuation, or draught issue. | Press reset. If it repeats, a Gas Safe engineer is needed. | Engineer |
| F.75 | No Pressure Change on Start-Up (Pump / Sensor) | No pressure change was detected when the boiler started, suggesting a pump fault or a faulty pressure sensor. | Check water pressure. If correct, an engineer is needed to inspect the pump and pressure sensor. | Engineer |
| F.20 | Safety Temperature Limiter Tripped | The boiler overheated. Typically a circulation fault, pump issue, or blocked heat exchanger. | Let the boiler cool for 30 minutes. Check pressure and valves. Reset. Recurring trips require an engineer. | Engineer |
| F.24 | Temperature Rising Too Quickly | Abnormally fast temperature rise, suggesting restricted water flow through the heat exchanger. | Check all heating valves are open. Check system filter if accessible. Call an engineer if unresolved. | Engineer |
| F.10 / F.11 | NTC Temperature Sensor Fault (Flow / Return) | F.10 = flow sensor fault; F.11 = return sensor fault. Sensor failure or wiring issue. | No DIY fix. Requires sensor replacement by a Gas Safe registered engineer. | Engineer |
Potterton Fault Codes
Applies to: Titanium, Performa, Prima, Gold, Promax, Sirius
Potterton and Baxi are sister brands sharing similar fault code systems. Codes are displayed as E-codes on the digital display. Always confirm codes against your specific model manual.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| E1 | Ignition / Flame Fault | Failed ignition or flame loss. Gas supply, ignition electrode, or flame sensor issue. | Check gas supply. Press reset. Repeated occurrences need a Gas Safe engineer. | Engineer |
| E2 | Fan Fault | The boiler fan is not operating correctly. Fan motor failure or PCB fault. | Check the flue terminal is clear. No DIY repair — call an engineer. | Engineer |
| E4 | Overheat Safety Lockout | The boiler has overheated and the safety thermostat has tripped. | Let it cool, check pressure, reset. Recurring overheats need engineer attention. | Engineer |
| E5 / E18 | Low Water Pressure | System pressure has fallen below the safe operating level. | Repressurise to 1–1.5 bar via the filling loop, then reset if needed. | DIY Check |
| E11 / E28 / E133 | Ignition Lockout / Safety Shutdown | Multiple failed ignition attempts or gas supply fault has caused a safety lockout. | Check gas supply is on. Reset once. Do not keep resetting — call an engineer if the code returns. | Engineer |
Alpha Boilers Fault Codes
Applies to: E-Tec, E-Tec Plus, InTec, CD, CD Eco
Alpha boilers display fault codes as E-XX on their digital display. Some older models use a flashing LED sequence instead of a numeric code. Always check the model-specific manual to confirm.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| E01 | Ignition Failure / Fan Not at Speed | The boiler could not establish ignition, often because the fan did not reach the required speed, or there is no gas. | Check gas supply. Reset. Recurring failures need a Gas Safe engineer. | Engineer |
| E02 | Overheat Safety Shutdown | The boiler has overheated. Pump fault or restricted water flow is likely. | Let the boiler cool. Check pressure and ensure all heating valves are open. Reset. Repeated overheats need engineer attention. | Engineer |
| E03 | NTC Temperature Sensor Fault | One or more temperature sensors has failed or has a wiring issue. | No DIY fix. Requires engineer diagnosis and sensor replacement. | Engineer |
| E04 | Flame Failure | The boiler ignited but lost the flame. Gas supply issue, dirty flame sensor, or draught problem. | Press reset. Persistent flame loss requires professional diagnosis. | Engineer |
| E05 | Gas Valve Fault | The gas valve is not opening or closing correctly, or is receiving incorrect signals from the PCB. | Do not attempt any DIY work on the gas valve. Call a Gas Safe registered engineer immediately. | Engineer |
| E06 | Low Water Pressure | System pressure has dropped below the minimum safe level. | Repressurise to 1–1.5 bar using the filling loop. Reset if needed. | DIY Check |
| E07 | Fan Fault | The fan is not running or not reaching the required speed before the ignition sequence can start. | Check the flue terminal for obstructions. An engineer is required for fan replacement. | Engineer |
Viessmann Fault Codes
Applies to: Vitodens 050-W, 100-W, 200-W, 222-F, 333-F
Viessmann uses alphanumeric diagnostic codes displayed on the front panel or accessible through the ViCare app. Code meanings can vary slightly between older and newer product ranges.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| A1 | Ignition / Burner Fault | The burner has shut down or failed to start. Gas supply issue, faulty igniter, or combustion fault. | Check gas supply. Reset the boiler. If the fault occurs again, call a Gas Safe engineer. | Engineer |
| A3 | Fan Fault | The fan is not reaching the required speed. Fan motor failure or PCB issue. | Check flue outlet is clear of obstructions. Requires engineer for fan replacement. | Engineer |
| D1 | Burner Safety Shutdown | The burner has repeatedly failed to light, triggering a safety shutdown. | Reset the boiler. If the fault recurs, call a Gas Safe registered engineer. | Engineer |
| E3 | Safety Temperature Limiter Tripped | The boiler has overheated and the safety limiter tripped. Typically a pump or circulation fault. | Let it cool, check pressure, reset. Recurring trips require engineer attention. | Engineer |
| E4 | No Flame Signal Detected | The flame sensor is not detecting a flame during or after the ignition sequence. Sensor or gas issue. | Press reset. If the fault persists, a Gas Safe engineer is needed. | Engineer |
| E7 | CH Flow Temperature Sensor Fault | The central heating flow sensor is faulty or has disconnected from the PCB. | No DIY fix. Requires sensor replacement by a qualified Gas Safe engineer. | Engineer |
| F1 | Flame Failure | The burner flame was lost unexpectedly during normal operation. | Reset and observe. Persistent flame loss requires professional investigation. | Engineer |
Ariston Fault Codes
Applies to: Alteas One, Genus One, Clas ONE, E-Combi, E-System, Net
Ariston boilers display numeric error codes on their LCD panels. Some models prefix the code with a flashing 'E'. Always refer to your specific model manual to confirm the exact meaning.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| 101 | Ignition Failure | The boiler failed to ignite. Gas supply interruption, faulty electrode, or gas valve issue. | Check gas supply is on. Reset. Repeated failures need a Gas Safe engineer. | Engineer |
| 102 | Flame Failure During Operation | The boiler lost its flame mid-cycle. Draught issue, gas supply fluctuation, or dirty flame sensor. | Press reset. If the fault recurs, call a Gas Safe engineer. | Engineer |
| 103 | Overheating | The boiler exceeded its maximum safe temperature. Circulation fault, pump issue, or restricted flow. | Let it cool, check pressure, reset. Recurring overheating must be investigated by an engineer. | Engineer |
| 104 | Low Water Pressure | System pressure is too low for safe operation. | Repressurise to 1–1.5 bar via the filling loop. Reset if required. | DIY Check |
| 105 | Fan Fault | The fan is not running or not reaching the correct speed. Fan motor failure or blocked flue. | Check the flue terminal. Requires engineer for repair. | Engineer |
| 109 | NTC Temperature Sensor Fault | A temperature sensor has failed or has an open/short circuit condition. | No DIY fix available. A Gas Safe engineer is required. | Engineer |
Ferroli Fault Codes
Applies to: Modena, Combi 30C, Domina, Optimax, BlueHelix, Antiga
Ferroli uses E-codes for live faults and A-codes for safety lockouts. A-code lockouts require a manual reset after the fault has been resolved.
| Code | Fault | Likely Cause | What to Try | Action |
|---|---|---|---|---|
| E01 / A01 | Ignition Failure / Safety Lockout | E01 = failed to ignite; A01 = lockout after multiple ignition failures. Gas, igniter, or flame sensor issue. | Check gas supply. Press reset. Repeated lockouts require a Gas Safe engineer. | Engineer |
| E02 | Flame Failure During Operation | The boiler lost its flame mid-cycle. Gas supply issue, combustion fault, or flame sensor problem. | Press reset. Recurring flame failure needs a Gas Safe engineer. | Engineer |
| E03 | Overheating | The boiler overheated. Pump failure, blocked heat exchanger, or restricted flow. | Let the boiler cool. Check pressure and valves. Reset. Call an engineer if it recurs. | Engineer |
| E04 | Flue / Thermofuse Fault | The flue overheat protection has tripped, often due to a blocked flue or combustion issue. | Check the external flue terminal for blockages. If the thermofuse has blown, an engineer is needed to replace it. | Engineer |
| E05 | Fan Fault | The fan is not running correctly. Fan motor failure or blocked flue. | Check flue terminal for obstructions. Requires engineer for fan repair. | Engineer |
| E06 | Low Water Pressure | System pressure has dropped below the minimum. Leak or normal pressure loss. | Repressurise to 1–1.5 bar using the filling loop. Reset if needed. | DIY Check |
| E15 | Condensate Pipe Blocked or Frozen | The condensate drain pipe is blocked or frozen, preventing condensate water from draining away. Very common in cold weather. | Check the pipe for ice or blockages. Pour warm (not boiling) water over any frozen section outside. Reset the boiler once clear. | DIY Check |
Practical Advice from Our Engineers
Common DIY steps and safety guidance from our Gas Safe registered team.
How to Repressurise Your Boiler
- The pressure gauge should read 1.0–1.5 bar when the system is cold
- Locate the filling loop — usually a grey or silver braided flexible hose beneath the boiler
- Open both filling loop valves slowly until the gauge reads 1.2–1.5 bar
- Close both valves fully and press the reset button
- If pressure drops again within a few days, there is likely a leak — call an engineer
How to Reset Your Boiler
- Find the reset button — usually red and labelled "Reset" or with a flame symbol
- Hold the button for 3–10 seconds (varies by brand; check your manual)
- Release and wait 30–60 seconds for the boiler to attempt a restart
- Do not reset more than twice — repeated resets can damage components or mask a serious fault
- If the fault code returns after two resets, call a Gas Safe engineer
When to Call an Engineer Immediately
- You can smell gas anywhere in the property — turn off gas at the meter and call 0800 111 999
- The boiler flame is yellow or orange instead of blue
- You or anyone in the home feels dizzy, sick, or has headaches — possible carbon monoxide
- The fault code returns after 2–3 reset attempts
- Any code relating to the gas valve, heat exchanger, or flame sensor
- The boiler is making unusual banging, clunking, or high-pitched whistling noises
Frozen Condensate Pipe (Winter)
- The condensate pipe is usually a white plastic pipe exiting the boiler to an outside drain
- Pour warm (not boiling) water over the section of pipe running outside
- Do not use a heat gun or boiling water — this can crack or damage the pipe
- Reset the boiler once the blockage clears and water flows freely
- Insulate the external section to prevent repeat freezing in cold spells
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