Boiler Fault Codes

If your boiler is showing a fault code, use the lists below to identify the issue. These codes are for quick reference — always check your boiler manual for full details.

GLOW-WORM

VAILLANT

NAVIEN

ALPHA

IDEAL BOILER FAULT CODES

IDEAL

If you smell gas, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.

This is a quick-reference guide only. Always check your boiler manufacturer’s instructions for the full list of codes and recommended actions.

Common Ideal Boiler Fault Codes

F — Boiler in frost mode.
FD — No water flow; this can sometimes be resolved by repressurising the system.
F1 — Low water pressure; repressurise to 1–1.5 bar.
F2 / FL — Flame loss; check the gas supply and try one reset.
F3 — Fan fault; requires a Gas Safe engineer.
F4 — Thermistor fault (temperature sensor).
F5 — Return thermistor fault.
F6 — Outside sensor fault.
F7 — Low mains voltage.
F8 — PCB fault.
F9 — Unconfigured or faulty PCB.
FU — Flow/return differential above 50°C; check valves.

Ideal L-Series Fault Codes

L1 — Low water flow or overheating.
L2 — Ignition lockout; check the condensate pipe during cold weather.
L3 — No-flow thermistor fault.
L4 — Flow thermistor fault.
L5 — Boiler return pipe issue or too many resets.
L6 — Flame-detection fault or false flame.
L9 / H9 / HA / LA — Overheating; check pressure and bleed radiators.
LF — Ignition fault.
LC — Five resets in 15 minutes; boiler lockout.
L-F — Boiler fails to ignite.
L-A — Boiler overheated.

Ideal C-Series & Other Codes

C0 — Central-heating signal issue.
C2 — Boiler chip-card fault.

When to Call a Gas Safe Engineer

Fan faults, such as F3
PCB faults, such as F8 or F9
Repeated ignition lockouts, such as L2 or F2
Overheat lockouts, such as L1, L9, HA or LA
Any smell of gas — leave immediately and call 0800 111 999

BAXI BOILER FAULT CODES

BAXI

Pressure & Water Flow Faults

E119 — Low Water Pressure: Pressure below 0.5 bar; repressurise via the filling loop.
E1 / E118 / E207 — No Flow, Airlock or Low Pressure: Airlock or low system pressure; bleed radiators and repressurise.
E117 — High Water Pressure: Pressure above 2.5–3 bar; release pressure via a radiator bleed valve.
E125 — Circulation Fault: Pump failure, airlock or blocked heat exchanger.

Ignition & Gas Supply Faults

E133 — Ignition Lockout: Failed ignition due to a gas-supply or ignition-electrode fault.
E168 — Flame Detection / Generic Lockout: Flame lost or unspecified lockout; often caused by sensor or condensate issues.

Overheat & Temperature Sensor Faults

E110 — Overheat: Overheat thermostat tripped; often related to the pump or sludge affecting circulation.
E199 — Temperature Sensor Fault: Sensor or wiring issue; requires an engineer.

Fan, Flue & Air Pressure Faults

E160 — Fan Fault: Flue fan not running correctly; requires an engineer.
E28 — Flue Fault: Blocked or faulty flue components.
E50 / E51 — Air Pressure / Condensate Issues: Often caused by frozen condensate pipes in cold weather.

Sensor & Electrical Faults

E78 — Diverter Valve Fault: Stuck or failing diverter valve; diaphragm or motor issue.
A-Series Codes (A.00.28, A.02.06, etc.) — Sensor disconnections, low system pressure or communication errors.

What You Can Safely Check Yourself

Low pressure (E119, E1) — Repressurise to 1–1.5 bar.
Airlocks — Bleed radiators.
Frozen condensate pipe — Safely thaw with warm water.
One reset only — If the same code returns, stop resetting and call a Gas Safe engineer.

When to Call a Gas Safe Engineer

Repeated lockouts, including E133, E168 or E160
Any gas, ignition, fan or flue-related fault
Persistent overheat faults, such as E110
Sensor or PCB communication faults, including A-series codes or E199

WORCESTER BOSCH BOILER FAULT CODES

WORCESTER

Ignition & Flame Detection Faults

EA / 227 — Ignition Lockout: No flame detected after ignition attempts. Possible causes include gas-supply issues, a frozen condensate pipe, faulty ignition electrode or a dirty flame sensor.
Action: Check the gas supply, thaw a frozen condensate pipe if applicable and try one reset. Call an engineer if it returns.
EA 229 — Flame Lost During Operation: Flame extinguishes mid-cycle.
Action: Engineer inspection required.
F7 — Flame Detected Before Ignition: Incorrect flame signal.
Action: Engineer-only repair.
FA 306 — False Flame Fault: Flame detected after the gas valve closes.
Action: Gas valve or PCB inspection required.

Overheat & Temperature Faults

E9 — Overheat Protection / High Flow Temperature: Water temperature is too high, often above 95°C. Causes may include low pressure, airlocks, pump failure or a blocked heat exchanger.
Action: Check pressure is 1–1.5 bar, bleed radiators and reset once. Call an engineer if recurring.
D4 — Flow Temperature Rising Too Fast: Circulation restriction caused by sludge, closed valves or airlocks.
Action: Engineer investigation required; a powerflush may be needed.

Pressure & Pump Faults

A1 / 281 — Pump Stuck or Low Water Pressure: Pump not circulating or system pressure too low.
Action: Repressurise to 1–1.5 bar and bleed radiators. Call an engineer if the pump fault persists.
1017 / CE 207 — Low System Pressure:
Action: Repressurise via the filling loop.

Fan & Airflow Faults

C7 — Fan Not Running on Start-up: Fan fails to reach the correct speed.
Action: Engineer-only repair.
C6 — Fan Speed Too High or Low: Dirty fan, faulty sensor or PCB issue.
Action: Reset once; an engineer should inspect and repair the fan.
C1 / C4 — Fan Stopping or Running Continuously: Wiring or PCB faults.
Action: Engineer inspection required.

Sensor Faults

D1 — Flow/Return Temperature Sensor Fault: Sensor reading out of range.
Action: Try one reset; call an engineer if recurring.
E2 — Flow Temperature NTC Sensor Fault: Faulty sensor or wiring.
Action: Engineer inspection and possible sensor replacement.
1021 B — Hot Water Sensor Fault:
Action: Engineer to replace the sensor.

Electrical, PCB & Control Module Faults

9U / F0 / A8 / CC — Control Box or Module Error: Loose wiring or defective PCB/HCM.
Action: Power-cycle once; module replacement may be required.
360 V — Incompatible Code Plug: Incorrect installation.
Action: Engineer to fit the correct plug.
B7 — Internal Control Board Fault:
Action: Engineer inspection required.

Worcester Bosch Fault Code Categories

Blocking errors — Boiler stops, but the code is hidden in the info menu.
Locking errors — Code is shown; boiler will not restart until reset.
Fault codes — Alphanumeric codes shown on the display.
Cause codes — Three-digit codes in the info menu.
Maintenance codes (H-series) — Boiler still runs but needs inspection.

Safe Checks You Can Carry Out

Repressurising the system
Thawing a frozen condensate pipe
Bleeding radiators
One reset attempt only

Call a Gas Safe Engineer For

Fan issues, such as C6 or C7
PCB/HCM faults, such as 9U, F0 or B7
Gas-valve faults, such as FA or F7
Repeated ignition lockouts, such as EA / 227

VOKERA BOILER FAULT CODES

VOKERA

General Status Code

_St — Auto-stop activated.
CL — Service required.
P — Preheating active.
Hours — Standby mode; boiler off.

Vokera A-Series Fault Codes

A01 — Ignition Failure: Gas supply, ignition electrode or gas-valve issue.
A02 — Limit Thermostat / Overheat Fault: Often caused by restricted water flow or pump issues.
A03 — Fan Fault: Fan is not removing combustion gases correctly; requires an engineer.
A04 — Low System Water Pressure: Can sometimes be resolved by repressurising.
A06 — Hot Water Temperature Sensor Fault: DHW thermistor issue.
A07 — Main Flow Temperature Sensor Fault: Overheating in flow water.
A08 — Return Temperature Sensor Fault: Return-water overheating.
A09 — Flue Temperature Sensor Fault: Flue thermistor problem.
A40 — Low Pressure: Common on many models.
A41 — Pressure Sensor Fault: Sensor malfunction.
A44 — Circulation Fault: Pump or water-flow issue.
A51 — Gas Valve Fault: Requires a Gas Safe engineer.
A77 — Flue Thermostat Triggered: Overheat in the flue area.

Vokera AL-Series Fault Codes (Mynute Models)

AL10, AL20, AL21, AL26, AL28, AL34 — Try resetting the boiler once.
AL40 / AL41 — Low pressure; refill the system and reset.
AL52, AL55, AL60, AL71, AL73, AL74, AL79 — Require professional diagnosis.

Vokera E-Series Fault Codes (Evolve Models)

E10 — Flame Lockout: Ignition or flame-detection issue.
E020 — Thermostat Fault: Overheating or sensor issue.
E030 — Fan Fault: Engineer required.
E040–E042 — Water Pressure / Transducer Fault
E060 — Hot Water Probe Fault
E090 — Flue Overheating
E091 — Heat Exchanger Blockage
E058 — Voltage / Power-Supply Issue
E085 / E094 / E095 — Combustion Fault: Call a Gas Safe engineer.

Safe Troubleshooting Steps

Reset once — Use the reset button and wait for the boiler to restart.
Check water pressure — It should normally be between 1.0 and 1.5 bar.
Check the gas supply — Ensure the meter is on and other gas appliances work.
Bleed radiators — This may help resolve overheating or flow issues.

If the fault returns after one reset, contact GAS365 Heating & Boiler Services.

Summary

Vokera boiler fault codes help identify issues such as ignition failure, low pressure, overheating, sensor faults and fan problems. Minor faults may be resolved by checking pressure or carrying out one reset, but persistent or combustion-related faults require professional attention.

GLOW-WORM BOILER FAULT CODES

GLOW-WORM

Glow-worm boilers display fault codes beginning with F followed by a number. These can indicate anything from low water pressure to ignition, sensor or safety-related faults.

Common Glow-worm Fault Codes

F22 / F9 — Low Water Pressure: Usually below 1 bar. Possible causes include leaks, bleeding radiators without topping up, or a faulty expansion vessel.
Action: Repressurise the system using the filling loop to 1–1.5 bar, then reset the boiler.
F28 / F29 — Ignition Fault: Often caused by a frozen condensate pipe.
Action: Carefully thaw the external condensate pipe with warm water, then reset the boiler once.
F75 — No Pressure Rise When Pump Runs: The boiler cannot detect a pressure rise when the pump runs. Possible causes include a stuck or failed pump, blocked pressure-sensor pipe, airlocks or sludge in the system.
Action: Bleed radiators and top up the system pressure. If unresolved, a Gas Safe engineer may need to replace the pump or sensor.
Other Fault Codes — Gas Valve, PCB, Sensor or Overheating Faults: These are safety-critical faults that can trigger a boiler lockout.
Action: Always contact a Gas Safe registered engineer.

Safety & Troubleshooting Tips

Reset once only — On Betacom models, turn the central-heating temperature control knob to the reset position, wait 30 seconds, then return it to normal operation.
Never open the boiler casing or attempt gas or electrical repairs yourself; this is dangerous and must be carried out by a qualified Gas Safe engineer.

VAILLANT BOILER FAULT CODES

VAILLANT

Common Vaillant Fault Codes

F22 — Low Water Pressure: System pressure has dropped below the minimum threshold, often below 0.6 bar.
Action: This can sometimes be resolved by topping up the pressure using the filling loop. Persistent pressure loss may indicate a leak and needs professional attention.
F27 — False Flame Detection: The boiler detects a flame when it should not, triggering a safety shutdown.
Action: Usually caused by a faulty flame sensor or main circuit board; call an engineer.
F28 — Ignition Failure: The boiler fails to ignite or detect a flame on start-up. Possible causes include a temporary gas-supply interruption, faulty electrode or frozen condensate pipe.
Action: Try one reset only. If it returns, contact a Gas Safe engineer.
F29 — Flame Loss During Operation: The flame is lost after ignition and re-ignition attempts fail. This can indicate intermittent gas or combustion issues.

Action: Engineer inspection required.

F75 — Pressure Sensor or Pump Issue: No pressure change detected when the pump starts. Possible causes include a faulty pressure sensor, defective pump or blockages in the system.
Action: Call a Gas Safe engineer.

Other Vaillant Fault Codes

F23 — Intermittent heating, noisy pump or an overheated pump.
F32 / F33 — Fan issue; fan not running or running erratically.
F49 — Controls unresponsive or intermittent.
F61 / F62 — Gas-valve errors or delayed shutdown.

Troubleshooting & Safety Tips

Reset once only — Minor faults can sometimes be cleared by turning the boiler off, waiting one minute and resetting it. Always follow the reset procedure in your user manual.
Low pressure (F22) — Competent users may be able to repressurise the system. Ensure the system is cold and follow safety instructions.
Professional help — Ignition, flame-detection and pump-related faults require a Gas Safe registered engineer. Never open the boiler casing or access internal components.
Regular maintenance — Annual servicing helps prevent common faults, including pressure drops, sludge build-up and sensor failures.

NAVIEN BOILER FAULT CODES

NAVIEN

Navien boilers display fault codes on the control panel to help identify issues within the system. These common codes can help homeowners understand when a simple check may be appropriate and when professional support is needed.

Common Navien Fault Codes

E003 — Ignition Failure: The boiler cannot produce a stable flame. Possible causes include an incorrect gas supply, faulty ignition leads or an issue with the flame-sensing electrode.
E001 — Heat Exchanger Overheating: Often caused by poor water flow, air trapped in the system or a faulty circulating pump.
E012 — Flame Loss: Can be caused by a temporary gas-supply interruption or a blocked condensate pipe.
E016 — Secondary Heat Exchanger Overheating: The secondary heat exchanger has reached an unsafe temperature.
E030 — Exhaust Gas Temperature Safety Lockout: The exhaust gas temperature has risen above approximately 65°C.
E046 — Exhaust System / Control Board Fault: The control board has detected an abnormality with the exhaust system.
E047 — Exhaust System / Control Board Fault: The control board has detected an abnormality with the exhaust system.
E060 — Control Board Fault: An abnormality has been detected with the control board.
E109 — Fan Motor Fault: Fan motor is not reaching the correct operating speed.
E110 — High Air Pressure: Air pressure within the combustion system is too high.
E127 — Air Pressure Sensor Fault: Problem with the Air Pressure Sensor (APS).
E205 — Heating Flow Thermistor Fault: Problem with the heating supply (flow) thermistor.
E218 — Heating Return Thermistor Fault: Problem with the heating return thermistor.
E302 — Low Water Pressure: The system may need repressurising.
E351 — Central Heating Pressure Fault: The central-heating circuit cannot reach or hold its target pressure.

When to Call a Gas Safe Engineer

Ignition, overheating, fan, exhaust-system, air-pressure, control-board and thermistor faults should be checked by a Gas Safe registered engineer. For E302 or E351, repressurising may help, but recurring pressure loss should always be professionally investigated.

ALPHA BOILER FAULT CODES

ALPHA

Alpha boilers display fault codes to help identify issues such as pressure loss, ignition failure and sensor faults.

Common Alpha Boiler Fault Codes

E01 — Ignition Failure: The boiler failed to ignite. Possible causes include a gas-supply issue or faulty ignition lead.
Action: Check the gas supply and ensure the meter is open. If the fault continues, contact a Gas Safe engineer.
E10 — Low Water Pressure: System pressure has dropped below the recommended level, usually below 0.6 bar.
Action: Repressurise using the filling loop. If the pressure continues to drop, there may be a leak that requires investigation.
E20 — Flame Detection Issue: The boiler cannot detect a flame. This may be due to a gas-supply problem or a faulty flame sensor.
Action: Contact a Gas Safe engineer; do not open the boiler casing or attempt internal repairs.
E37 — Incorrect System Pressure: The boiler cannot read the correct system pressure.
Action: Ensure the system is properly filled. Ongoing pressure loss may indicate a leak.
E96 — Fan or Flue Blockage: Problem with the fan or a blockage in the flue.
Action: Professional diagnosis and repair required.

General Troubleshooting Steps

Check the gas supply — Ensure the gas supply is active and other gas appliances are working.
Check the pressure gauge — Maintain pressure between 1 and 1.5 bar when the system is cold.
Reset once only — A reset can clear a temporary fault. If the same fault returns, stop resetting and contact a Gas Safe engineer.

Boiler Fault Codes

If your boiler is showing a fault code, use the lists below to identify the issue. These codes are for quick reference — always check your boiler manual for full details.

IDEAL BOILER FAULT CODES

If you smell gas, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.

This is a quick-reference guide only. Always check your boiler manufacturer’s instructions for the full list of codes and recommended actions.

Common Ideal Boiler Fault Codes

F — Boiler in frost mode.

FD — No water flow; this can sometimes be resolved by repressurising the system.

F1 — Low water pressure; repressurise to 1–1.5 bar.

F2 / FL — Flame loss; check the gas supply and try one reset.

F3 — Fan fault; requires a Gas Safe engineer.

F4 — Thermistor fault (temperature sensor).

F5 — Return thermistor fault.

F6 — Outside sensor fault.

F7 — Low mains voltage.

F8 — PCB fault.

F9 — Unconfigured or faulty PCB.

FU — Flow/return differential above 50°C; check valves.

​​

Ideal L-Series Fault Codes

L1 — Low water flow or overheating.

L2 — Ignition lockout; check the condensate pipe during cold weather.

L3 — No-flow thermistor fault.

L4 — Flow thermistor fault.

L5 — Boiler return pipe issue or too many resets.

L6 — Flame-detection fault or false flame.

L9 / H9 / HA / LA — Overheating; check pressure and bleed radiators.

LF — Ignition fault.

LC — Five resets in 15 minutes; boiler lockout.

L-F — Boiler fails to ignite.

L-A — Boiler overheated.

​​

Ideal C-Series & Other Codes

C0 — Central-heating signal issue.

C2 — Boiler chip-card fault.

​​

When to Call a Gas Safe Engineer

Fan faults, such as F3

PCB faults, such as F8 or F9

Repeated ignition lockouts, such as L2 or F2

Overheat lockouts, such as L1, L9, HA or LA

Any smell of gas — leave immediately and call 0800 111 999

BAXI BOILER FAULT CODES

Pressure & Water Flow Faults

E119 — Low Water Pressure: Pressure below 0.5 bar; repressurise via the filling loop.

E1 / E118 / E207 — No Flow, Airlock or Low Pressure: Airlock or low system pressure; bleed radiators and repressurise.

E117 — High Water Pressure: Pressure above 2.5–3 bar; release pressure via a radiator bleed valve.

E125 — Circulation Fault: Pump failure, airlock or blocked heat exchanger.

​​

Ignition & Gas Supply Faults

E133 — Ignition Lockout: Failed ignition due to a gas-supply or ignition-electrode fault.

E168 — Flame Detection / Generic Lockout: Flame lost or unspecified lockout; often caused by sensor or condensate issues.

​​

Overheat & Temperature Sensor Faults

E110 — Overheat: Overheat thermostat tripped; often related to the pump or sludge affecting circulation.

E199 — Temperature Sensor Fault: Sensor or wiring issue; requires an engineer.

​​

Fan, Flue & Air Pressure Faults

E160 — Fan Fault: Flue fan not running correctly; requires an engineer.

E28 — Flue Fault: Blocked or faulty flue components.

E50 / E51 — Air Pressure / Condensate Issues: Often caused by frozen condensate pipes in cold weather.

​​

Sensor & Electrical Faults

E78 — Diverter Valve Fault: Stuck or failing diverter valve; diaphragm or motor issue.

A-Series Codes (A.00.28, A.02.06, etc.) — Sensor disconnections, low system pressure or communication errors.

​​

What You Can Safely Check Yourself

Low pressure (E119, E1) — Repressurise to 1–1.5 bar.

Airlocks — Bleed radiators.

Frozen condensate pipe — Safely thaw with warm water.

One reset only — If the same code returns, stop resetting and call a Gas Safe engineer.

​​

When to Call a Gas Safe Engineer

Repeated lockouts, including E133, E168 or E160

Any gas, ignition, fan or flue-related fault

Persistent overheat faults, such as E110

Sensor or PCB communication faults, including A-series codes or E199

WORCESTER BOSCH BOILER FAULT CODES

Ignition & Flame Detection Faults

EA 229 — Flame Lost During Operation: Flame extinguishes mid-cycle. Action: Engineer inspection required.

EA 229 — Flame Lost During Operation: Flame extinguishes mid-cycle.
Action: Engineer inspection required.

F7 — Flame Detected Before Ignition: Incorrect flame signal.
Action: Engineer-only repair.

FA 306 — False Flame Fault: Flame detected after the gas valve closes.
Action: Gas valve or PCB inspection required.

E9 — Overheat Protection / High Flow Temperature: Water temperature is too high, often above 95°C. Causes may include low pressure, airlocks, pump failure or a blocked heat exchanger.
Action: Check pressure is 1–1.5 bar, bleed radiators and reset once. Call an engineer if recurring.

D4 — Flow Temperature Rising Too Fast: Circulation restriction caused by sludge, closed valves or airlocks.
Action: Engineer investigation required; a powerflush may be needed.

A1 / 281 — Pump Stuck or Low Water Pressure: Pump not circulating or system pressure too low.
Action: Repressurise to 1–1.5 bar and bleed radiators. Call an engineer if the pump fault persists.

1017 / CE 207 — Low System Pressure:
Action: Repressurise via the filling loop.

C7 — Fan Not Running on Start-up: Fan fails to reach the correct speed.
Action: Engineer-only repair.

C6 — Fan Speed Too High or Low: Dirty fan, faulty sensor or PCB issue.
Action: Reset once; an engineer should inspect and repair the fan.

C1 / C4 — Fan Stopping or Running Continuously: Wiring or PCB faults.
Action: Engineer inspection required.

D1 — Flow/Return Temperature Sensor Fault: Sensor reading out of range.
Action: Try one reset; call an engineer if recurring.

E2 — Flow Temperature NTC Sensor Fault: Faulty sensor or wiring.
Action: Engineer inspection and possible sensor replacement.

1021 B — Hot Water Sensor Fault:
Action: Engineer to replace the sensor.

9U / F0 / A8 / CC — Control Box or Module Error: Loose wiring or defective PCB/HCM.
Action: Power-cycle once; module replacement may be required.

360 V — Incompatible Code Plug: Incorrect installation.
Action: Engineer to fit the correct plug.

B7 — Internal Control Board Fault:
Action: Engineer inspection required.