Boiler Problems & Fault Finder
This guide helps homeowners identify common boiler faults before calling a heating engineer.
Boiler Fault Code Finder
Worcester Bosch Fault Codes
| Code | Description | Repair Guidance |
|---|---|---|
| EA | No flame detected |
Possible causes• Gas supply turned off• Ignition electrode fault • Condensate pipe blockage • Gas valve issue • PCB fault |
| E9 | Overheat thermostat triggered |
Possible causes• Pump failure• Blocked heat exchanger • System sludge • Low water circulation |
| A1 | Low system pressure |
Possible causes• System needs repressurising• Leak in heating circuit • Faulty pressure sensor |
Ideal Boiler Fault Codes
| Code | Description | Repair Guidance |
|---|---|---|
| F1 | Low water pressure |
Possible causes• System pressure below 1 bar• Leak in system • Pressure sensor issue |
| L2 | Ignition lockout |
Possible causes• Gas supply off• Ignition electrode fault • Gas valve issue |
| F3 | Fan fault |
Possible causes• Fan wiring issue• Fan motor failure • PCB communication problem |
Vaillant Boiler Fault Codes
| Code | Description | Repair Guidance |
|---|---|---|
| F22 | Low water pressure / dry fire |
Possible causes• System pressure too low• Leak in heating system • Pressure sensor fault |
| F28 | Ignition failure |
Possible causes• Gas supply turned off• Gas valve fault • Ignition electrode problem |
| F75 | Pump not detecting pressure rise |
Possible causes• Pump stuck• Pressure sensor fault • Air trapped in system |
Baxi Boiler Fault Codes
| Code | Description | Repair Guidance |
|---|---|---|
| E118 | Low water pressure |
Possible causes• System pressure below recommended level• Leak in heating system • Pressure sensor fault |
| E133 | Ignition lockout |
Possible causes• Gas supply off• Ignition electrode fault • Gas valve issue |
| E168 | PCB fault |
Possible causes• Control board failure• Electrical supply issue |
Boiler Fault Finder
What problem are you experiencing?
Quick Boiler Diagnostic Checks
Step 1 – Check boiler pressure
Most boilers should operate between 1.0 – 1.5 bar.
Most boilers should operate between 1.0 – 1.5 bar.
Step 2 – Check power supply
Ensure the boiler and fused spur switch are on.
Ensure the boiler and fused spur switch are on.
Step 3 – Check thermostat & programmer
Make sure heating is scheduled and thermostat calling for heat.
Make sure heating is scheduled and thermostat calling for heat.
Step 4 – Check radiators
Bleed radiators and ensure both valves are open.
Bleed radiators and ensure both valves are open.
Step 5 – Check condensate pipe
Frozen pipes are common in winter.
Frozen pipes are common in winter.
Step 6 – Reset boiler
Most boilers have a reset button on the front.
Most boilers have a reset button on the front.
Common Boiler Faults
No Heating
Possible causes
- Thermostat not calling for heat
- Heating zone valve fault
- Circulation pump failure
- Diverter valve fault (combi boilers)
Checks
- Check thermostat batteries
- Check thermostat settings
- Check heating programmer
- Ensure radiator valves open
No Hot Water
- Diverter valve failure
- Plate heat exchanger blocked
- Hot water temperature sensor fault
- Programmer settings incorrect
Low Boiler Pressure
- Recently bled radiators
- Heating system leak
- Expansion vessel problem
- Pressure relief valve opening
Re-pressurise using the filling loop to 1–1.5 bar.
Radiators Not Heating
- Air trapped in radiators
- Sludge build-up
- Pump not circulating
- Thermostatic radiator valve stuck
Boiler Making Loud Noises
- Limescale in heat exchanger
- Pump problems
- Air in heating system
- Low water pressure
Common Boiler Error Codes
Worcester Bosch
| Error | Description | Possible Cause |
|---|---|---|
| EA | Ignition failure | Gas supply or ignition electrode |
| C6 | Fan fault | Fan not operating |
| D5 | Temperature sensor fault | Faulty NTC sensor |
| E9 | Overheat lockout | Pump failure or restricted circulation |
Vaillant
| Error | Description | Possible Cause |
|---|---|---|
| F22 | Low pressure | Water pressure too low |
| F28 | Ignition failure | Gas supply issue |
| F75 | Pump or pressure sensor fault | No pressure increase detected |
Ideal Boilers
| Error | Description | Cause |
|---|---|---|
| L2 | Ignition lockout | Gas supply issue |
| F1 | Low pressure | System pressure too low |
| F4 | Thermistor fault | Temperature sensor issue |
Safety Note
If your boiler repeatedly locks out, leaks water, or you smell gas, contact a qualified Gas Safe engineer immediately.
If your boiler repeatedly locks out, leaks water, or you smell gas, contact a qualified Gas Safe engineer immediately.
Complete Boiler Fault Code Database
Reference guide to common fault codes for Worcester Bosch, Ideal, Vaillant and Baxi boilers.
Worcester Bosch Fault Codes
| Code | Description | Possible Cause |
|---|---|---|
| A1 | Low system pressure | System needs repressurising |
| A7 | DHW temperature sensor fault | Thermistor wiring issue |
| A9 | Hot water sensor fault | Sensor failure |
| B1 | Coding plug error | PCB communication issue |
| C6 | Fan speed fault | Fan wiring or failure |
| C7 | Fan running too slow | Fan motor problem |
| C1 | Fan not running | Fan or PCB fault |
| D1 | Flow sensor fault | Sensor wiring issue |
| D5 | Return sensor fault | Thermistor failure |
| E2 | Flow temperature sensor | Sensor malfunction |
| E9 | Overheat thermostat triggered | Pump or circulation issue |
| EA | No flame detected | Gas supply or ignition issue |
| F0 | Internal error | PCB fault |
| F7 | Flame detected when off | Electrode fault |
| F9 | Temperature sensor error | Sensor failure |
| H11 | Communication error | Control board issue |
| H13 | Sensor reading implausible | Sensor malfunction |
| EA227 | Ignition failed | Gas valve issue |
| EA338 | Heat exchanger too hot | Circulation fault |
Ideal Boiler Fault Codes
| Code | Description | Possible Cause |
|---|---|---|
| F1 | Low water pressure | System needs filling |
| F2 | Flame loss | Ignition issue |
| F3 | Fan fault | Fan motor failure |
| F4 | Flow thermistor fault | Sensor failure |
| F5 | Return thermistor fault | Sensor wiring problem |
| F6 | Outside sensor fault | Sensor disconnected |
| L1 | Flow temperature overheat | Pump failure |
| L2 | Ignition lockout | Gas supply issue |
| L3 | Fan speed error | Fan malfunction |
| L4 | Ignition attempt failure | Electrode issue |
| L5 | False flame detection | Flame sensor fault |
| L6 | Ignition fault | Gas valve problem |
| H1 | Flow thermistor issue | Sensor reading incorrect |
| H2 | Return thermistor issue | Thermistor failure |
| H3 | PCB communication error | Control board fault |
Vaillant Boiler Fault Codes
| Code | Description | Possible Cause |
|---|---|---|
| F00 | Flow temperature sensor fault | Sensor wiring issue |
| F01 | Return temperature sensor fault | Thermistor failure |
| F10 | DHW sensor short circuit | Sensor failure |
| F11 | DHW sensor open circuit | Wiring issue |
| F20 | Overheat triggered | Poor circulation |
| F22 | Dry fire / low pressure | Low water level |
| F23 | Temperature difference too high | Pump issue |
| F24 | Poor circulation | Blockage or pump fault |
| F25 | Flue gas temperature high | Flue blockage |
| F27 | False flame detection | Electrode fault |
| F28 | Ignition failure | No gas supply |
| F29 | Flame failure during operation | Gas valve issue |
| F32 | Fan speed error | Fan fault |
| F49 | Low voltage fault | Electrical supply issue |
| F61 | Gas valve fault | Gas valve wiring |
| F62 | Gas valve delay fault | Gas valve sticking |
| F63 | PCB internal error | Main board failure |
| F64 | Electronics overheating | Control board issue |
| F65 | Electronics temperature high | PCB overheating |
| F75 | Pressure sensor fault | Pump not detecting pressure rise |
Baxi Boiler Fault Codes
| Code | Description | Possible Cause |
|---|---|---|
| E10 | Outdoor sensor fault | Sensor wiring |
| E20 | Central heating sensor fault | Thermistor issue |
| E28 | Flue sensor error | Flue temperature high |
| E50 | Hot water sensor fault | Thermistor issue |
| E110 | Safety thermostat triggered | Overheating |
| E117 | Water pressure too high | Overfilled system |
| E118 | Water pressure low | Needs repressurising |
| E119 | Low pressure | Leak or pressure drop |
| E125 | Circulation fault | Pump or blockage |
| E128 | Flame failure | Gas supply problem |
| E129 | Ignition failure | Electrode issue |
| E133 | Ignition lockout | No gas supply |
| E135 | Ignition fault | Ignition electrode |
| E160 | Fan fault | Fan motor failure |
| E168 | PCB fault | Main board issue |
| E193 | Flue temperature fault | Flue blockage |
Download the Homeowner Boiler Troubleshooting Guide
Download our free guide explaining the most common boiler problems homeowners experience.