BoilerFaultCodesWhat's wrong with your boiler?
Choose the symptom that best describes the problem. We'll point you to the most likely fault codes and what to do next.
No hot water
Heating works but taps run cold, or hot water is intermittent.
No heating
Radiators are cold even when the boiler appears to be running.
Low boiler pressure
Pressure gauge reads below 1 bar. Boiler may show a fault code or cut out.
Boiler keeps turning off
Boiler fires up then cuts out repeatedly, or goes to lockout.
Boiler making noise
Banging, gurgling, whistling or kettling sounds from the boiler or pipes.
Frozen condensate pipe
Common in winter. Boiler shows a fault code and won't fire. Often a gurgling sound.
Boiler pressure too high
Pressure gauge reads above 2.5 bar. May cause pressure relief valve to discharge.
Boiler leaking water
Water dripping from the boiler casing, pipes, or pressure relief valve.
Boiler won't ignite
Boiler attempts to light but flame doesn't establish. Often shows ignition fault code.
Radiators cold at top
Radiators warm at the bottom but cold at the top — air trapped in the system.
Flue or fan fault
Boiler detects a problem with the flue gas or fan speed.
Boiler overheating
Boiler cuts out on overheat thermostat. May feel very hot to touch.
Common boiler problems explained
No hot water
Heating works but taps run cold, or hot water is intermittent.
Common causes
- › Diverter valve fault
- › DHW sensor failure
- › Low flow rate
- › Thermistor fault
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
No heating
Radiators are cold even when the boiler appears to be running.
Common causes
- › Pump failure
- › Zone valve stuck
- › Low pressure
- › Diverter valve fault
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Low boiler pressure
Pressure gauge reads below 1 bar. Boiler may show a fault code or cut out.
Common causes
- › System leak
- › Bleed valve left open
- › Expansion vessel fault
- › Pressure relief valve discharge
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler keeps turning off
Boiler fires up then cuts out repeatedly, or goes to lockout.
Common causes
- › Flame sensor fault
- › Gas supply issue
- › Overheating
- › Fan or flue blockage
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler making noise
Banging, gurgling, whistling or kettling sounds from the boiler or pipes.
Common causes
- › Limescale buildup
- › Air in system
- › Low flow rate
- › Pump cavitation
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Frozen condensate pipe
Common in winter. Boiler shows a fault code and won't fire. Often a gurgling sound.
Common causes
- › External condensate pipe frozen solid in cold weather
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler pressure too high
Pressure gauge reads above 2.5 bar. May cause pressure relief valve to discharge.
Common causes
- › Overfilled system
- › Expansion vessel waterlogged
- › Filling loop left open
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler leaking water
Water dripping from the boiler casing, pipes, or pressure relief valve.
Common causes
- › Pressure too high
- › Faulty pump seal
- › Corroded heat exchanger
- › Loose connections
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler won't ignite
Boiler attempts to light but flame doesn't establish. Often shows ignition fault code.
Common causes
- › Gas supply issue
- › Faulty ignition lead
- › Spark electrode worn
- › Gas valve fault
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Radiators cold at top
Radiators warm at the bottom but cold at the top — air trapped in the system.
Common causes
- › Air trapped in radiators
- › Needs bleeding
- › Pump circulation issue
Flue or fan fault
Boiler detects a problem with the flue gas or fan speed.
Common causes
- › Fan failure
- › Blocked flue
- › Air pressure switch fault
- › Condensate blockage
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Boiler overheating
Boiler cuts out on overheat thermostat. May feel very hot to touch.
Common causes
- › Pump failure
- › Blocked heat exchanger
- › Low flow rate
- › Thermostat fault
Fault codes to look for
These codes appear on different brands — search your boiler brand above for the exact meaning.
Always use a Gas Safe engineer
Gas work must only be carried out by a Gas Safe registered engineer. If you smell gas, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.