BoilerFaultCodesWhat's wrong with your boiler?
Pick the symptom that best matches the problem. We'll point you to the most likely fault codes and the next step to take.
Can't see what's happening to your boiler? Try a different approach
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 keeps losing pressure
You top up but pressure drops again within days, there's a leak somewhere.
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.
Radiators cold at bottom
Hot top, cold bottom: sludge build-up blocking flow, not trapped air.
Yellow or orange flame
Safety-critical, indicates incomplete combustion and possible carbon monoxide risk.
Pilot light keeps going out
Older boilers, pilot flame won't stay lit when you relight it.
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 : the part that switches the boiler between heating and hot water
- DHW sensor failure : the sensor that measures hot-tap water temperature
- Low flow rate : not enough water moving through the boiler when you open a tap
- Thermistor fault : a temperature sensor inside the boiler
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
No heating
Radiators are cold even when the boiler appears to be running.
Common causes
- Pump failure : the pump that pushes hot water round the radiators has stopped
- Zone valve stuck : the motorised valve that routes water to your heating zone
- Low pressure : the gauge reads under 1 bar
- Diverter valve fault : the part that switches between heating and hot water
Fault codes to look for
These codes appear on different brands. Search your boiler brand 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 : water escaping somewhere in the pipes or radiators
- Bleed valve left open : a radiator bleed screw left undone after letting air out
- Expansion vessel fault : the sealed cushion that absorbs heating-water expansion has lost its charge
- Pressure relief valve discharge : a safety valve has dumped water outside (look for a pipe out the wall dripping)
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
Boiler keeps turning off
Boiler fires up then cuts out repeatedly, or goes to lockout.
Common causes
- Flame sensor fault : the sensor that confirms the burner has actually lit
- Gas supply issue : gas pressure or supply to the boiler
- Overheating : water leaving the boiler hotter than it should
- Fan or flue blockage : the flue (the pipe out of the boiler) or fan can't breathe
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
Boiler making noise
Banging, gurgling, whistling or kettling sounds from the boiler or pipes.
Common causes
- Limescale buildup : chalky mineral deposits inside the boiler's heat exchanger
- Air in system : pockets of air in the radiators or pipes
- Low flow rate : not enough water moving through the boiler
- Pump cavitation : air bubbles forming inside the pump
Fault codes to look for
These codes appear on different brands. Search your boiler brand 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 : the plastic pipe taking acidic water out of the boiler has iced up
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
Boiler keeps losing pressure
You top up but pressure drops again within days, there's a leak somewhere.
Common causes
- Radiator valve leak : a slow drip from a radiator inlet or outlet
- Expansion vessel failure : the sealed cushion inside the boiler has lost its air charge
- Faulty pressure relief valve : a safety valve passing water it shouldn't (check for the pipe out the wall dripping)
- Cracked heat exchanger : the main heat-transfer block has split; this is a serious fault
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
Boiler leaking water
Water dripping from the boiler casing, pipes, or pressure relief valve.
Common causes
- Pressure too high : gauge reads over 2.5 bar; the boiler may be releasing water through the safety valve
- Faulty pump seal : a rubber seal inside the pump has perished
- Corroded heat exchanger : the heat-transfer block has rusted through; usually means replacement
- Loose connections : a fitting somewhere has worked loose
Fault codes to look for
These codes appear on different brands. Search your boiler brand 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 : no gas reaching the burner
- Faulty ignition lead : the wire that delivers the spark has degraded
- Spark electrode worn : the small metal probe that creates the spark is past its life
- Gas valve fault : the valve that meters gas to the burner
Fault codes to look for
These codes appear on different brands. Search your boiler brand 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 : pockets of air at the top of a radiator that stop hot water from filling it
- Needs bleeding : opening the small screw on top of the radiator to let air out
- Pump circulation issue : the pump isn't pushing water hard enough to reach the top
Radiators cold at bottom
Hot top, cold bottom: sludge build-up blocking flow, not trapped air.
Common causes
- Sludge build-up : rust and debris that has settled in the bottom of the radiator
- System needs a power flush : a professional cleanout of the whole heating system
- Magnetic filter blocked : the inline filter that catches iron debris is full
Yellow or orange flame
Safety-critical, indicates incomplete combustion and possible carbon monoxide risk.
Common causes
- Blocked burner : soot or debris clogging the part where gas burns
- Dirty heat exchanger : the main heat-transfer block has built up deposits
- Blocked flue : the pipe taking exhaust gases outside is restricted
- Incorrect gas pressure : gas supply pressure is wrong for the boiler
Pilot light keeps going out
Older boilers, pilot flame won't stay lit when you relight it.
Common causes
- Worn thermocouple : the safety sensor that keeps the gas on when the pilot is lit
- Draught : a gust of air blowing out the pilot, often from a nearby vent
- Fouled pilot burner : dirt or insects in the tiny pilot burner
- Gas supply issue : gas pressure or supply to the pilot
Boiler overheating
Boiler cuts out on overheat thermostat. May feel very hot to touch.
Common causes
- Pump failure : the pump that circulates hot water has stopped, so heat can't leave the boiler
- Blocked heat exchanger : sludge or limescale stopping water moving through the heat block
- Low flow rate : not enough water moving through the boiler
- Thermostat fault : the main temperature sensor is misreading
Fault codes to look for
These codes appear on different brands. Search your boiler brand for the exact meaning.
Doesn't match any of the above?
Search by fault code or model number, or speak to a Gas Safe engineer directly.
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.