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
Heating & hot water
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.
No heating but hot water works
Taps run hot, radiators stay cold. The boiler itself is fine — something is stopping heat reaching the radiators.
Hot water lukewarm or fluctuating
Water runs warm rather than hot, or swings between hot and cold mid-shower.
Boiler won't ignite
Boiler attempts to light but flame doesn't establish. Often shows ignition fault code.
Pressure problems
Low boiler pressure
Pressure gauge reads below 1 bar. Boiler may show a fault code or cut out.
Boiler keeps losing pressure
You top up but pressure drops again within days, there's a leak somewhere.
Pressure too high
Gauge reads over 2.5 bar, or climbs every time the heating runs. May drip from the overflow pipe outside.
Cutting out & power
Boiler keeps turning off
Boiler fires up then cuts out repeatedly, or goes to lockout.
Boiler overheating
Boiler cuts out on overheat thermostat. May feel very hot to touch.
Boiler runs constantly
The boiler never seems to switch off, even when the house is warm or the schedule says off.
Display blank / no power
The boiler is completely dead: no display, no lights, nothing happens.
Pilot light keeps going out
Older boilers, pilot flame won't stay lit when you relight it.
Noises
Leaks, smells & flue
Smell of gas
Act now. A gas smell is an emergency, not a fault-finding job.
Boiler leaking water
Water dripping from the boiler casing, pipes, or pressure relief valve.
Yellow or orange flame
Safety-critical, indicates incomplete combustion and possible carbon monoxide risk.
Burning smell from boiler
A hot, dusty or plastic smell when the boiler runs.
Steam or plume from flue
A white cloud from the outside flue pipe when the boiler runs.
Radiators
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.
Some radiators cold, others hot
Heating works, but certain rooms never warm up — often the furthest radiators from the boiler.
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.
No heating but hot water works
Taps run hot, radiators stay cold. The boiler itself is fine — something is stopping heat reaching the radiators.
Common causes
- Room thermostat fault : dead batteries, wrong schedule, or a thermostat not calling for heat
- Diverter valve stuck : jammed in the hot-water position, so heating never gets a turn
- Zone valve fault : the motorised valve feeding the radiators is stuck closed
- Pump failure : hot water rises by design, but radiators need the pump working
Hot water lukewarm or fluctuating
Water runs warm rather than hot, or swings between hot and cold mid-shower.
Common causes
- Scaled-up plate heat exchanger : limescale coating the part that heats tap water, common in hard-water areas
- Diverter valve partially stuck : not fully switching over, so only some heat reaches the tap water
- Flow rate too high : the tap is drawing water faster than the boiler can heat it, worse in winter
- DHW thermistor misreading : the hot-water temperature sensor is drifting
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.
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 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.
Pressure too high
Gauge reads over 2.5 bar, or climbs every time the heating runs. May drip from the overflow pipe outside.
Common causes
- Filling loop left open : the valve used to top up pressure hasn't been fully closed and is letting mains water seep in
- Expansion vessel failure : with its air charge gone, pressure swings sharply as the water heats
- Overfilled after bleeding : topped up too far when repressurising — bleed a radiator briefly to drop it
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 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.
Boiler runs constantly
The boiler never seems to switch off, even when the house is warm or the schedule says off.
Common causes
- Room thermostat fault : the boiler is never told the house is warm enough — check batteries and placement
- Zone valve stuck open : the motorised valve feeding the heating can't close, so the boiler keeps serving it
- Diverter valve stuck : permanently heating the hot-water side
- Pump overrun fault : a few minutes of running after shutdown is normal; constant running is not
Display blank / no power
The boiler is completely dead: no display, no lights, nothing happens.
Common causes
- Tripped fuse or RCD : check the switched fused spur next to the boiler and your consumer unit
- Blown internal fuse : a fuse on the boiler's own control board
- PCB fault : the boiler's control board (its electronic brain) has failed
- Frost or damp damage : moisture reaching the electronics, common in garages and outbuildings
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 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.
Smell of gas
Act now. A gas smell is an emergency, not a fault-finding job.
What to do, in order
- Do not touch light switches, sockets or anything electrical, and put out naked flames.
- Open windows and doors to ventilate.
- Turn off the gas at the meter if you can reach it safely (the lever should sit across the pipe).
- Leave the property and call the National Gas Emergency Service on 0800 111 999 (free, 24/7).
- Do not go back inside until you're told it's safe. Never use a boiler you suspect of leaking gas.
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.
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
Burning smell from boiler
A hot, dusty or plastic smell when the boiler runs.
Common causes
- Dust burning off : harmless and brief when the heating first runs after summer
- Electrical fault : a plastic or fishy smell can mean overheating wiring — switch off and call an engineer
- Overheating component : a failing pump or fan running hot
- Debris on the heat exchanger : fluff or debris scorching on hot surfaces
Steam or plume from flue
A white cloud from the outside flue pipe when the boiler runs.
Common causes
- Normal condensation : condensing boilers make a visible plume in cold weather — this is how they're meant to work
- Excessive plume with a fault code : combined with error codes or dripping, it can point to a combustion or condensate problem
- Plume nuisance : if it blows across a window or a neighbour's door, an engineer can fit a plume redirection kit
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
Some radiators cold, others hot
Heating works, but certain rooms never warm up — often the furthest radiators from the boiler.
Common causes
- System needs balancing : adjusting each radiator's valves so the nearest ones don't take all the flow
- TRV pin stuck : the small pin under the thermostatic valve head has seized shut
- Sludge in long runs : debris settling in the pipework serving the furthest radiators
- Pump speed too low : not enough push to reach the whole circuit
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.