Common symptoms and fault codes
Symptoms and quick diagnosis
Boiler losing pressure
Top up via filling loop to 1.0–1.5 bar. If it drops again within days, you likely have a hidden leak or failed expansion vessel.
Radiators cold at the bottom
Sludge build-up. A power flush + magnetic filter usually restores full heat.
Radiators cold at the top
Trapped air — bleed the radiator. If it returns, you may need a power flush or system inhibitor top-up.
No hot water but heating works
Typically diverter valve or DHW thermistor — common, quick fix on most combis.
Hot water lukewarm
Scaled plate heat exchanger or failing diverter — descale or replace.
Banging or kettling noises
Limescale or sludge on the heat exchanger. Don't ignore — it shortens boiler life.
Whistling or gurgling
Air in the system or undersized pump. Bleed rads and check pump speed setting.
Pilot light keeps going out
Thermocouple or gas valve. Don't keep relighting; call a Gas Safe engineer.
Pressure rising on its own
Failed filling loop or expansion vessel pressure loss. Easy fix for an engineer.
Boiler ignites then cuts out
Flame loss — check gas supply, ignition lead and flame sensor. Often a PCB fault on older units.
Condensate pipe frozen
Common in winter. Pour warm (not boiling) water on the external pipe, then reset.
Boiler keeps locking out
Repeated lockouts point to ignition, gas valve or PCB fault. Stop resetting — book an engineer.
Smell of gas
Open windows, turn off the gas at the meter and call the National Gas Emergency line on 0800 111 999 immediately.
Leaking from underneath
Pump seal, heat exchanger or pressure relief valve. Turn the boiler off and call us.
Fault codes by brand
Worcester Bosch
- EA 227 · Flame loss — gas supply, ignition lead or PCB
- EA 229 · Flame detected when it shouldn't be — gas valve
- EA 338 · Ignition fault — spark generator or gas valve
- E9 · Heating flow temperature too high — pump or sludge
- H07 · Pressure low — top up via filling loop
- D1 286 · Return sensor fault
- C6 269 · Fan speed out of range
- A1 · Pump not circulating — air or blockage
Vaillant
- F22 · Low water pressure — repressurise system
- F23 · Large temperature difference flow/return — pump
- F24 · Rapid temperature rise — pump airlock or blockage
- F28 · Ignition failure — gas valve or PCB
- F29 · Flame lost during operation
- F62 · Gas valve shutdown delay fault
- F75 · Pump or pressure sensor fault
- F83 · Flow/return temperature change too small
Baxi
- E110 · Boiler overheat — pump or system blockage
- E118 · Low water pressure
- E125 · Insufficient circulation
- E133 · Ignition lockout — gas supply or igniter
- E160 · Fan fault
- E168 · PCB communication error
- E28 · Flue sensor fault
Ideal Heating
- F1 · Low pressure — repressurise
- F2 · Flame loss
- F3 · Fan fault
- F4 · Flow thermistor fault
- F5 · Return thermistor fault
- F9 · PCB fault
- L2 · Ignition lockout — failed to detect flame
- L5 · Repeat overheat lockout
Glow-worm
- F1 · Low water pressure
- F4 · No flame established
- F9 · Boiler timing out on heating
- F22 · Low water pressure
- F28 · Ignition fault
- F29 · Flame loss during operation
- F75 · Pressure sensor / pump fault
Potterton
- E110 · Overheat lockout
- E125 · Circulation issue
- E133 · Ignition lockout
- E160 · Fan speed fault
- E168 · Internal PCB error
Viessmann
- F.4 · No flame detected
- F.5 · Air pressure switch
- F.10 · Outdoor sensor fault
- F.20 · Safety shut-off — overheat
- F.22 · Pump fault — dry running
- F.38 · Return sensor short circuit
Alpha
- E1 · Ignition failure
- E2 · Overheat
- E3 · Fan / flue fault
- E4 · Flow thermistor
- E10 · Low pressure
Main
- E110 · Overheat
- E125 · Poor circulation
- E133 · Ignition lockout
- E160 · Fan fault
Ferroli
- A01 · Ignition lockout
- A02 · Flame on with gas valve closed
- A03 · Overheat
- F37 · Low pressure
- F39 · External probe fault
Vokera
- A01 · Ignition lockout
- A02 · False flame signal
- A03 · Overheat
- A04 · Low pressure
- A06 · DHW thermistor fault
Ariston
- 1 01 · Overheat
- 1 03 · Insufficient circulation
- 1 04 · Insufficient circulation — pump
- 1 P1 · Pressure low
- 5 01 · No flame detected
- 6 01 · Flue gas blocked / fan fault
