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.
No hot water but heating works
Typically diverter valve or DHW thermistor — common, quick fix on most combis.
Banging or kettling noises
Limescale or sludge on the heat exchanger. Don't ignore — it shortens boiler life.
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.
Fault codes by brand
Worcester Bosch
- EA 227 · Flame loss — usually gas supply, ignition lead, or PCB
- EA 338 · Ignition fault — spark generator or gas valve
- H07 · Pressure low — top up via filling loop
Vaillant
- F22 · Low water pressure — repressurise system
- F75 · Pump or pressure sensor fault
- F28 · Ignition failure — gas valve or PCB
Baxi
- E110 · Boiler overheat — pump or system blockage
- E133 · Ignition lockout
- E168 · PCB communication error
Ideal Heating
- F1 · Low pressure
- F2 · Flame loss
- L2 · Ignition lockout
Glow-worm
- F22 · Low water pressure
- F28 · Ignition fault
- F75 · Pressure sensor / pump
Potterton
- E110 · Overheat lockout
- E125 · Circulation issue
Viessmann
- F.5 · Air pressure switch
- F.4 · No flame detected
- F.22 · Pump fault
Alpha
- E1 · Ignition failure
- E2 · Overheat
- E10 · Low pressure
Main
- E133 · Ignition lockout
- E110 · Overheat
Ferroli
- A01 · Ignition lockout
- A02 · Flame on with gas valve closed
- F37 · Low pressure
Vokera
- A01 · Ignition lockout
- A03 · Overheat
Ariston
- 1 01 · Overheat
- 1 03 · Insufficient circulation
- 1 P1 · Pressure low
