BoilerFaultCodesThe 6 most common faults
01
A1 High DIY-safe
Your boiler has detected that it is running without enough water inside, which is often caused by a leak or a pump that has jammed or failed.
What to try first
Check the pressure gauge on the boiler and top it up to 1.5 bar using the filling loopInspect visible pipework and radiators for any signs of water leaksReset the boiler to see if the error clears after repressurising
02
A8 High Engineer
Your boiler has lost its connection to your external room thermostat or heating controls, meaning it doesn't know when to turn on or off.
Full guide for A803
F1 High Engineer
Your boiler has stopped working because an internal safety fuse has failed, cutting off power to the system's control board.
Full guide for F104
F2 High Engineer
Your boiler has stopped working because an internal electrical fuse has blown, cutting off the power needed for it to operate.
Full guide for F205
No lights and no functions (with demand) High Engineer
Your boiler has completely lost electrical power and is not responding at all, even when you turn the heating or hot water on.
Full guide for No lights and no functions (with demand)06
No DHW function (CH OK) High Engineer
Your boiler is still keeping your home warm, but it has stopped heating up your water for taps and showers.
Full guide for No DHW function (CH OK)on the Worcester Bosch 24i / 28i Junior
Central heating pumpPump capacitorAutomatic air ventRF ReceiverInternal Wiring HarnessExternal Room Thermostat
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Worcester Bosch shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue