BoilerFaultCodesThe 6 most common faults
01
A1 High DIY-safe
Your boiler has detected that the water pump is not moving water correctly, which often happens because there is air trapped in the system or the pressure is too low.
What to try first
Check your boiler's pressure gauge and ensure it reads between 1 and 1.5 barIf the pressure is below 1 bar, use the external filling loop to top up the systemBleed your radiators to remove any trapped air that might be preventing water circulation
02
C4 High Engineer
Your boiler's internal fan is stuck running continuously, which prevents the system from starting up correctly or shutting down as it should.
Full guide for C403
d1 High Engineer
Your boiler has stopped working because it is unable to accurately measure the temperature of the water returning to it, so it has shut down as a precaution.
Full guide for d104
A8 High Engineer
Your boiler's electronic brains are struggling to talk to each other, so the system has shut down to prevent incorrect operation.
Full guide for A805
F0 High Engineer
Your boiler has detected an internal software or electronic communication error that prevents it from starting up safely.
Full guide for F006
FC High Engineer
Your boiler's electronic control system is failing to communicate with the display screen, meaning the unit cannot show status information or process commands.
Full guide for FCon the Worcester Bosch Ri 25 HE
Central heating pumpPressure relief valveExpansion vesselFan assemblyPrinted Circuit Board (PCB)Return temperature sensor
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