BoilerFaultCodesThe 6 most common faults
01
A7 High Engineer
Your boiler has lost the ability to accurately measure the temperature of your hot water, so it has shut down to prevent overheating.
Full guide for A702
A8 High Engineer
Your boiler has lost the ability for its internal parts to 'talk' to each other, meaning it has safely shut down until the connection is restored.
Full guide for A803
B1 High Engineer
Your boiler's internal computer is struggling to recognise its own configuration data, which is preventing it from starting up correctly.
Full guide for B104
C1 High Engineer
Your boiler is struggling to breathe properly because its internal ventilation fan is not spinning fast enough to clear fumes safely.
Full guide for C105
C4 High Engineer
Your boiler's internal fan is stuck in the 'on' position and will not turn off, which is preventing the boiler from starting your heating or hot water correctly.
Full guide for C406
C6 High Engineer
Your boiler has detected that the internal fan is spinning faster than it should, so it has safely shut itself down to prevent any issues.
Full guide for C6on the Worcester Bosch Greenstar System
NTC Temperature SensorWiring harnessPrinted Circuit Board (PCB)Code plugFan assembly
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