BoilerFaultCodesThe 6 most common faults
01
A5 High Engineer
Your boiler is having trouble measuring the temperature of the water stored in its internal hot water tank, which means it cannot reliably provide hot water.
Full guide for A502
A7 High Engineer
Your boiler is having trouble sensing the temperature of your hot water supply, which means it cannot heat your taps or shower correctly.
Full guide for A703
A8 High Engineer
Your boiler's internal computer is having trouble talking to its main control unit, which prevents it from operating correctly.
Full guide for A804
b1 High Engineer
Your boiler's internal computer is failing to communicate with a small identity chip, which prevents it from knowing how to operate correctly.
Full guide for b105
b7 High Engineer
Your boiler's main control computer is experiencing an internal electronic fault and has stopped the system from operating to keep you safe.
Full guide for b706
C6 High Engineer
Your boiler has detected that the internal fan responsible for safely venting exhaust gases is not working correctly, causing the system to stop heating to ensure your safety.
Full guide for C6on the Worcester Bosch Highflow CDi
NTC Temperature SensorWiring harnessDomestic Hot Water (DHW) NTC sensorControl unitPCBCode plug
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