BoilerFaultCodesThe 6 most common faults
01
A7 High Engineer
Your boiler is struggling to accurately measure the temperature of the water used for your taps and shower, which will likely prevent you from getting hot water.
Full guide for A702
AC High Engineer
Your boiler's computer is having trouble communicating with one of its internal control parts, causing the system to stop working as a safety precaution.
Full guide for AC03
b1 High Engineer
Your boiler's internal control computer has lost its connection to a secondary plug, which acts like a security key to tell the system which specific version of the boiler it is working with.
Full guide for b104
C1 High Engineer
Your boiler is struggling to push air through its system correctly, so it has stopped working as a safety precaution to ensure it burns fuel properly.
Full guide for C105
d3 High Engineer
Your boiler has lost the signal from the external control or bridge connection that tells it to start running, preventing the system from heating up.
Full guide for d306
E2 High Engineer
Your boiler has stopped working because a sensor that measures the water temperature is providing an incorrect reading, causing the system to shut down as a safety precaution.
Full guide for E2on the Worcester Bosch Greenstar ZWB7
Hot water NTC sensorDHW thermistor harnessMain electronic control unit (PCB)Wiring harnessCoding plugMain printed circuit board
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