BoilerFaultCodesThe 6 most common faults
01
21 High Engineer
Your boiler's internal computer is having trouble processing signals from its electronic sensors, which has caused the system to shut down for safety.
Full guide for 2102
25 High Engineer
Your boiler's internal computer chips are failing to communicate correctly because of a software conflict or a hardware memory error.
Full guide for 2503
26 High Engineer
Your boiler is struggling to keep its flame lit and has shut down for safety after failing to stay ignited five times in a row.
Full guide for 2604
30 High Engineer
Your boiler has stopped working because it has detected an electrical fault with the internal sensor that monitors air temperature.
Full guide for 3005
31 High Engineer
Your boiler has lost its connection to the sensor that monitors air temperature, preventing it from heating up safely.
Full guide for 3106
34 High Engineer
Your boiler is not receiving enough electrical power from your home's supply to operate safely.
Full guide for 34on the Johnson Starley C16d
Main PCB (Printed Circuit Board)Control ModuleESYS Control BoardMain PCBFlame sensing electrodeIgnition control 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 Johnson Starley 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