BoilerFaultCodesThe 6 most common faults
01
E18 High DIY-safe
Your boiler has stopped working because the waste water produced during heating cannot drain away correctly, usually due to a blocked or faulty pump.
What to try first
Check the external white plastic drainage pipe for any signs of freezing or blockages.Ensure any flexible hoses connected to the condensate pump are not kinked or trapped.If the outlet pipe is frozen, gently pour warm (not boiling) water over the pipe to thaw it.
02
21 High Engineer
Your boiler's main internal computer has developed a communication fault and can no longer control the heating safely.
Full guide for 2103
25 High Engineer
Your boiler's internal computer chips are failing to communicate because their software versions no longer match.
Full guide for 2504
26 High Engineer
Your boiler is struggling to keep its burner lit and has shut down for safety after failing to detect a steady flame several times.
Full guide for 2605
30 High Engineer
Your boiler has stopped working because it thinks the air in the heating ducts is overheating due to a faulty electrical sensor.
Full guide for 3006
31 High Engineer
Your boiler has stopped working because it cannot detect the temperature of the air inside the heating system.
Full guide for 31on the Johnson Starley C26d
Condensate pumpFloat switch sensorCondensate discharge pipeworkMain PCB (ESYS Control Board)ESYS Control BoardPCB Display Module
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