BoilerFaultCodesThe 6 most common faults
01
4 High Engineer
Your boiler is failing to light the flame needed to provide you with heating and hot water.
Full guide for 402
5 High Engineer
Your boiler is struggling to detect that the central heating system is active, which is likely preventing your radiators from warming up.
Full guide for 503
6 High Engineer
Your boiler is struggling to light or detect a steady flame, which is preventing your heating and hot water from working.
Full guide for 604
8 High Engineer
Your boiler has stopped working because the internal fan, which safely exhausts fumes, is not spinning at the correct speed.
Full guide for 805
10 High Engineer
Your boiler's internal temperature sensor has stopped sending information, causing the system to shut down for safety.
Full guide for 1006
11 High Engineer
Your boiler has stopped working because one of its internal temperature sensors is unable to send a correct reading to the main control board.
Full guide for 11on the Atmos Intersystem HE26
Ignition electrodeIgnition leadGas valveFlow switchDHW sensorControl 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 Atmos 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