BoilerFaultCodesThe 6 most common faults
01
F1 High Engineer
Your boiler has attempted to start several times but has failed to ignite the flame, so it has stopped working for your safety.
Full guide for F102
F4 High Engineer
Your boiler is failing to stay lit after the initial ignition, meaning it cannot provide heat or hot water and has shut down for safety.
Full guide for F403
F5 High Engineer
Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or safety hazards.
Full guide for F504
F6 High Engineer
Your boiler has lost the ability to measure the temperature of the water used for your radiators, so it has shut down as a precaution to prevent overheating.
Full guide for F605
F10 High Engineer
Your boiler is struggling to accurately measure the temperature of the water returning to it, so it has stopped working as a safety precaution until the faulty sensor is corrected.
Full guide for F1006
F11 High Engineer
Your boiler's computer is struggling to communicate with internal sensors, meaning it cannot safely start the heating or hot water.
Full guide for F11on the Vaillant Ecomax Pro 18E / 28E
Ignition electrodeGas valveIgnition leadPCBFlame rectification probeNTC sensor
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 Vaillant 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