BoilerFaultCodesThe 6 most common faults
01
F9 High Engineer
Your boiler's air circulation system is failing to start up correctly, which is preventing the unit from safely igniting and providing heating or hot water.
Full guide for F902
F4 High Engineer
Your boiler is failing to light the flame properly, meaning it cannot provide heating or hot water until the ignition process is resolved.
Full guide for F403
30 High Engineer
Your boiler has stopped working because it is unable to accurately measure the temperature of the water inside the system due to a faulty electrical sensor connection.
Full guide for 3004
38 High Engineer
Your boiler has stopped working because it can no longer accurately read the temperature of the water inside the system.
Full guide for 3805
51 High Engineer
Your boiler has stopped working because it is unable to accurately measure the temperature of the water inside the system due to a faulty electrical sensor connection.
Full guide for 5106
52 High Engineer
Your boiler's heat sensor has developed a wiring or electrical fault, which means the system cannot accurately measure the temperature of the water and has shut down as a precaution.
Full guide for 52on the Viessmann Vitodens 100-W WB1B
Fan assemblyWiring harnessPCB (Printed Circuit Board)Ignition electrodeGas valveIonisation probe
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 Viessmann 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