BoilerFaultCodesThe 6 most common faults
01
Fd High DIY-safe
Your boiler's internal computer has crashed due to an ignition failure or electrical interference, preventing it from starting the burner.
What to try first
Press the reset button or turn the boiler off and back on at the fused spur switchEnsure large electronic devices or powerful wireless routers are not sitting directly next to the boilerCheck if other gas appliances in your home are working to ensure gas is reaching the property
02
F5 High Engineer
Your boiler has stopped working because it cannot detect the internal fan that safely clears combustion gases.
Full guide for F503
F9 High Engineer
Your boiler has detected a communication or power issue with its internal fan, which is crucial for safely venting exhaust gases.
Full guide for F904
F4 High Engineer
Your boiler has failed to ignite or stay lit, meaning you currently have no heating or hot water.
Full guide for F405
0b High Engineer
Your boiler has detected that water is not flowing through it properly, which is likely because there is trapped air in the system or a sensor is misreading the flow.
Full guide for 0b06
30 High Engineer
Your boiler has stopped working because a sensor that monitors its temperature is giving a faulty signal, causing the system to shut down for safety.
Full guide for 30on the Viessmann Vitodens 100 Compact
Ignition electrodesIgnition leadsElectronic control unit (PCB)Fan assemblyWiring harnessIgnition electrode
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