BoilerFaultCodesThe 6 most common faults
01
0C High Engineer
The boiler has stopped because its burner is blocked or not getting a steady enough electricity supply, so it cannot light and heat your home or water safely until it is checked.
Full guide for 0C02
30 High Engineer
Your boiler has stopped because its sensor that measures the water temperature is reporting a fault, and it may also have a blockage in the burner, so it has shut itself down to stay safe until it's checked.
Full guide for 3003
38 High Engineer
Your boiler has shut itself down because it has detected a problem with a temperature sensor and a possible blockage around the burner, so it needs a qualified engineer to check it.
Full guide for 3804
F9 High Engineer
The boiler's fan, which pushes air through the boiler to burn gas safely, isn't starting, so the boiler has shut itself down as a precaution and won't heat your home or water until it's checked.
Full guide for F905
F4 High Engineer
The boiler tried to light its burner but couldn't establish a flame, so it has stopped and is waiting for a reset, which usually means no heating or hot water until it's checked.
Full guide for F406
50 High Engineer
Your boiler has stopped heating your hot water because it has detected a fault with the sensor that measures the temperature in your hot water cylinder, and a qualified engineer will need to check it.
Full guide for 50on the Viessmann Vitodens 111-W B1LD 26kW
Burner assemblyBurner gasketIgnition electrodeFanGas valveBoiler water temperature sensor (NTC)
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
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