BoilerFaultCodesThe 6 most common faults
01
F02 High DIY-safe
Your boiler has likely overheated or a sensor has detected an electrical fault, causing the system to shut down for safety.
What to try first
Wait 20 minutes for the boiler to cool down naturally.Check that your radiator valves are open and not blocked.Press the 'Reset' button on the boiler control panel once to see if the error clears.
02
F03 High Engineer
Your boiler is unable to create the spark needed to light the gas and start the heating process.
Full guide for F0303
F04 High Engineer
Your boiler has stopped working because it cannot detect the flame needed to heat your water and radiators safely.
Full guide for F0404
F05 High Engineer
Your boiler has stopped working because the fan that removes waste gases is not spinning at the correct speed.
Full guide for F0505
F07 High Engineer
Your boiler has shut down because it thinks the water inside is getting too hot, which is usually caused by a blockage or a pump issue preventing water from circulating properly.
Full guide for F0706
F08 High Engineer
Your boiler has detected an internal electrical fault where a component controlling the gas supply has stuck in the 'on' position, preventing the system from operating safely.
Full guide for F08on the Viessmann Vitodens 025-W
Flow temperature sensorHigh limit thermostatInternal pumpIgnition electrodeIgnition leadIgnition transformer
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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