BoilerFaultCodesThe 6 most common faults
01
F4 High Engineer
Your boiler is failing to light the flame properly, meaning it cannot provide heating or hot water for your home.
Full guide for F402
20 High Engineer
Your boiler has lost its connection to the temperature sensor that monitors the main water flow, likely due to a faulty electrical component or sensor.
Full guide for 2003
28 High Engineer
Your boiler has lost connection with a temperature sensor that monitors the main water flow, meaning it cannot accurately regulate how much heat to send to your radiators.
Full guide for 2804
30 High Engineer
Your boiler has stopped working because its internal temperature gauge has failed, causing the system to lock the burner for safety.
Full guide for 3005
38 High Engineer
Your boiler has stopped working because it can no longer accurately measure its own internal temperature due to a broken wire or faulty sensor.
Full guide for 3806
50 High Engineer
Your boiler is unable to heat your water because the temperature sensor for your hot water cylinder has failed or has a wiring fault.
Full guide for 50on the Viessmann Vitodens 300-W
Ignition electrodesIonisation leadGas valveIgnition transformerSystem flow temperature sensor (NTC)Sensor lead/wiring harness
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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