Viessmann Vitodens 300-W Common Problems & Fault Codes (2026 Guide)

The 6 most common faults

b5 High DIY-safe

Your boiler has experienced an internal communication error or software glitch within its main electronic control unit.

What to try first

Locate the power switch or fused spur for the boilerTurn the power off for 60 secondsTurn the power back on and see if the code clears

Est. cost: £350-650
Parts: Main PCB (Control Board), Coding Card

Full guide for b5

F2 High DIY-safe

Your boiler has switched itself off because it has reached a dangerous temperature and triggered a safety cut-out to prevent overheating.

What to try first

Check that all radiator valves are fully open to allow water to flow.Verify that the boiler pressure is between 1.0 and 1.5 bar on the gauge.Press the 'Reset' button on the control panel once the boiler has cooled down.

Est. cost: £120-350
Parts: Central heating pump, Overheat thermostat (Limit probe), Diverter valve

Full guide for F2

F9 High Engineer

Your boiler is failing to start because the internal fan is not spinning at the correct speed to safely clear the exhaust gases.

Est. cost: £250-£550
Parts: Extraction fan unit, Electronic control PCB, Fan wiring harness

Full guide for F9

F4 High Engineer

Your boiler is failing to light the flame properly, meaning it cannot provide heating or hot water for your home.

Est. cost: £120-350
Parts: Ignition electrodes, Ionisation lead, Gas valve

Full guide for F4

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.

Est. cost: £140-230
Parts: System flow temperature sensor (NTC), Sensor lead/wiring harness

Full guide for 20

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.

Est. cost: £120-210
Parts: System flow temperature sensor, Flow sensor wiring harness, Low loss header sensor

Full guide for 28

Parts most often replaced

on the Viessmann Vitodens 300-W

Main PCB (Control Board)Coding CardCentral heating pumpOverheat thermostat (Limit probe)Diverter valveExtraction fan unit

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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See the full 55-code list for the Viessmann Vitodens 300-W