Common problems guide

The most common Viessmann Vitodens 343-F WS4A problems

The faults most likely to send a Viessmann Vitodens 343-F WS4A into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
59 documented codes
6 most common
0 DIY-safe checks
6 engineer needed

The 6 most common faults

01
F9 High Engineer

The boiler's exhaust fan is not starting up properly, so it has stopped itself as a safety precaution and will need a qualified engineer to check it.

Est. cost
£250-450
Parts
Combustion fan assembly, Fan power and signal wiring harness, Control PCB (main circuit board)
Full guide for F9
02
F4 High Engineer

Your boiler tried to start but could not light its burner, so it has stopped and is not producing heat or hot water until a qualified engineer checks it.

Est. cost
£120-350
Parts
Ignition electrode, Ionisation (flame sensing) electrode, Ignition module or control PCB
Full guide for F4
03
Eb High Engineer

Your boiler has detected a problem with how it sets up its gas burning controls, so it may have stopped or be running less well, and a registered engineer should check it.

Est. cost
£120-350
Parts
Ionisation electrode, Connecting cable, Flue seal or gasket
Full guide for Eb
04
30 High Engineer

Your boiler has shut itself down because a temperature sensor that tells it how hot the water is has failed, so it cannot safely light, which means your heating and hot water may not be working until a qualified engineer repairs it.

Est. cost
£150-280
Parts
Boiler water temperature (flow) sensor, Sensor wiring or connector
Full guide for 30
05
38 High Engineer

Your boiler has shut itself down for safety because its burner has stopped working properly, and a wiring fault in one of its temperature sensors is likely the cause, so a qualified engineer should check it.

Est. cost
£150-300
Parts
Boiler water temperature sensor, Sensor lead or wiring harness
Full guide for 38
06
50 High Engineer

Your boiler has stopped heating your hot water because it has lost track of the water temperature in your cylinder, and a qualified engineer will need to replace the faulty sensor so hot water can return.

Est. cost
£120-220
Parts
Cylinder temperature sensor, Sensor wiring harness or connector, Control unit circuit board (if wiring is fine but fault persists)
Full guide for 50
Parts most often replaced

on the Viessmann Vitodens 343-F WS4A

Combustion fan assemblyFan power and signal wiring harnessControl PCB (main circuit board)Ignition electrodeIonisation (flame sensing) electrodeIgnition module or control PCB

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
See the full 59-code list for the Viessmann Vitodens 343-F WS4A

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →