Common problems guide

The most common Viessmann Vitodens 100-W B1HF-25 problems

The faults most likely to send a Viessmann Vitodens 100-W B1HF-25 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

151 documented codes
6 most common
0 DIY-safe checks
6 engineer needed

The 6 most common faults

01
49 High Engineer

The boiler has stopped because it can't read the temperature of the gases leaving through the flue, which it needs to check for safe operation, so a qualified engineer should look at it soon.

Est. cost
£120-250
Parts
Flue gas temperature sensor, Flue sensor wiring harness, Flue sensor connector
Full guide for 49
02
50 High Engineer

Your boiler has spotted a problem with the sensor that monitors the temperature of the hot gases leaving through the flue, so it has likely shut itself down as a safety precaution and will need a qualified engineer to check it.

Est. cost
£120-220
Parts
Flue gas temperature sensor, Flue sensor wiring harness, Main control PCB (if the sensor is fine but the board is faulty)
Full guide for 50
03
59 High Engineer

Your boiler's internal fan is not starting, so the boiler cannot safely switch on to heat your home or water, and a qualified engineer should check it.

Est. cost
£180-420
Parts
Fan assembly, Fan connecting cable or wiring harness, Fan power supply or control board
Full guide for 59
04
62 High Engineer

Your boiler's automatic self-check on how it burns gas did not finish successfully, so it has paused as a safety precaution.

Est. cost
£90-250
Parts
Fan assembly, Gas valve, Ionisation electrode
Full guide for 62
05
63 High Engineer

Your boiler has shut itself down because the hot gases leaving through the flue are running hotter than it's designed to allow, which usually points to a blockage or a faulty safety sensor rather than a serious failure.

Est. cost
£120-300
Parts
Flue gas temperature limiter, Flue gas temperature sensor, Flue terminal or flue pipe section
Full guide for 63
06
64 High Engineer

The boiler's burner started but then lost its flame and shut itself down as a safety precaution, so your heating and hot water may be off until it is checked.

Est. cost
£120-300
Parts
Ionisation electrode, Ignition electrode, Gas valve
Full guide for 64
Parts most often replaced

on the Viessmann Vitodens 100-W B1HF-25

Flue gas temperature sensorFlue sensor wiring harnessFlue sensor connectorMain control PCB (if the sensor is fine but the board is faulty)Fan assemblyFan connecting cable or 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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See the full 151-code list for the Viessmann Vitodens 100-W B1HF-25

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