Common problems guide

The most common Viessmann Vitodens 100-W WB1C 35kW System Boiler problems

The faults most likely to send a Viessmann Vitodens 100-W WB1C 35kW System Boiler 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.
26 documented codes
6 most common
0 DIY-safe checks
6 engineer needed

The 6 most common faults

01
30 High Engineer

Your boiler has lost its reading of the water temperature and has shut down as a safety precaution, so it needs a qualified engineer to check the sensor.

Est. cost
£120-220
Parts
Boiler water temperature sensor (NTC sensor), Sensor wiring harness or connector
Full guide for 30
02
38 High Engineer

Your boiler has stopped its burner from firing because it has lost reliable temperature readings from one of its internal sensors, so it can't safely produce heat or hot water until a qualified engineer checks it.

Est. cost
£150-280
Parts
Boiler water temperature sensor (NTC), Sensor wiring lead or harness, Sensor well or seal
Full guide for 38
03
50 High Engineer

Your boiler has detected a faulty reading from the sensor that measures the hot water tank temperature, so it has stopped heating your hot water while your central heating may still work.

Est. cost
£120-220
Parts
Cylinder temperature sensor (DHW sensor), Sensor cable or plug connector, Control unit (Vitotronic) if the fault persists after sensor replacement
Full guide for 50
04
F9 High Engineer

The boiler's internal fan, which is needed to safely start the burner, isn't switching on, so the boiler has stopped and will likely need a qualified engineer to inspect it before it can heat your home or water again.

Est. cost
£200-400
Parts
Combustion fan assembly, Fan connecting cable or harness, Main PCB or fan control board
Full guide for F9
05
F4 High Engineer

Your boiler tried to light its burner but did not detect a flame, so it has stopped to stay safe, and it will need a qualified engineer to find out why.

Est. cost
£120-350
Parts
Ionisation electrode, Ignition electrode, Ignition module
Full guide for F4
06
51 High Engineer

The boiler has detected a faulty temperature sensor on the hot water outlet pipe, so it has stopped working as a safety precaution until a engineer checks it.

Est. cost
£150-250
Parts
Flow (outlet) temperature sensor, Sensor wiring harness, Control PCB (rarely needed)
Full guide for 51
Parts most often replaced

on the Viessmann Vitodens 100-W WB1C 35kW System Boiler

Boiler water temperature sensor (NTC sensor)Sensor wiring harness or connectorBoiler water temperature sensor (NTC)Sensor wiring lead or harnessSensor well or sealCylinder temperature sensor (DHW sensor)

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 26-code list for the Viessmann Vitodens 100-W WB1C 35kW System Boiler

Related guides

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