Common problems guide

The most common Viessmann Vitodens 100-W WB1A problems

The faults most likely to send a Viessmann Vitodens 100-W WB1A 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.
201 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
P.4 High DIY-safe

Your boiler has detected that the water pressure in your heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop, usually a small silver braided hose connecting your cold mains pipe to your heating pipework.Open the valves on the filling loop slowly until you hear water entering the system.Monitor the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves tightly to prevent over-pressurisation.

Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for P.4
02
F.74 High DIY-safe

Your boiler has stopped heating because the water pressure inside the system has dropped below the required level.

What to try first

Locate the external filling loop, which is a flexible silver hose connected between two valves under or near the boiler.Open the valves slowly to allow water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close the valves firmly and check for any visible leaks on your radiators or pipes.

Est. cost
£0-90
Parts
Pressure relief valve, Pressure sensor, Expansion vessel
Full guide for F.74
03
F.161 High Engineer

Your boiler's computer has detected an internal communication error and is unable to read its own memory, meaning the system cannot run correctly.

Est. cost
£250-450
Parts
Main PCB, Control Unit
Full guide for F.161
04
30 High Engineer

Your boiler has stopped working because a sensor that monitors its temperature is giving a faulty signal, causing the system to shut down for safety.

Est. cost
£150-250
Parts
Boiler temperature sensor, Wiring harness
Full guide for 30
05
38 High Engineer

Your boiler has lost the signal from one of its internal temperature sensors, causing it to stop firing because it does not know how hot it is.

Est. cost
£120-220
Parts
Temperature sensor, Wiring harness
Full guide for 38
06
68 High Engineer

Your boiler has lost communication with a sensor that monitors the water returning to it, which prevents the system from heating your home properly.

Est. cost
£120-200
Parts
Return temperature sensor, Wiring harness
Full guide for 68
Parts most often replaced

on the Viessmann Vitodens 100-W WB1A

Filling loopPressure relief valvePressure sensorExpansion vesselMain PCBControl 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
See the full 201-code list for the Viessmann Vitodens 100-W WB1A

Related guides

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