Viessmann Vitodens 100-W Fault codes & diagnostics

37 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

CombiSystem Natural GasLPG 19-35 kW 2016-present
GC Numbers 47-819-5047-819-5147-819-5241-819-4841-819-4941-819-5041-819-5141-819-4041-819-4141-819-4347-819-4047-819-4147-819-42
Download the Viessmann Vitodens 100-W manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 31 High 2 Medium 2 Low

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
41-819-40 Vitodens 100-W B1HC-19 2016–present
41-819-41 Vitodens 100-W B1HC-26 2016–present
41-819-43 Vitodens 100-W B1HC-35 2016–present
47-819-40 Vitodens 100-W B1KC-26 2016–present
47-819-41 Vitodens 100-W B1KC-30 2016–present
47-819-42 Vitodens 100-W B1KC-35 2016–present

All 37 documented codes

Tap any card for details

10

Your boiler is struggling to accurately read the outside temperature and is instead running in a constant mode to ensure you do not lose heat.

Low Engineer only £120-200

Parts commonly replaced

  • Outdoor temperature sensor
  • Wiring loom

Related codes

18

Your boiler has lost the signal from the weather sensor outside your home and is now running on a default setting.

Low Engineer only £120-200

Parts commonly replaced

  • Outdoor temperature sensor
  • External wiring loom

Related codes

20

Your boiler has stopped working because it is unable to accurately measure the temperature of the water inside the system.

High Engineer only £150-250

Parts commonly replaced

  • Flow temperature sensor
  • Wiring harness
28

Your boiler has lost the connection to the sensor that measures the water temperature, so it has shut down as a precaution to prevent overheating.

High Engineer only £150-250

Parts commonly replaced

  • Flow temperature sensor
  • Wiring harness

Related codes

30

Your boiler has stopped working because a temperature monitoring sensor is sending an incorrect signal, causing the system to shut down for safety.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

38

Your boiler has switched itself off because it has lost the electronic connection to a temperature sensor, preventing it from safely monitoring its own heat level.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor
  • Wiring harness

Related codes

40

Your boiler has stopped heating your water because it is receiving an incorrect signal from the temperature sensor inside your water cylinder.

High Engineer only £120-200

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness
48

Your boiler has stopped working because the connection between the water tank sensor and the control system has been disrupted.

High Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness
50

Your boiler has stopped heating your hot water tank because it is receiving an incorrect signal from the temperature sensor.

High Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness

Related codes

51

Your boiler has stopped heating your tap water because it is getting a faulty signal from the sensor that measures the water temperature.

High Engineer only £120-200

Parts commonly replaced

  • DHW temperature sensor

Related codes

Showing 11–20 of 37
58

Your boiler is struggling to detect the temperature of your hot water cylinder because there is a loose or damaged electrical connection.

Medium Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness

Related codes

59

Your boiler has stopped producing hot water because a sensor inside is giving an incorrect reading, likely due to a loose wire or a faulty component.

High Engineer only £120-220

Parts commonly replaced

  • DHW temperature sensor

Related codes

60

Your boiler has detected a fault with the sensor that monitors the water temperature returning to the unit, causing it to stop running for safety and efficiency.

High Engineer only £120-£200

Parts commonly replaced

  • Return NTC temperature sensor
  • Wiring harness

Related codes

68

Your boiler has lost its connection to the sensor that measures the water temperature coming back into the unit, which often results in the system shutting down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return temperature sensor (NTC)
  • Wiring harness / lead connector

Related codes

A1

Your boiler has stopped working because the water pressure inside the system is too low to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (usually a silver braided hose under the boiler)
  2. Open the valves to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar
  3. Close both valves tightly and restart the boiler

Parts commonly replaced

  • Pressure relief valve
  • Filling loop
  • Pressure sensor
A3

Your boiler has switched itself off because a sensor inside detects that the exhaust system or internal temperature isn't configured exactly as it should be.

High Engineer only £120-250

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness
  • PCB

Related codes

A7

Your boiler is struggling to accurately detect the temperature of your hot water cylinder, which may result in your hot water not heating up correctly.

Medium Engineer only £120-220

Parts commonly replaced

  • Cylinder temperature sensor
  • Wiring harness

Related codes

A9

Your boiler has detected an internal electronic error and has safely shut itself down to prevent further issues.

High DIY-safe £150-350

What to check first

  1. Check that your thermostat is powered on and has fresh batteries if applicable.
  2. Restart the boiler using the power switch or reset button to refresh the connection.
  3. Ensure the thermostat wiring is securely plugged into its base if it is a wall-mounted unit.

Parts commonly replaced

  • Main circuit board (PCB)
  • Wiring harness

Related codes

b1

Your boiler's electronic command center is having trouble talking to its main internal brain, which means the system has safely shut down to prevent incorrect operation.

High Engineer only £200-450

Parts commonly replaced

  • Programming unit
  • Main PCB
  • Connecting cable harness

Related codes

b5

Your boiler's main electronic brain has encountered a internal software error, meaning it has safely shut itself down to prevent any further issues.

High Engineer only £250-500

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Showing 21–30 of 37
b7

Your boiler has stopped working because it cannot correctly identify the internal key used to manage its settings.

High Engineer only £150-250

Parts commonly replaced

  • Coding plug
  • Main control board

Related codes

E3

Your boiler has detected a safety interruption in its internal system and has shut down to prevent any unsafe operation.

High Engineer only £150-350

Parts commonly replaced

  • Air pressure switch
  • Flue gas sensor
  • Control board (PCB)
  • Wiring harness

Related codes

E4

Your boiler has detected an electrical problem with its power supply that is preventing it from safely lighting the flame.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Power supply module

Related codes

E5

Your boiler has detected an internal electronic malfunction and has safely shut itself down to prevent further issues.

High Engineer only £150-350

Parts commonly replaced

  • Main circuit board (PCB)
  • Ionisation electrode
  • Wiring harness

Related codes

E6

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (a silver flexi-hose with two valves) under your boiler.
  2. Open the valves slowly until the pressure gauge on the boiler shows between 1.0 and 1.5 bar.
  3. Close both valves firmly and restart the boiler.

Parts commonly replaced

  • Pressure relief valve
  • Pressure gauge
  • Expansion vessel

Related codes

F0

Your boiler has detected an internal electronic malfunction and has shut itself down as a safety precaution, meaning you currently have no heating or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Control Board (PCB)
  • Wiring Harness
  • Gas Valve

Related codes

F2

Your boiler has switched itself off because it has detected that it is running too hot and needs a professional to check it is venting heat correctly.

High Engineer only £150-350

Parts commonly replaced

  • Temperature limiter
  • Primary heat exchanger
  • Pump
  • NTC sensors

Related codes

F3

Your boiler is detecting a flame inside before it has actually started, which usually means the safety sensors are confused or there is a moisture issue inside the unit.

Emergency Engineer only £150-300

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Control board

Related codes

F4

Your boiler is failing to ignite the flame needed to produce heat and hot water, so it has shut down as a safety precaution.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ionisation probe
  • Control unit

Related codes

F5

Your boiler has detected an issue with the sensors that ensure air is moving correctly, so it has safely shut itself down to prevent unsafe operation.

High Engineer only £150-350

Parts commonly replaced

  • Differential pressure sensor
  • Air pressure switch
  • Fan assembly
  • Flue system components

Related codes

Showing 31–37 of 37
F5/F9

Your boiler is failing to start because it cannot detect that the internal fan, which clears exhaust gases, is spinning correctly.

High Engineer only £180-350

Parts commonly replaced

  • Fan unit
  • Fan control cable
  • Printed Circuit Board (PCB)

Related codes

F7

Your boiler has detected an issue with the sensors that monitor the airflow, so it has safely shut down to prevent it from operating incorrectly.

High Engineer only £180-350

Parts commonly replaced

  • Differential pressure sensor
  • Venturi assembly
  • Wiring harness

Related codes

F8

Your boiler has detected that the internal gas valve is failing to shut off properly, which indicates a serious safety risk regarding how the unit manages fuel.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F9

Your boiler is failing to start because it cannot detect enough airflow to safely ignite the flame.

High Engineer only £150-350

Parts commonly replaced

  • Fan unit
  • PCB
  • Venturi
FA

Your boiler has detected that the internal exhaust fan is not switching off correctly when it should, which is a safety mechanism to prevent the boiler from starting under unsafe conditions.

High Engineer only £250-450

Parts commonly replaced

  • Fan unit
  • Combined gas valve and venturi
  • Boiler control board
Fd

Your boiler has detected an internal electronic error and has safely shut itself down to prevent operation while it is malfunctioning.

High Engineer only £200-450

Parts commonly replaced

  • Burner control unit
  • PCB
  • Ignition transformer

Related codes

FE

Your boiler's electronic control system has detected a critical internal error, which means it has safely shut down to prevent damage and will not provide heating or hot water.

High Engineer only £250-500

Parts commonly replaced

  • Coding card
  • Burner control unit (PCB)

Related codes

Avoid the next call-out bill

Monthly cover means no unexpected bills when your boiler breaks down.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.