Viessmann Vitodens 100-W WB1C 35kW System Boiler Fault codes & diagnostics

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

System Natural GasLPG 2012-2015 Discontinued
GC Number 41-819-29
Download the Viessmann Vitodens 100-W WB1C 35kW System Boiler manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 21 High 1 Medium 2 Low
Just want the most common Viessmann Vitodens 100-W WB1C 35kW System Boiler faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 26 documented codes

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5A

Your boiler has lost the signal from the sensor that tells it how hot the water leaving the boiler is, so it has shut itself down as a safety precaution and will likely need a qualified engineer to fix it.

High Engineer only £120-250

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it.
  2. Wait about 10 minutes, then restore power and switch the boiler back on.
  3. If the fault returns, do not open the casing or disconnect wiring. Contact a Gas Safe registered engineer.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor cable or wiring harness
  • Control PCB (if the sensor and wiring test fine)

Related codes

Full explanation for 5A
10

The boiler has noticed a problem with the sensor that measures the outdoor temperature, so it is running at a fixed level instead of adjusting to the weather, but it should still heat your home and water.

Low DIY-safe £90-180

What to check first

  1. Turn off the boiler at its on/off switch and, if you can, at the fused spur or isolator supplying it. Do not open the boiler casing.
  2. Find the outside temperature sensor, usually a small white or grey box on a north or shaded wall, and check it for visible damage, cracks, or water inside the housing.
  3. Follow the sensor cable from the sensor as far as you can see. Look for cuts, crushed or pinched sections, or cable that has been stapled too tightly, and check that the connections at the sensor are secure and dry.
  4. Turn the power back on and press the reset button on the boiler, or switch the boiler off and on again. If the code returns, or you found damage, contact a Gas Safe registered engineer to test or replace the sensor and cable.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor cable

Related codes

Full explanation for 10
18

The boiler has lost the signal from its outside weather sensor, so it can't adjust heating to the weather and is running at a fixed setting instead, which is usually not an emergency but should be checked.

Low DIY-safe £90-180

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur, then wait five minutes.
  2. Find the outside temperature sensor, usually a small white or grey box on a north or shaded outside wall, and check its cable for visible damage, chafing, or a cut where it enters the wall.
  3. If the sensor cable is accessible at the sensor end, check that its plug or connector is fully pushed in and not corroded. Do not open the boiler casing or touch any gas components.
  4. Switch the boiler back on and check whether the code clears. If it returns, leave the boiler running at its fixed setting and book a Viessmann-approved engineer to test the sensor and replace it or its lead.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor lead or cable

Related codes

Full explanation for 18
30

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.

High Engineer only £120-220

What to check first

  1. Switch off the boiler at its on/off switch and at the fused spur or isolator, then wait 5 minutes.
  2. Switch the power back on and press the reset button on the boiler once, or use the reset option on the display.
  3. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up using the filling loop only if the manufacturer's instructions say to.
  4. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler water temperature sensor (NTC sensor)
  • Sensor wiring harness or connector

Related codes

Full explanation for 30
38

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.

High Engineer only £150-280

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop, following the boiler manual.
  2. Make sure the gas supply tap to the boiler is fully open and the boiler is switched on at its power point.
  3. Press the reset button once (or use the reset option on the display), then wait about 10 minutes to see whether the boiler restarts. If the fault code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor wiring lead or harness
  • Sensor well or seal

Related codes

Full explanation for 38
50

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.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its mains isolator or fused spur and wait about 10 minutes.
  2. Switch it back on and check whether code 50 returns after the boiler has run for a while.
  3. Do not open the boiler casing or the cylinder. If the code comes back, contact a Gas Safe registered engineer to test and replace the cylinder temperature sensor.

Parts commonly replaced

  • Cylinder temperature sensor (DHW sensor)
  • Sensor cable or plug connector
  • Control unit (Vitotronic) if the fault persists after sensor replacement

Related codes

Full explanation for 50
51

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.

High Engineer only £150-250

What to check first

  1. Switch the boiler off at its power supply or isolator switch and wait 10 minutes.
  2. Switch the power back on and check whether the boiler restarts and the fault code clears.
  3. If the code returns or the boiler will not run, stop and book a Gas Safe registered engineer. Do not open the boiler casing or disconnect the sensor yourself.

Parts commonly replaced

  • Flow (outlet) temperature sensor
  • Sensor wiring harness
  • Control PCB (rarely needed)

Related codes

Full explanation for 51
52

The boiler has detected a faulty temperature sensor that measures the hot water heading out to your radiators, so it has stopped working properly and needs a qualified engineer to check it.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on to see if the fault clears.
  2. Check the boiler pressure gauge and top it up only if it is below 1 bar and the manual says it is safe to do so.
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, as the sensor and its wiring must be checked by a professional.

Parts commonly replaced

  • Flow temperature sensor
  • Sensor wiring harness
  • Main PCB (control board), if the sensor circuit is damaged

Related codes

Full explanation for 52
58

Your boiler has lost track of the temperature of your hot water cylinder, so it has stopped heating stored hot water until the sensor is checked and repaired.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch, wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check that the boiler's display is not showing other fault codes alongside F58, and note any that appear.
  3. Check that your heating and hot water are still working through taps and radiators, then contact a Gas Safe registered engineer if the code returns.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or lead
  • Sensor wiring connector

Related codes

Full explanation for 58
59

Your boiler has lost track of how hot the water leaving it is, so it has shut down to stay safe, and it will usually need a sensor replaced by a Gas Safe engineer.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur for 5 minutes, then switch it back on to see if the fault clears.
  2. Check the boiler's display for any other codes or messages and note them down for the engineer.
  3. If the fault returns straight away, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring lead or connector

Related codes

Full explanation for 59
Showing 11–20 of 26
A9

Your boiler has lost its connection to your room thermostat or heating controller, so it may not respond to your heating settings, though this is usually a wiring or communication problem that can often be fixed.

Medium DIY-safe £90-250

What to check first

  1. Turn your heating controller or room thermostat off, then back on, and wait a few minutes for it to reconnect to the boiler.
  2. Check and replace the batteries in your room thermostat or controller if it runs on them, and check it is set to the correct mode.
  3. Look at the wiring at the thermostat end only (not inside the boiler) to confirm the wires are firmly connected and not damaged, then power the boiler back on and see if the code clears.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • Thermostat wiring or terminal connector
  • Boiler control PCB (uncommon)

Related codes

Full explanation for A9
b0

Your boiler has stopped because a safety sensor that checks the exhaust gases leaving the flue is giving a faulty reading, so it has shut down as a precaution and will need a qualified engineer to check it.

High Engineer only £120-220

What to check first

  1. Check the boiler pressure gauge and top it up to between 1 and 1.5 bar if it is low, following the filling instructions in the user manual.
  2. Press the reset button or use the reset option on the boiler's display once, then wait 10 minutes to see whether it restarts.
  3. Check that the flue terminal outside the house is not blocked by leaves, nesting material, snow or debris, and clear it only if you can reach it safely.
  4. If the fault returns, stop trying to reset it and book a Gas Safe registered engineer. Do not open the boiler casing or try to replace any parts yourself.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness or plug
  • Main control PCB (if the sensor circuit is damaged)

Related codes

Full explanation for b0
b8

The boiler has lost the signal from a safety sensor that monitors the exhaust gases, so it has shut itself down as a precaution and will need a Gas Safe engineer to check it.

High Engineer only £120-220

What to check first

  1. Check the boiler display to confirm the code is B8 and note whether heating and hot water are both off.
  2. Turn the boiler off at its power switch or fused spur, wait 1 minute, then turn it back on to try a reset.
  3. Make sure the flue terminal outside the property is not blocked by leaves, debris, nests or snow, then check whether the boiler restarts.
  4. If the code returns or the boiler stays off, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor lead or wiring harness
  • Main control PCB (if the lead is fine but the fault persists)

Related codes

Full explanation for b8
E3

Your boiler has shut itself down because one of its built-in safety checks has detected a problem, and it needs a qualified engineer to find out why before it can safely run again.

High Engineer only £120-350

What to check first

  1. Check the boiler display to confirm the code is E3, then wait a few minutes.
  2. Press the reset button on the front of the boiler, or switch the boiler off at its power supply for 30 seconds and back on, to try one restart.
  3. Check the flue pipe outside your home for visible blockages such as leaves, nests or debris, and check that any condensate pipe is not frozen.
  4. If the fault returns or the boiler will not restart, stop resetting it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas thermostat
  • Air pressure switch
  • Combustion fan
  • Condensate trap

Related codes

Full explanation for E3
E5

Your boiler's control system has detected an internal problem and has stopped itself as a precaution, so it needs a qualified engineer to check it.

High Engineer only £120-450

What to check first

  1. Turn the boiler off at the control panel and at the isolator switch, wait 10 minutes, then turn it back on to see if the code clears.
  2. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when cold; if it is lower, top it up using the filling loop as shown in the manual.
  3. If the code returns, or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Flame detection electrode
  • Fan assembly
  • Gas valve

Related codes

Full explanation for E5
F0

The boiler has detected an internal electronic problem and has shut itself down as a precaution, so it may have stopped providing heating and hot water until it is reset and checked.

High Engineer only £200-450

What to check first

  1. Check that the boiler has not lost power and that the gas supply tap is fully open.
  2. Press and hold the reset button on the front of the boiler for about 2 seconds, or switch it off at the isolator or fused spur for 30 seconds and then back on.
  3. Wait 10 minutes and check whether the boiler restarts and the display clears the F0 code. If the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Fan assembly
  • Flue gas sensor

Related codes

Full explanation for F0
F1

Your boiler has switched itself off because the hot gases leaving through the flue got hotter than it is designed to allow, which usually points to a blockage or a problem in the flue or heat exchanger that needs checking by a qualified engineer.

High Engineer only £120-350

What to check first

  1. Turn the boiler off at its on/off switch and wait five minutes before trying to restart it.
  2. Check that your carbon monoxide alarm is working and in place near the boiler, and open a window if you notice any unusual smells.
  3. From outside, check that the flue terminal (the pipe or grille on an outside wall) is not blocked by debris, nests, leaves or snow, and clear anything you can reach safely from the ground.
  4. Restart the boiler once. If the fault returns, leave it off and book a Gas Safe registered engineer rather than repeating the reset.

Parts commonly replaced

  • Flue gas temperature limiter
  • Flue gas temperature sensor
  • Flue terminal or flue pipe seals
  • Heat exchanger (in more serious cases)

Related codes

Full explanation for F1
F2

Your boiler has shut itself down as a precaution because it got too hot, and it should be safe to restart once it has cooled and the cause is checked.

High Engineer only £100-250

What to check first

  1. Wait at least 15-20 minutes for the boiler to cool down completely before doing anything else.
  2. Check the boiler pressure gauge; if it reads below about 1 bar, top it up using the filling loop as shown in the manual, then close the loop again.
  3. Make sure the heating is not blocked, for example by thermostatic radiator valves that are fully closed, and that the boiler's own isolation valves are open.
  4. Press the reset button on the boiler (or use the reset option on the display) once it has cooled. If the fault clears and the boiler runs normally, keep an eye on it. If F2 returns, stop using it and book a Gas Safe engineer.

Parts commonly replaced

  • Temperature limiter (STB)
  • Heating circulation pump
  • Flow temperature sensor (NTC)

Related codes

Full explanation for F2
F3

The boiler has detected a flame when it should not be lit, so it has stopped as a safety precaution and will need a qualified engineer to check it before it is used again.

Emergency Engineer only £100-400

Parts commonly replaced

  • Ionisation (flame sensor) electrode
  • Gas valve assembly
  • Control PCB
  • Burner gasket

Related codes

Full explanation for F3
F4

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.

High Engineer only £120-350

What to check first

  1. Check that your gas supply is on: look at your gas meter and confirm the emergency control valve and any supply isolation valve are open.
  2. Check whether other gas appliances, such as the hob, work normally. If they do not, contact your gas supplier, not a heating engineer.
  3. Press the reset button on the boiler's control panel once, wait a few minutes, and see whether it lights. If it fails again or the code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Ionisation electrode
  • Ignition electrode
  • Ignition module
  • Gas valve
  • Condensate siphon

Related codes

Full explanation for F4
Showing 21–26 of 26
F8

Your boiler has shut itself down for safety because its gas supply valve isn't closing properly, so it needs a Gas Safe registered engineer to inspect it before it's used again.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve assembly
  • Gas valve solenoid coil
  • Boiler control PCB

Related codes

Full explanation for F8
F9

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.

High Engineer only £200-400

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait about 5 minutes.
  2. Switch it back on and see whether the boiler restarts or shows the same F9 code.
  3. Check that the flue outlet outside is not blocked by leaves, debris or nesting material, and that the condensate pipe is not frozen or blocked.
  4. If the code returns, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan connecting cable or harness
  • Main PCB or fan control board
  • Gas valve (if the fan and gas valve are checked together)

Related codes

Full explanation for F9
FA

The boiler's fan is still turning when it should have stopped, so the boiler has likely shut itself down as a safety precaution and will need a qualified engineer to check it.

High Engineer only £200-450

What to check first

  1. Switch the boiler off at its power supply or isolator switch, wait about 10 minutes, then switch it back on to reset it.
  2. Check that the flue outlet on the outside wall is clear of leaves, nests, snow or other obstructions.
  3. Check that the condensate drain pipe is not blocked or frozen, and that the boiler is not in a cold, damp location where pipes could freeze.
  4. If the fault returns after the reset, stop trying to restart it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan power or signal cable
  • Main PCB (control board)

Related codes

Full explanation for FA
FC

The boiler has stopped because its fan control isn't working properly, so it can't safely run until a qualified engineer checks and repairs it.

High Engineer only £250-550

What to check first

  1. Check that the boiler has power, the pressure gauge reads about 1-1.5 bar, and the flue outside is not blocked.
  2. Press the reset button once, wait 10 minutes, and see if the boiler restarts.
  3. If the fault code returns, stop trying to reset it and book a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Control unit (PCB)
  • Fan power and signal cable

Related codes

Full explanation for FC
Fd

Your boiler's internal control system has detected a fault and has shut down the burner, so it likely isn't producing heat or hot water until a Gas Safe engineer looks at it.

High Engineer only £200-450

What to check first

  1. Check that your gas supply and other gas appliances are working, and that the boiler pressure gauge reads between 1 and 1.5 bar.
  2. Switch the boiler off at the isolator or fused spur for 30 seconds, then switch it back on.
  3. Press the reset button once. If the Fd code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (PCB)
  • Ignition electrode
  • Flame sensor (ionisation electrode)
  • Gas valve

Related codes

Full explanation for Fd
FF

Your boiler has stopped because its internal control electronics have detected a fault, so it is shutting itself down until a qualified engineer can check it.

High Engineer only £250-450

What to check first

  1. Turn the boiler off at its on/off switch, then switch off the spur or isolator supplying it and wait five minutes.
  2. Switch the power back on and press the reset button on the boiler if it is lit or flashing, then wait a few minutes to see if it restarts.
  3. Check that your gas supply is on by confirming that a gas hob or other gas appliance works, and check that the boiler's pressure gauge is in the normal range (around 1 to 1.5 bar when cold).
  4. If the code returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit (PCB)
  • Ignition electrode
  • Ionisation electrode
  • Gas valve

Related codes

Full explanation for FF