Viessmann Vitodens 111-W B1LD 26kW Fault codes & diagnostics

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

Combi Natural GasLPG 2016-present
GC Number 47-819-43
Download the Viessmann Vitodens 111-W B1LD 26kW 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.
4 Emergency 21 High 1 Medium 2 Low
Just want the most common Viessmann Vitodens 111-W B1LD 26kW faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 28 documented codes

Tap any card for details

0A

Your boiler has shut itself off because its flame is producing too much carbon monoxide, which means the burner is probably blocked and needs a Gas Safe engineer to check it before the boiler is used again.

Emergency Engineer only £150-600

Parts commonly replaced

  • Burner
  • Burner gasket
  • Ignition electrode
  • Heat exchanger (if damaged)
  • Flue terminal (if blocked)

Related codes

Full explanation for 0A
0C

The boiler has stopped because its burner is blocked or not getting a steady enough electricity supply, so it cannot light and heat your home or water safely until it is checked.

High Engineer only £120-350

What to check first

  1. Check that other appliances in the home have power and that no fuse or RCD has tripped, then reset the boiler once using its reset button or control panel.
  2. Check that the flue outlet on the outside wall is clear of leaves, nests, snow or debris, and that the condensate pipe is not frozen or blocked.
  3. If the boiler still shows 0C after one reset, leave it switched off at the isolator and contact a Gas Safe registered engineer to clean or replace the burner and check the electrical supply.

Parts commonly replaced

  • Burner assembly
  • Burner gasket
  • Ignition electrode
  • Fan
  • Gas valve

Related codes

Full explanation for 0C
10

The boiler has lost the signal from its outside weather sensor, so it can't adjust heating to the weather, but it can usually still heat your home and water.

Low DIY-safe £90-180

What to check first

  1. Turn the boiler off at its power switch, wait 30 seconds, then turn it back on to clear the fault and see if it returns.
  2. Look at the outside temperature sensor on the exterior wall, usually a small white or grey box near a window or under the eaves, and check that its cable is not cut, pinched, or damaged and that the cover is not cracked or letting in water.
  3. If the cable is damaged, or the fault comes back after the reset, contact a Gas Safe registered engineer to test or replace the sensor. Do not open the boiler casing yourself.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 10
18

Your boiler has lost the signal from the sensor that measures the outside temperature, so it can't adjust its heating to the weather, but it will usually still heat your home and water.

Low DIY-safe £100-200

What to check first

  1. Turn the boiler off at its power switch, wait 30 seconds, then switch it back on to see if the code clears.
  2. Locate the outside temperature sensor, usually a small white or grey box on a shaded north or north-facing wall, and check that its cable is not cut, pinched, or loose at either end.
  3. Check that the cable has not been damaged by weather, pests, or garden work, and tighten any loose connections at the sensor end only.
  4. If the code returns after a reset, leave it alone and book a Gas Safe registered engineer to check or replace the sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 18
30

Your boiler has stopped because its sensor that measures the water temperature is reporting a fault, and it may also have a blockage in the burner, so it has shut itself down to stay safe until it's checked.

High Engineer only £120-220

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the manual, then stop.
  2. Press the reset button or use the reset option on the boiler display once. If the fault returns, do not keep resetting it.
  3. Make sure the condensate pipe is not frozen or blocked, and check that the boiler has power and the room thermostat is calling for heat.

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Burner cleaning kit or burner gasket

Related codes

Full explanation for 30
38

Your boiler has shut itself down because it has detected a problem with a temperature sensor and a possible blockage around the burner, so it needs a qualified engineer to check it.

High Engineer only £150-350

What to check first

  1. Check the boiler pressure gauge; it should read between 1 and 1.5 bar when the boiler is cold. If it is low, top it up using the filling loop as shown in the manual.
  2. Wait about 10 minutes, then press and hold the reset button for a few seconds to restart the boiler once.
  3. If the fault code returns, or the boiler will not restart, stop trying and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor lead or wiring harness
  • Burner gauze or burner assembly
  • Condensate trap (if blocked)

Related codes

Full explanation for 38
50

Your boiler has stopped heating your hot water because it has detected a fault with the sensor that measures the temperature in your hot water cylinder, and a qualified engineer will need to check it.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its isolator switch or fused spur and wait one minute.
  2. Switch the boiler back on and check whether the code clears or returns on the display.
  3. If the code returns, leave the boiler off and book a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor (B3)
  • Sensor cable or connector

Related codes

Full explanation for 50
51

Your boiler has detected a faulty temperature sensor, so it has stopped producing hot water for taps and showers, though it may still be able to heat your radiators.

High Engineer only £120-220

What to check first

  1. Check the boiler pressure gauge; it should read between 1.0 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the handbook, then close the loop.
  2. Switch the boiler off at its power supply, wait 30 seconds, then switch it back on to clear the fault code.
  3. If the code returns, stop and call a Gas Safe registered engineer, as the sensor needs testing and replacing.

Parts commonly replaced

  • Outlet temperature sensor (NTC)
  • Sensor wiring harness
  • Hot water cylinder sensor (if fitted)

Related codes

Full explanation for 51
58

Your boiler has stopped heating your hot water tank because it has lost the signal from the sensor that measures the water temperature, so it cannot tell how hot the water is.

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. Check that the cable from the cylinder temperature sensor is firmly plugged into the boiler's control board and that the sensor cable at the cylinder is not loose, cut, or damaged.
  3. Switch the power back on, press the reset button on the boiler (or use the reset option on the control panel), and check whether the code clears and hot water returns.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or plug connector

Related codes

Full explanation for 58
59

Your boiler has lost contact with a small sensor that checks how hot the water leaving it is, so it has stopped heating water for your taps and showers, though it is not a safety risk.

High Engineer only £120-220

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur, wait 30 seconds, then switch it back on.
  2. Check the boiler pressure gauge reads between 1.0 and 1.5 bar when cold, and top up using the filling loop only if the boiler manual says so.
  3. If the code 59 returns, leave the boiler off and call a Gas Safe registered engineer, as the sensor and its wiring need inspecting and replacing by a qualified person.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring lead or plug
  • Gas Safe engineer call-out and labour

Related codes

Full explanation for 59
Showing 11–20 of 28
A3

The boiler has not been able to finish its startup checks because one of its safety sensors in the flue pipe is not fitted in the correct position, so it will not run until an engineer corrects it.

High Engineer only £80-180

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor clip or seal

Related codes

Full explanation for A3
A9

Your boiler is set up to receive heating instructions from a smart or connected room controller, but it isn't getting a signal from that controller, so it may not be controlling the heating as intended.

Medium Engineer only £80-180

What to check first

  1. Turn the boiler off at its on/off switch, wait about 30 seconds, then turn it back on to see if the code clears.
  2. Check that the room thermostat or smart controller has power, its batteries are fresh, and it is switched on and set to heating.
  3. Look at the low-voltage wires at the thermostat and at the boiler's control terminals, if you can see them safely, and check nothing is loose or damaged. Do not open the boiler casing.

Parts commonly replaced

  • OpenTherm or room thermostat
  • OpenTherm connection cable or wiring
  • Boiler control PCB (uncommon)

Related codes

Full explanation for A9
b0

Your boiler has shut itself down as a safety precaution because it has detected a possible blockage in the burner or flue, so please turn it off at the switch and arrange for a Gas Safe engineer to check it before using it again.

Emergency Engineer only £120-280

What to check first

  1. Turn the boiler off at its on/off switch and at the fused spur or isolator, then make sure the room is well ventilated and open a window.
  2. From outside, look at the flue terminal (the pipe or cap exiting the wall or roof) and check for visible blockages such as nests, leaves, debris or snow, and remove them only if it is safe and you can reach them without a ladder or risk.
  3. Leave the boiler off for 10 minutes, then switch it back on and try one reset using the reset button on the boiler. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer. Do not keep repeating resets.

Parts commonly replaced

  • Flue gas temperature sensor
  • Burner gasket
  • Flue terminal cap
  • Burner assembly

Related codes

Full explanation for b0
b7

Your boiler has shut itself down for safety because its internal control system has detected a fault, and it needs a qualified engineer to check and repair it.

High Engineer only £200-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or isolator, then wait 10 minutes.
  2. Switch the power back on and press the reset button on the front of the boiler, if it has one.
  3. Check the pressure gauge reads between 1 and 2 bar. If it is low, top it up using the filling loop as shown in the manual.
  4. If the boiler still shows b7 after resetting, stop and book a Gas Safe registered engineer. Do not attempt to open or repair the control unit yourself.

Parts commonly replaced

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

Related codes

Full explanation for b7
b8

Your boiler has shut itself down because its burner or flue air path looks blocked, and a sensor that checks the exhaust temperature is reporting a fault, so it is stopping safely until someone checks it.

High Engineer only £150-350

What to check first

  1. Check the boiler display for any other fault codes and note them down before you do anything else.
  2. Turn the boiler off at its on/off switch, wait 10 minutes, then turn it back on once to see whether the code clears. Do not repeat this more than once or twice.
  3. From outside, check the flue terminal (the pipe or grille on the outside wall) for leaves, debris, bird nests or snow and remove only what you can reach safely from the ground or a secure step.
  4. Check the boiler pressure gauge is between 1 and 1.5 bar when cold, and top up using the filling loop only if you know how to do this safely.
  5. If the fault returns, or you smell gas or hear a CO alarm, leave the property, open windows and call the National Gas Emergency Service on 0800 111 999 or a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Burner gauze or burner assembly
  • Combustion chamber gasket
  • Flue terminal cleaning or replacement

Related codes

Full explanation for b8
E3

Your boiler has shut itself down for safety because its gas flame isn't being detected or its safety checks have failed, so it isn't producing heating or hot water until it's looked at.

High Engineer only £90-250

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar. If it's low, top it up using the filling loop only if you know how, otherwise leave this to an engineer.
  2. Press the reset button or use the reset option on the display, then wait 5 minutes for the boiler to restart.
  3. Check that other gas appliances in the home are working, and that the boiler's flue outlet and air vents are not blocked by debris, snow or vegetation.
  4. If the boiler shows E3 again after one reset, stop resetting it and call a Gas Safe registered engineer. Repeated resets can hide a fault that needs proper repair.

Parts commonly replaced

  • Flame detection (ionisation) electrode
  • Ignition electrode and lead
  • Gas valve
  • Fan assembly
  • Main PCB (control board)

Related codes

Full explanation for E3
E5

Your boiler has shut itself down because its burner appears to be blocked or has an internal fault, so it's stopped producing heat and hot water until a qualified engineer checks it.

High Engineer only £150-400

What to check first

  1. Check that the gas supply is on by confirming the gas meter valve is open and other gas appliances are working.
  2. Check the boiler pressure gauge and top up through the filling loop if it reads below 1 bar, following the manufacturer's instructions.
  3. Press the reset button once, wait 5 minutes, and see if the boiler restarts. If E5 returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner gasket
  • Burner assembly
  • Ignition electrode

Related codes

Full explanation for E5
F0

Your boiler has shut itself down as a safety precaution because its gas burner appears to be blocked or it has detected an internal fault, so it needs a qualified engineer to check it before it can run safely again.

High Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, following the manufacturer's instructions.
  2. Press the reset button once (or use the reset function on the controls) and wait 10 minutes to see if the boiler restarts.
  3. If the fault returns or the boiler will not restart, turn off the boiler and call a Gas Safe registered engineer. Do not attempt to clean or open the burner yourself.

Parts commonly replaced

  • Burner assembly
  • Burner gasket
  • Fan assembly
  • Ignition electrode
  • Flue cleaning kit

Related codes

Full explanation for F0
F1

Your boiler has shut itself down as a safety precaution because the hot gases leaving through the flue are running hotter than they should, so you may have no heating or hot water until it is checked.

High Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge and top it up to the recommended level (usually around 1.0 to 1.5 bar) if it is low, following the filling instructions in the manual.
  2. Press the reset button once on the boiler and wait a few minutes to see whether it restarts. Do not keep pressing reset repeatedly.
  3. From ground level, check that the flue terminal outside the property is clear of leaves, nests, debris or anything blocking it, and that nothing is stored close to it.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue gas temperature limiter
  • Flue seals or gaskets
  • Burner assembly
  • Heat exchanger (if damaged)

Related codes

Full explanation for F1
F2

Your boiler has shut itself down as a safety precaution because it got too hot, and it will need to be checked before it can heat your home again.

High Engineer only £100-300

What to check first

  1. Check the boiler pressure gauge and make sure it reads between 1 and 1.5 bar when cold. If it is low, stop here and call an engineer.
  2. Make sure the heating is switched on, the thermostat is calling for heat, and the radiator valves are open.
  3. Press the reset button once on the boiler. If it fails again or the fault returns quickly, stop and book a Gas Safe registered engineer.

Parts commonly replaced

  • Circulation pump
  • Flow temperature sensor (NTC)
  • Temperature limiter (STB)

Related codes

Full explanation for F2
Showing 21–28 of 28
F3

The boiler has detected a flame when it should not be burning, so it has shut itself down as a safety precaution, and it needs a Gas Safe engineer to check it before it is used again.

Emergency Engineer only £90-400

What to check first

  1. Turn the boiler off at its on/off switch and, if you can do so safely, turn off the gas supply at the isolation valve near the boiler.
  2. Do not keep trying to reset the boiler, and do not use any gas appliance that might be affected until it has been checked.
  3. Open windows and doors to ventilate the room, and if you smell gas, leave the property and call the National Gas Emergency number on 0800 111 999 from outside.
  4. Call a Gas Safe registered engineer to inspect the gas valve, flame sensor and burner before the boiler is restarted.

Parts commonly replaced

  • Ionisation flame sensor electrode
  • Gas valve (gas block)
  • Ignition electrode
  • Control PCB

Related codes

Full explanation for F3
F4

The boiler tried to light its burner but couldn't establish a flame, so it has stopped and is waiting for a reset, which usually means no heating or hot water until it's checked.

High Engineer only £120-350

What to check first

  1. Check that the gas supply is on: confirm the gas meter or isolation valve is open and that other gas appliances, such as the hob, are working.
  2. Check the boiler pressure gauge and top it up using the filling loop only if it reads below about 1 bar, following the boiler manual.
  3. Press the reset button once and wait 10 minutes. If the fault returns or the boiler locks out again, stop and call a Gas Safe registered engineer. Do not keep resetting it.

Parts commonly replaced

  • Ignition electrode
  • Ionisation (flame sensing) electrode
  • Gas valve (gas train)
  • Ignition module / control PCB

Related codes

Full explanation for F4
F8

The boiler has shut itself down as a safety precaution because its gas supply did not switch off quickly enough after the flame went out, so it needs a Gas Safe registered engineer to check it before it is used again.

Emergency Engineer only £250-500

Parts commonly replaced

  • Gas valve (fuel valve)
  • Gas/air ratio control unit
  • Ignition and ionisation electrode
  • Control PCB

Related codes

Full explanation for F8
F9

The boiler's fan, which pushes air through the boiler to burn gas safely, isn't starting, so the boiler has shut itself down as a precaution and won't heat your home or water until it's checked.

High Engineer only £250-450

What to check first

  1. Turn the boiler off at its isolator switch and wait 10 minutes, then turn it back on and press the reset button once on the boiler's display. If F9 returns, do not keep resetting it.
  2. Check that the external flue terminal (the pipe outlet on an outside wall) isn't blocked by leaves, nests, snow or debris, and clear any obstruction you can reach safely.
  3. Check that the boiler has mains power and that the water pressure on the gauge is between 1 and 1.5 bar, topping up only if the manufacturer's filling instructions say it is safe to do so.

Parts commonly replaced

  • Combustion fan assembly
  • Fan connecting cable or harness
  • Main control PCB (circuit board)

Related codes

Full explanation for F9
FA

The boiler has stopped its burner because it has detected a fault, so it may not be heating your home or providing hot water until a qualified engineer checks it.

High Engineer only £150-350

What to check first

  1. Check the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when cold. If it is well below 1.0 bar, top it up using the filling loop as described in your manual, then stop.
  2. Wait about 10 minutes, then press and hold the reset button for about 3 seconds, or use the reset option on the control panel.
  3. If the boiler restarts and stays on, keep an eye on it. If the fault code returns or the boiler shuts down again, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Ignition electrode
  • Gas valve
  • Combustion chamber gasket
  • Control PCB

Related codes

Full explanation for FA
FC

Your boiler has shut itself down for safety because its burner is blocked and a part that controls the fan is not working properly, so it needs a qualified engineer to check and repair it.

High Engineer only £180-420

What to check first

  1. Check that the outside flue terminal and any air vents are clear of leaves, nests, snow or other debris.
  2. Try resetting the boiler once using the reset button or control panel, then watch whether it stays running.
  3. If the fault code returns or the boiler shuts down again, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Burner head
  • Burner gasket
  • Fan control unit (PCB)

Related codes

Full explanation for FC
Fd

Your boiler has shut itself down for safety because the burner is not lighting or staying lit properly, and it will need a qualified engineer to check it.

High Engineer only £120-450

What to check first

  1. Check the boiler pressure gauge. It should read between 1 and 1.5 bar when cold. If it is low, top it up using the filling loop as shown in the manual.
  2. Make sure the condensate pipe is not frozen or blocked, and that the boiler's external flue and air vents are clear of leaves, snow or debris.
  3. Switch the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then switch it back on and press the reset button once. If it fails again, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit
  • Ionisation (flame sensor) electrode
  • Burner gauze or burner assembly
  • Gas valve
  • Fan

Related codes

Full explanation for Fd
FF

Your boiler has shut itself down for safety because its burner has become blocked or its control system has detected a fault, so it isn't producing heat or hot water until it's checked.

High Engineer only £150-450

What to check first

  1. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar, then check that the gas supply and other gas appliances are working normally.
  2. Make sure nothing is blocking the boiler's air vent or flue outlet, and that the boiler area is clear and well ventilated.
  3. Press and hold the reset button on the boiler control panel for about one second to try a single restart, and wait a few minutes to see whether the boiler fires up. If the FF code returns, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Burner control unit
  • Ignition electrode
  • Ionisation (flame sensing) electrode
  • Burner gasket
  • Fan assembly
  • Gas valve

Related codes

Full explanation for FF
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