Viessmann Vitodens 111-W B1LA 26kW Fault codes & diagnostics

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

Combi Natural GasLPG 2012-2016 Discontinued
Download the Viessmann Vitodens 111-W B1LA 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.
2 Emergency 18 High 3 Medium 2 Low
Just want the most common Viessmann Vitodens 111-W B1LA 26kW faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 25 documented codes

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0C

The boiler has detected that the electricity supply reaching it is weaker than it should be, so it has paused to protect itself until the power becomes steady again.

Medium Engineer only £80-200

What to check first

  1. Check whether other appliances or room lights are dimming or flickering, and note whether the problem happens only at certain times of day.
  2. Switch off high-power appliances such as electric showers, kettles, ovens, or washing machines that may be drawing a lot of current, then wait a few minutes.
  3. Turn the boiler off at its isolator or mains switch, wait 30 seconds, then switch it back on and press the reset button on the boiler's control panel if the fault still shows.
  4. If the code returns or the boiler keeps cutting out, contact your electricity network operator (DNO) to check the supply to your property, or call a qualified electrician to inspect the socket, wiring, and consumer unit.

Parts commonly replaced

  • Power supply circuit board
  • Mains supply fuse
  • Mains filter

Related codes

Full explanation for 0C
10

The boiler's outdoor weather sensor is sending a faulty signal, so it can't adjust heating to the weather, but it will usually keep heating and hot water working normally.

Low DIY-safe £90-180

What to check first

  1. Turn the boiler off at the on/off switch, wait 5 minutes, then turn it back on and check whether the fault clears.
  2. Find the outdoor temperature sensor, a small white or grey box on an outside wall, usually on the north or shaded side and often under a cover. Check that its cable is not pinched, cut, or chewed, and that the sensor is not damaged or full of water.
  3. If the cable is damaged, do not repair it yourself. Contact a Gas Safe registered engineer, as the boiler connection and sensor replacement should be done by a qualified person.
  4. If the fault returns after a reset, book a Gas Safe engineer to test or replace the outdoor sensor.

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 outdoor temperature, but it will usually keep heating your home and water at a fixed setting.

Low Engineer only £100-200

What to check first

  1. Turn the boiler off at its on/off switch and wait a few minutes.
  2. Check the outdoor sensor on the outside wall (usually north-facing) for visible damage, loose wiring, or a cable pulled out of its clip.
  3. Turn the boiler back on and check whether code 18 clears. If it returns, book a Gas Safe registered engineer to test or replace the sensor.

Parts commonly replaced

  • Outside temperature sensor
  • Outside sensor cable
  • Sensor connector

Related codes

Full explanation for 18
30

Your boiler has stopped because it is getting an impossible temperature reading from one of its internal sensors, so it has shut down heating and hot water as a safety precaution.

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 your manual, then close the loop.
  2. Switch the boiler off at its power switch, wait 30 seconds, then switch it back on to see if the fault clears. If it returns, do not keep resetting it.
  3. Note the fault code and check whether heating or hot water is still working at other times. Then contact a Gas Safe registered engineer, as the sensor itself needs replacing and the casing must not be opened by a homeowner.

Parts commonly replaced

  • Boiler water temperature sensor (NTC)
  • Sensor wiring harness or connector
  • Control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 30
38

The boiler has lost the connection to one of its temperature sensors, so it can't read the water temperature properly and has likely stopped heating your home and hot water until it's looked at.

High Engineer only £120-220

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to see if the code clears
  2. Check the boiler's pressure gauge and top up with the filling loop if it reads below 1 bar, following the manual
  3. If the code returns, leave the boiler off and contact a Gas Safe registered engineer, giving them the code 38

Parts commonly replaced

  • Boiler water temperature sensor (flow sensor)
  • Sensor wiring harness or connector
  • Main PCB (only if the fault persists after the sensor is replaced)

Related codes

Full explanation for 38
50

The boiler has detected a faulty signal from the sensor that measures the temperature of the hot water cylinder, so it may stop heating your stored hot water until the sensor or its wiring is checked.

Medium Engineer only £120-220

What to check first

  1. Switch off the boiler at its power supply and the fused spur, then wait a few minutes.
  2. Check the cable from the boiler to the cylinder sensor for visible damage, pinched wires, or loose plugs, without disconnecting anything inside the boiler casing.
  3. Restore power, then reset the boiler using its reset button or control menu, and check whether fault 50 returns. If it does, call a Gas Safe registered engineer.

Parts commonly replaced

  • Cylinder temperature sensor (DHW sensor)
  • Sensor cable or wiring harness

Related codes

Full explanation for 50
51

Your boiler has detected a problem with one of its internal temperature sensors, so it may stop heating your home or producing hot water until a qualified engineer replaces or repairs the faulty part.

High Engineer only £120-250

What to check first

  1. Check that the boiler pressure on the display is between 1.0 and 1.5 bar, and top it up using the filling loop only if it is below 1.0 bar.
  2. Turn the boiler off at the switch, wait 30 seconds, then turn it back on to see whether the fault clears.
  3. If the code returns, stop and book a Gas Safe registered engineer, as the sensor and its wiring must be checked inside the boiler.

Parts commonly replaced

  • Outlet temperature sensor (flow sensor)
  • Sensor wiring harness
  • Control PCB (only if the sensor and wiring are fine)

Related codes

Full explanation for 51
58

Your boiler has lost contact with the sensor that checks the temperature of your hot water cylinder, so it may not be heating your stored hot water properly, though your central heating may still work.

High Engineer only £120-250

What to check first

  1. Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on to see if the fault clears.
  2. If the fault returns, check the cylinder sensor cable is not visibly damaged, cut through or pulled loose where it runs from the cylinder area to the boiler, and do not open the boiler casing.
  3. Note the exact fault code and any other messages on the display, then book a Gas Safe registered engineer to test and replace the sensor if needed.

Parts commonly replaced

  • Cylinder temperature sensor
  • Sensor cable or connector

Related codes

Full explanation for 58
59

The boiler has lost its reading of how hot the water leaving it is, so it has stopped as a precaution and will need a qualified engineer to check and replace a small sensor.

High Engineer only £120-220

What to check first

  1. Check that the boiler is on and the heating controls are set to call for heat.
  2. Check the boiler pressure gauge and top up with the filling loop if it reads below 1 bar.
  3. Try one reset using the reset button or the control panel, then wait 10 minutes to see if the boiler restarts.
  4. If the fault returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing or work on the sensor yourself.

Parts commonly replaced

  • Outlet (flow) temperature sensor
  • Sensor wiring harness or plug
  • Control PCB (only if the sensor and wiring test good)

Related codes

Full explanation for 59
A9

Your boiler has lost its connection to your room thermostat or controls, so it may not be responding to heating requests correctly, but this is usually a fixable wiring or thermostat issue.

Medium DIY-safe £90-250

What to check first

  1. Check that your room thermostat has working batteries, or that its display is lit if it is mains or wired powered.
  2. Look at the thermostat wiring at both ends and make sure no wires are loose or disconnected. Switch the boiler off at the isolator or fused spur before touching any terminals.
  3. Confirm the thermostat is an OpenTherm-compatible model and that it is set to OpenTherm mode, then switch the boiler back on and reset it using the reset button or by power cycling it.
  4. If the code returns, leave the boiler off and contact a Gas Safe registered engineer.

Parts commonly replaced

  • OpenTherm room thermostat or controller
  • Thermostat connection cable
  • Boiler control unit communication module

Related codes

Full explanation for A9
Showing 11–20 of 25
b0

The boiler has lost track of how hot its exhaust gases are, so it has switched itself off as a precaution and will need a qualified engineer to check the sensor.

High Engineer only £120-220

What to check first

  1. Check the pressure gauge on the boiler display and make sure it reads between 1.0 and 1.5 bar; top it up only if the manufacturer's instructions for your model say so.
  2. Turn the boiler off at its power switch or isolator, wait 5 minutes, then switch it back on to see whether it restarts.
  3. If the fault code returns or the boiler stays off, stop trying to fix it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue sensor wiring harness
  • Main control PCB (only if the fault is traced to the board)

Related codes

Full explanation for b0
b8

Your boiler has stopped because it can no longer read the temperature of the gases leaving through the flue, which it needs to check for safety, so a qualified engineer should look at it soon.

High Engineer only £120-220

What to check first

  1. Check the boiler display for any other fault codes and note them down for the engineer.
  2. Press the reset button once, or use the reset option on the boiler controls, and wait a few minutes to see whether it restarts.
  3. From outside, check that the flue outlet is not blocked by leaves, nests, debris or snow, and that nothing is covering it.
  4. If the code returns or the boiler will not restart, stop trying and book a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Sensor wiring lead or harness
  • Control PCB (only if the sensor and wiring test good)

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, so it needs a qualified engineer to find out why before it will run again.

High Engineer only £120-350

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. If it is low, follow the filling instructions in your user manual.
  2. Check that the condensate pipe is not frozen or blocked, and that the flue terminal outside is clear of debris, snow or nesting material.
  3. Reset the boiler using the reset button or by following the reset steps in your manual. If the fault returns, do not keep resetting it. Call a Gas Safe registered engineer.

Parts commonly replaced

  • Air pressure switch
  • Safety temperature limiter (STB)
  • Condensate siphon
  • Fan assembly
  • Flue gas temperature sensor

Related codes

Full explanation for E3
E5

Your boiler's control system has detected an internal problem with itself, which usually means it has shut down to protect its parts, so it may not be heating your home or providing hot water until it is checked.

High Engineer only £150-450

What to check first

  1. Switch the boiler off at its on/off switch and at the fused spur or circuit breaker, then wait 5 minutes.
  2. Switch the power back on and check whether the boiler restarts and the code clears.
  3. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, and top up only if the manual says it is safe to do so.
  4. If the code returns or the boiler will not restart, stop and call a Gas Safe registered engineer.

Parts commonly replaced

  • Control unit (PCB)
  • Ignition electrode
  • Flame sensor
  • Fan assembly

Related codes

Full explanation for E5
F0

Your boiler's internal control system has detected a fault and may have stopped working until it is reset or repaired.

High Engineer only £250-550

What to check first

  1. Turn off the boiler at its on/off switch and at the fused spur or isolator supplying it, then wait 10 minutes.
  2. Switch the power back on and check whether the boiler restarts and the F0 code clears.
  3. If the code returns or the boiler will not run, stop trying to reset it and call a Gas Safe registered engineer.

Parts commonly replaced

  • Main PCB (control unit)
  • Power supply unit

Related codes

Full explanation for F0
F1

Your boiler has shut itself down because the hot gases leaving through the flue are running hotter than it is designed to allow, which is a safety protection rather than a sign that it has been permanently damaged.

High Engineer only £120-300

What to check first

  1. Switch the boiler off at its control panel and at the fused spur or isolator, then wait about 15 minutes.
  2. With the boiler off, check the outside flue terminal for anything blocking it, such as leaves, nests, debris or a damaged cover, and clear it only if you can do so safely from the ground or a stable position.
  3. Switch the boiler back on and press the reset button once. If it locks out with F1 again, or you notice a gas smell, a hissing sound or a carbon monoxide alarm going off, stop, leave the property if needed, and call a Gas Safe registered engineer.

Parts commonly replaced

  • Flue gas temperature sensor
  • Flue gas temperature limiter
  • Flue terminal or flue pipe section
  • Combustion fan assembly

Related codes

Full explanation for F1
F2

Your boiler has shut itself down because the heating water got hotter than it should, and a built-in safety cut-out stopped it to protect the system.

High Engineer only £120-300

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 your manual.
  2. Make sure all radiator and heating valves are open and the boiler's circulation pump is running, as low water flow is a common cause of overheating.
  3. Bleed any radiators that are cold at the top, as trapped air can reduce circulation.
  4. Let the boiler cool for at least 15 to 20 minutes, then try resetting it from the control panel, but only if your manual says the user can reset it.
  5. If the fault returns, stop using the boiler and call a Gas Safe registered engineer.

Parts commonly replaced

  • Safety temperature limiter (STB)
  • Circulation pump
  • Flow temperature sensor

Related codes

Full explanation for F2
F3

Your boiler has detected a flame just as it is starting up, when no flame should be there, which can point to a fault that needs a qualified gas engineer to check safely.

Emergency Engineer only £150-400

Parts commonly replaced

  • Gas valve
  • Ionisation electrode
  • Control PCB

Related codes

Full explanation for F3
F4

The boiler has switched itself off because it couldn't detect its gas flame lighting properly, so it's stopped providing heating and hot water until it's checked.

High Engineer only £90-250

What to check first

  1. Check that other gas appliances, such as the cooker or hob, still work, to confirm the gas supply is on. If they don't, contact your gas supplier or the National Gas Emergency Service on 0800 111 999.
  2. Check the boiler's pressure gauge. 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, following your manual.
  3. Check that the flue outlet on the outside wall is clear of leaves, debris or nests, and that the condensate pipe is not frozen or blocked.
  4. Press the reset button once (or switch the boiler off and on at the mains). If the fault clears and returns, or the boiler won't restart, stop and call a Gas Safe registered engineer. Do not open the casing or work on the burner, gas valve or electrodes yourself.

Parts commonly replaced

  • Ionisation (flame sensing) electrode
  • Ignition electrode
  • Gas valve
  • Control PCB
  • Gas filter

Related codes

Full explanation for F4
F8

The boiler's gas shut-off valve isn't closing quickly enough when the boiler stops, so it has shut down as a safety precaution and needs a qualified engineer to check it before it's used again.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve assembly
  • Gas valve seals and O-rings
  • Control PCB (main circuit board)

Related codes

Full explanation for F8
Showing 21–25 of 25
F9

Your boiler's fan isn't spinning up to the speed it needs before it lights, so it has stopped as a safety precaution and will try again only after a reset.

High Engineer only £200-500

What to check first

  1. Switch the boiler off at the isolator or fused spur for 10 minutes, then switch it back on and press the reset button once.
  2. Check that the flue outlet on the outside wall is not blocked by leaves, nests, snow or debris, and clear any blockage you can safely reach.
  3. Check that the condensate pipe is not frozen or blocked, and if it is, thaw it gently with warm water.
  4. If the boiler still shows F9 after one reset, stop trying and call a Gas Safe registered engineer.

Parts commonly replaced

  • Combustion fan assembly
  • Fan speed sensor (Hall sensor)
  • Main control PCB

Related codes

Full explanation for F9
FA

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

High Engineer only £180-450

What to check first

  1. Turn the boiler off at its on/off switch and wait about 10 minutes, then switch it back on to see if the fault clears.
  2. Check the flue terminal outside the property for blockages such as leaves, nests or debris, and clear any you can safely reach.
  3. Check the boiler pressure gauge reads between 1 and 1.5 bar when cold, topping up only if the manufacturer's filling instructions are followed.
  4. If the fault returns, stop resetting it and contact a Gas Safe registered engineer.

Parts commonly replaced

  • Fan assembly
  • Fan speed sensor
  • Main control PCB

Related codes

Full explanation for FA
FC

Your boiler has shut itself down because its fan, which helps it burn gas safely, is not working correctly, so it needs a qualified engineer to check it.

High Engineer only £250-450

What to check first

  1. Check the boiler display for the full fault code and note any other messages.
  2. Switch the boiler off at its on/off switch and at the fused spur, wait 10 minutes, then switch it back on once.
  3. Check that the flue terminal outside the property is not blocked by leaves, nests, snow or debris, and clear it if it is safe to reach.
  4. If the fault returns, stop and call a Gas Safe registered engineer. Do not open the boiler casing.

Parts commonly replaced

  • Combustion fan assembly
  • Fan power and signal cable or connector
  • Main control PCB
  • Flue gas sensor or pressure switch

Related codes

Full explanation for FC
Fd

Your boiler has shut itself down for safety because its burner control has detected a fault, so it will need a qualified engineer to find and fix the cause before it can run normally again.

High Engineer only £150-400

What to check first

  1. Check the boiler pressure gauge and top it up to between 1.0 and 1.5 bar if it is low, using the filling loop as shown in the manual.
  2. Make sure the gas supply tap to the boiler is fully open and that other gas appliances are working.
  3. Turn the boiler off at its power switch, wait 30 seconds, turn it back on, and press the reset button once if the display allows it.
  4. If the fault returns or the boiler will not reset, stop and contact a Gas Safe registered engineer.

Parts commonly replaced

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

Related codes

Full explanation for Fd
FF

Your boiler has shut itself down because its burner is not lighting or staying lit safely, so it needs a qualified engineer to check the gas and ignition system before it will restart properly.

High Engineer only £120-350

What to check first

  1. Check the boiler pressure gauge; it should read between 1 and 2 bar when cold. If it is low, top it up using the filling loop as shown in the manual.
  2. Press the reset button on the front of the boiler once, wait about 10 minutes, and see whether the heating and hot water come back on.
  3. Make sure the gas supply is on and that other gas appliances, such as the hob, are working. If they are not, contact your gas supplier rather than the boiler engineer.
  4. Check that the condensate pipe outside is not frozen or blocked, as this can cause the boiler to lock out in cold weather.
  5. If the fault code returns after one reset, stop trying and book a Gas Safe registered engineer. Repeated resets can cause further damage.

Parts commonly replaced

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

Related codes

Full explanation for FF