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The right solution for your boiler fault code

In this guide, we’re here to tell you exactly what Vaillant error codes mean, and the steps needed to take to fix them.
Woman with back to camera in laundry room with ecoTEC plus boiler on wall

If your boiler displays a fault code, it isn’t always something to worry about. It could be a simple fix that you can do yourself or it may require the knowledge of an expert.

Often resetting your boiler can resolve a fault or temporarily get your boiler working again.

Find your boiler manual here which will help you to reset your boiler.

We strongly advise that you do not gain access inside your boiler as this can be dangerous. It’s important that if you smell gas, you need to vacate your property immediately and call the National Gas Emergency Service number on 0800 111 999.

OVERVIEW OF THE VAILLANT FAULT CODES LIST

What’s The Problem?

  • F.0

    The flow temperature sensor has been interrupted

    Possible cause:

    • Fault in the electrical connection for the flow temperature sensor
    • Flow temperature sensor defective

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Click here find your user manual which will help you to reset your boiler.

  • F.1

    The return temperature sensor has been interrupted

    Possible cause:

    • Fault in the electrical connection for the return temperature sensor
    • Return temperature sensor defective

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.
      Often resetting your boiler can resolve a fault or temporarily get your boiler working again.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Click here to find your user manual which will help you to reset your boiler.

  • F.10

    The flow temperature sensor has short-circuited

    Possible cause:

    • Fault in the electrical connection for the flow temperature sensor
    • Flow temperature sensor defective

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Click here to find your user manual which will help you to reset your boiler.

  • F.11

    The return temperature sensor has short-circuited

    Possible cause:

    • Fault in the electrical connection for the return temperature sensor.
    • Return temperature sensor defective.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.12

    F.12 The domestic hot water connection temperature sensor has short-circuited

    Possible cause:

    • Fault in the electrical connection for the domestic hot water temperature sensor.
    • Domestic hot water temperature sensor defective.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.13

    The cylinder temperature sensor has short-circuited

    Possible cause:

    • Fault in the electrical connection for the cylinder temperature sensor
    • Cylinder temperature sensor defective

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.20

    The safety thermal cut out has operated shutting the boiler off

    Possible cause:

    • System water pressure below normal range of 1 to 1.5 bar
    • Fault in the electrical connection for the flow and/or return temperature sensor
    • Flow and/or return temperature sensor defective
    • Circulation fault, pump, valve, blockage
    • Water pressure sensor defective
    • Volume flow sensor defectiveIgnition cable, ignition plug or ignition electrode

    How to fix it:

    • Check the water pressure and refill the system if needed.
    • Look for leaks and check the system for any issues.
    • Ensure radiator valves are open, and external pumps and zone valves are working correctly.
    • Make sure there are no airlocks in the system.
    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.

    To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.22

    Low water pressure in the system

    Possible cause:

    • The water pressure in your system is below the normal range of 1 to 1.5 bar
    • A problem with the water pressure sensor
    • Loose, disconnected, or faulty wiring for the pump or pressure sensor
    • Water pressure sensor faulty
    • The pump may not be working properly (Open vented boilers only).
    • Internal expansion vessel faulty

    How to fix it:

    If your system water pressure is showing less than 1 bar:

    This can be solved without an engineer if you are competent to do so. The following guidance and resources are used at your own risk, Vaillant accepts no liability as a result.

    If you need further support, please contact our homeowner technical support line on 0344 736 0049.

    If your system water pressure is showing more than 1 bar:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.23

    The temperature spread between the flow/return is too great

    Possible cause:

    • Air in the product
    • Fault in the electrical connection for the flow and/or return temperature sensor.
    • Flow and/or return temperature sensor defective.
    • Circulation fault, pump, valve, blockage.
    • Internal expansion vessel defective.

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.24

    The temperature increases too quickly

    Possible cause:

    • Air in the product
    • System pressure too low
    • Fault in the electrical connection for the flow and/or return temperature sensor
    • Flow and/or return temperature sensor defective
    • Circulation fault, pump, valve, blockage
    • Internal expansion vessel defective

    How to fix it:

    • If you have an open vented boiler this is likely a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.
    • If you have a system or combination boiler this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.25

    The flue gas temperature is too high

    Possible cause:

    • Circulation fault, pump, valve, blockage
    • Wiring harness defective
    • Flue gas temperature monitor has triggered

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.27

    A flame signal has been detected while the burner is off

    Possible cause:

    • Gas solenoid valve not leaktight
    • PCB defective

    How to fix it:

    • This is a case for an expert. Don’t try to solve it yourself as it can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler

  • F.28

    The flame signal was not detected during the ignition phase.

    Possible cause:

    • Not enough gas reaching the boiler (for example, low gas pressure or insufficient credit on a pre-payment meter.
    • Condensate has backed up in the combustion chamber due to a blocked condensate pipe.
    • The ignition electrode may be faulty.
    • The PCB (circuit board) could be faulty.
    • There may be an issue with the flue system.

    How to fix it:

    • Check if other gas appliances in your home are working.
    • If you have a pre-payment gas meter, ensure you have enough credit and top up if needed.
    • During cold weather, check for ice blockages in any outdoor condensate pipes.

    Watch the video to learn how to defrost your frozen condensate pipe.

    • Outside of the above, this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.29

    The ignition following a flame loss during operation was unsuccessful.

    Possible cause:

    • The gas supply is interrupted, insufficient gas pressure.
    • Fault in the flue gas route caused by recirculation or flue gas blockage.
    • Poor electrical earthing.
    • Condensate discharge pipe blocked.
    • Control electrode got contact with the burner.

    How to fix it:

    • Check if other gas appliances in your home are working.
    • If you have a pre-payment gas meter, ensure you have enough credit and top up if needed.
    • During cold weather, check for ice blockages in any outdoor condensate pipes.

    Watch the video to learn how to defrost your frozen condensate pipe.

    • Outside of the above, this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.32

    Fan fault

    Possible cause:

    • Fan defective.
    • Plug or harness defective.
    • PCB issue

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.35

    The air/ flue pipe is blocked

    Possible cause

    • Gas flow pressure too low.
    • Fault in the flue gas route caused by recirculation or flue gas blockage.
    • Condensate discharge pipe blocked.
    • Insufficient combustion air supply.
    • Control electrode defective.

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.52

    Mass flow sensor fault

    Possible cause:

    • The mass flow sensor/Venturi is not connected electrically.
    • The plug is defective/not plugged in.
    • The slot is defective (loose connection).
    • Mass flow sensor/Venturi defective.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.53

    Combustion regulation fault

    Possible cause:

    • The combustion regulation has detected a fault.
    • Gas pressure too low.
    • Incorrect coding resistor used when operating with natural gas.
    • Gas valve defective.
    • Filter below venturi blocked.
    • Mass flow sensor defective.

    How to fix it:

    • Check there is gas to other appliances in the property.
    • Check there is sufficient credit in the case of a pre-payment type gas meter.
    • In severe weather conditions, check for ice blockages in any externally ran condensate pipework.

    Watch the video to learn how to defrost your frozen condensate pipe.

    • Outside of the above, this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.54

    Fault in the gas supply (in combination with F.28/F.29)

    Possible cause:

    • There is insufficient gas supply to operate the unit.

    • Gas isolator cock(s) closed.

    • Gas flow pressure is too low.

    • Gas valve defective

    How to fix it:

    • Check there is gas to other appliances in the property.

    • Check there is sufficient credit in the case of a pre-payment type gas meter.

    • In severe weather conditions, check for ice blockages in any externally ran condensate pipework.

    Watch the video to learn how to defrost your frozen condensate pipe.

    • Outside of the above, this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.57

    The boiler is not burning correctly in comfort mode

    Possible cause:

    • Fault in the flue, recirculation of flue products.

    • Condensate discharge pipe blocked.

    • Gas pressure too lowCable harness damaged or defective.

    • PCB defective.

    • Fan defective.

    How to fix it:

    • Check there is gas to other appliances in the property.

    • Check there is sufficient credit in the case of a pre-payment type gas meter.
    • In severe weather conditions, check for ice blockages in any externally ran condensate pipework.

    Watch the video to learn how to defrost your frozen condensate pipe.

    • Outside of the above, this is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.62

    The flame switch-off was detected after a delay.

    Possible cause:

    • Gas valve defective.
    • PCB defective.
    • Ignition electrode defective.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.70

    The Device Specific Number (DSN) is incorrect, missing or does not match the coding resistor

    Possible cause:

    • Device Specific Number not set / is incorrect.

    • Interruption in the cable harness.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.72

    The temperature spread between the flow and return temperature sensor is invalid

    Possible cause:

    • Flow and/or return temperature sensor defective.
    • Circulation fault, pump, valve, blockage.

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.75

    The pressure change is too low when the heat pump is started up

    Possible cause:

    • System pressure below normal range of 1 to 1.5 bar
    • Air in the product/heating system
    • Water pressure sensor defective
    • Internal heating pump defective
    • Open circuit in cable harness

    How to fix it:

    If your system water pressure is showing less than 1 bar:

    This can be solved without an engineer if you are competent to do so. The following guidance and resources are used at your own risk, Vaillant accepts no liability as a result.

    If you need further support, please contact our homeowner technical support line on 0344 736 0049.

    If your system water pressure is showing more than 1 bar:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.
    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.83

    When the burner starts, little or no temperature increase is registered at the flow or return temperature sensor

    Possible cause:

    • System pressure too low.

    • Circulation fault, pump, valve, blockage.

    • Flow/return temperature sensor fault.

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.
    • To solve it please contact your heating installer or find your nearest installer.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.84

    The temperature differences between the flow and return temperature sensors return implausible values

    Possible cause:

    • Flow and return pipe work crossed.

    • Circulation fault, pump, valve, blockage.

    • Flow and return temperature sensors incorrectly fitted or on the wrong pipe.

    How to fix it:

    • This is a case for a Vaillant installer in the first instance, as it can relate to how the boiler is working with the heating system.

    Vaillant will attend to any fault code, however we reserve the right to charge a call out fee for non-product faults.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

  • F.88

    The electrical connection to the gas valve has been interrupted

    Possible cause:

    • Gas valve assembly connected incorrectly.

    • Short circuit in the cable harness.

    How to fix it:

    • This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

    • To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

    How to fix it:

      This is a case for a Vaillant engineer. Don’t try to solve it yourself as opening the boiler casing can be dangerous.

      To solve it please contact our customer service team on 0330 100 3143 or visit myVaillant Web to arrange a visit from a Vaillant engineer.

    Often resetting your boiler can resolve a fault or temporarily get your boiler working again. Find your user manual which will help you to reset your boiler.

If your fault code isn’t listed above please contact 0344 736 0049 for further assistance

HOW TO HANDLE COMMON FAULT CODES Play it safe: Call an expert

Many faults require a Vaillant engineer or Vaillant installer. They will have the expert knowledge to diagnose the issue and hopefully be able to clear the fault code.

installer carrying out work on boiler

Two ways for a quick solution

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VAILLANT SPECIALISTS
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Enjoy the comfort of always being on the safe side

Learn more about Vaillant services

Always stay on top of boiler maintenance

  1. Book a boiler repair via myVaillant Web

  2. Modernise your home with a heat pump

  3. Servicing your boiler - What to know