Vaillant Boiler Fault Codes

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Vaillant Boiler Fault Codes

When a Vaillant boiler displays a code beginning with the letter F, then it means there is a fault. Knowing what each fault code means can help you to diagnose and solve the problem.

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Common Vaillant Boiler Fault Codes


eBUS Fault

The eBUS may have short circuited, the eBUS could be overloaded or two power supplies with different polarities are using the eBUS.


Mass Flow Sensor Connection Fault

Either the mass flow sensor has become disconnected, the plug is not connected correctly or there’s an issue with the mass flow sensor operation.


Mass Flow Sensor Fault

Either the mass flow sensor is faulty, the filter below the venturi filter cap is wet or blocked, or the gas flow pressure is too low.


Gas Pressure Fault (also applicable to F.28/F.29)

No gas or not enough gas supply pressure, or the gas valve is closed.


Mass Flow Sensor Regulation Fault

The gas valve is defective, or the cable harness to the gas valve is defective.


Comfort Safety Mode Fault

The ignition electrode is corroded.


Gas Valve Regulation Fault

This could be because of a short circuit/short to earth in cable harness for the gas valve or a defective gas valve, such as the coils shorted to earth. It could also signify defective electronics.


Gas Valve Switch Off Delay

There could be a delay in the gas valve shutdown, a delay in extinguishing the flame signal or a gas valve leak.



There are defective electronics.


Electronics/NTC Fault

The supply or return NTC short circuited, or there are defective electronics.


Electronic Temperature Fault

The electronics are overheating due to external influences, defective electronics.


Electronics or Flame Fault

There’s an implausible flame signal, defective electronics.


Unstable Flame Signal Fault

There could be air in the gas, the gas flow pressure could be too too low or the wrong air ratio may be present. It could also be linked to the condensate route being blocked, the wrong gas restrictor being used, the ionisation flow being interrupted, poor flue gas recirculation or a faulty condensate route.


Invalid Device Specific Number (DSN)

The display and PCB may have been replaced at same time and the DSN not set, set incorrectly or it could be missing an output range coding resistor.


Flow NTC Fault

The flow temperature sensor is signalling constant value, either due to it being incorrectly positioned at the supply pipe or it being defective.


Flow/Return NTC Faults

The temperature difference between the flow and return NTC is too great, or the flow and/or return temperature sensor defective.


Water Pressure Sensor Signal is in the Wrong Range

There’s been an interruption or short circuit of the water pressure sensor, there’s been an interruption or short circuit to GND in the supply line to the water pressure sensor, or the water pressure sensor is somewhat defective.


Water Pressure Sensor Signal Outside Correct Range

Either the cable to the water pressure sensor has short circuited at 5V/24 V or there’s an internal fault in the water pressure sensor.


No Pressure Change Detection When Starting Pump

The water pressure sensor or the pump is defective, there’s air in the heating installation or the water pressure in the boiler is too low. This can be resolved by checking the adjustable bypass and connecting the external expansion vessel to return.


Overheating Protection on Primary Heat Exchanger Has Responded

The cable or cable connections for the safety fuse in the primary heat exchanger are defective, the or primary heat exchanger is defective.


Flue Non-Return Flap or Condensate Pump Fault

Either the cable to the water pressure sensor has short circuited at 5V/24 V or there’s an internal fault in the water pressure sensor.


Interruption to DHW Outlet Sensor at the External Controller

The UK link box is connected but the hot water NTC is not bridged.


Flow or Return Temperature Sensor Temperature Change Fault

When the burner starts, the temperature change registered by the flow or return temperature sensor is non-existent or it’s too small, or there’s not enough water in the boiler. The flow or return temperature sensor may not be in the correct position.


Flow or Return Temperature Sensor Difference Implausible

The flow and return temperature sensors return implausible values, the flow and return temperature sensors have been inverted or the flow and return temperature sensors have not been fitted correctly.


Flow and Return Temperature Sensors Incorrectly Fitted

The flow or return temperature sensors have been fitted to the same pipe or wrong pipe.


Coding Resistor Fault

The coding resistor on the PCB does not match the entered gas family. This can be resolved by checking the resistor, repeating the gas family check and entering the correct gas family.


Gas Group Fault

The combustion quality is outside of the permitted range. This could be because of a wrong gas restrictor, recirculation issues, the wrong gas family, or the internal pressure measuring point in venturi is blocked. A gas family check is prematurely terminated.

Are you a landlord looking for a plumber?

If you’re a landlord and you need experienced and swift action taken to replace an old cranky boiler, please don’t hesitate to give us a ring on 01908 034194. We’ll work quickly to get the problem resolved and keep your tenants happy.

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Areas we cover

  • Aylesbury
  • Bedford
  • Leighton Buzzard
  • Northampton
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  • Towcester
  • Woburn Sands
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