Everything A Boiler User Must Know About Vaillant F.6 Boiler Fault Code

For homeowners relying on Vaillant boilers for their heating and hot water, encountering a fault code can be dangerous.  Boilers are very sophisticated devices that may face any defect at any moment. One such common error is the F.6 fault code. Every boiler user must know what this fault is all about. They should also know how to fix the faulty code. This blog post explains the meaning of the F.6 fault code. It also works well to identify its potential causes and provide a guideline on how to address them.

What Does the F.6 Fault Code Mean?

The F.6 error on your Vaillant boiler indicates a problem with the flow thermistor. The flow thermistor is a sensor that monitors the temperature of the water within the heating system. If the data from the sensor is not within expected ranges—either too high or too low—your boiler will display an F.6 fault code. This usually means the boiler cannot safely operate as it cannot accurately gauge the temperature of the water circulating through the system.

Common Causes of the F.6 Fault Code

To effectively troubleshoot and fix the F.6 fault code, it's important to understand what typically causes it. Here are the most likely reasons your boiler is displaying this error:

Faulty Flow Thermistor:

The thermistor itself might be faulty or damaged, leading to incorrect temperature readings.

Wiring Issues:

Problems with the wiring connections between the thermistor and the boiler’s control board can cause incorrect readings or signal losses.

Sludge or Scale Build-up:

Over time, sludge or scale can accumulate in the boiler, affecting water flow and temperature readings.

System Water Pressure Problems:

Incorrect system water pressure can lead to abnormal boiler temperatures, which might trigger the F.6 code.

Troubleshooting the F.6 Fault Code

If your Vaillant boiler displays an F.6 fault code, here are some steps you can take to resolve it:


  • Check the Flow Thermistor: Examine the thermistor for any signs of damage or wear. If it appears to be faulty, replacing it is usually straightforward.
  • Inspect the Wiring: Look at the wiring connections to the thermistor. Make sure they are secure and not damaged. Loose or corroded connections can often cause the F.6 error.
  • Flush the System: If sludge or scale is suspected, system flushing can help. This process clears out any build-up within the pipes and the boiler, improving the overall efficiency and function.
  • Adjust the Water Pressure: Verify the water pressure in your heating system. If it's too high or too low, adjusting it to the correct level might clear the fault code.
  • Reset the Boiler: Sometimes, a simple reset can resolve the issue. Depending on your model, you can turn the boiler off and on again or press the reset button.
  • Call a Professional: If you are not comfortable performing these checks yourself, or if the above steps do not resolve the problem, it’s wise to contact a qualified heating engineer to take a look.

Preventing Future F.6 Errors

Maintaining your boiler can prevent recurring issues like the F.6 fault code. Regular maintenance checks and following a few simple tips can keep your heating system in optimal condition:

  • Annual Servicing: Have your boiler serviced by a professional every year to ensure all components are functioning correctly.
  • Regular Monitoring: Check the boiler's pressure and temperature readings regularly. This can help you spot any irregularities early.
  • Clean System: Consider scheduling periodic flushing of your heating system to remove any internal debris and prevent sludge or scale build-up.



The Vaillant F.6 fault code indicates an issue with the flow thermistor, but it can be resolved efficiently with the right approach. Understanding what causes the error and how to troubleshoot it restores your boiler’s functionality and extends its lifespan. Always ensure your boiler is maintained regularly, and don’t hesitate to seek professional help for complex issues. This proactive approach will keep your Vaillant boiler running smoothly and comfortable in your home.

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