Viessmann F4 Fault

Viessmann F4 Fault

The Viessmann F4 fault code typically indicates a flame signal issue after the gas has been shut off. This means the boiler’s control system has detected a flame presence when there should be none, which could suggest a problem with the flame detection system or a lingering flame due to a gas valve leak or delayed shutdown.

Causes of the F4 Fault Code

  1. Faulty Flame Detector: The flame detector may be malfunctioning, incorrectly sensing a flame even when the burner is off. This could be due to dirt, damage, or a technical fault in the detector itself.
  2. Delayed Gas Valve Closure: If the gas valve doesn’t close immediately after the burner shuts off, gas may continue to flow briefly, causing a flame presence that triggers the F4 code.
  3. Leaking Gas Valve: A leaking gas valve can allow gas to pass through even when it’s supposed to be closed, potentially leading to a flame presence after shutdown.
  4. Control Unit Issues: Problems with the boiler’s control unit could result in incorrect signals being sent or received, including false flame detection.

Troubleshooting and Resolving the F4 Fault Code

  1. Inspect and Clean the Flame Detector: Start by cleaning the flame detector to ensure it’s not giving a false reading due to dirt or debris. If the detector is damaged or faulty, it will need to be replaced.
  2. Check the Gas Valve: Test the gas valve for immediate closure upon shutdown and for any signs of leakage. This task should be performed by a qualified technician due to the risks involved with handling gas components.
  3. Reset the Boiler: After any corrective action, reset the boiler to see if the F4 fault clears. This can often recalibrate the system and clear temporary errors.
  4. Consult a Professional: Given the potential complexity and safety concerns of the issues indicated by an F4 fault, it’s advisable to consult a Gas Safe registered engineer. They can accurately diagnose the problem, whether it’s with the flame detector, gas valve, or control unit, and undertake the necessary repairs or adjustments.
  5. Regular Maintenance: Prevent future occurrences of the F4 fault through regular professional maintenance. This ensures all components, especially those related to gas flow and flame detection, are in good working condition.


The F4 fault code on a Viessmann boiler indicates a problem that requires attention to ensure the safety and efficiency of your heating system. Due to the nature of the issues related to this fault code, professional diagnosis and repair are recommended. Regular servicing can help prevent such faults by keeping the boiler and its components in optimal condition.