Viessmann A4 Fault

Viessmann A4 Fault

The Viessmann A4 fault code indicates a problem related to the return temperature sensor. Specifically, it signifies that the sensor is detecting a temperature that’s either too high for the system to operate safely or outside the expected range. This could be due to several reasons, affecting the efficiency and safety of your boiler’s operation.

Causes of the A4 Fault Code

  1. Incorrect System Configuration: If the heating system is not configured correctly, it could cause the return water temperature to exceed the maximum allowable limit.
  2. Faulty Return Temperature Sensor: The sensor itself might be malfunctioning, providing inaccurate temperature readings to the boiler’s control unit.
  3. Blocked or Restricted Flow: A blockage or restriction within the heating system can cause the return water temperature to increase excessively. This might be due to sludge or debris in the pipes.
  4. Pump Failure: If the circulation pump is failing or not working correctly, it can lead to inadequate water circulation, causing the return temperature to rise.
  5. Radiator Issues: Problems with radiators, such as being improperly balanced or partially closed valves, can affect the flow of water and lead to increased return temperatures.

Troubleshooting and Resolving the A4 Fault Code

  1. Check System Configuration: Ensure that the heating system is correctly configured and that all settings are appropriate for your home’s heating requirements.
  2. Inspect the Return Temperature Sensor: Verify that the sensor is functioning correctly. If you suspect it’s faulty, it should be replaced by a professional.
  3. Identify and Clear Blockages: Look for signs of blockage or restricted flow within the system. A professional heating engineer can flush the system to remove sludge or debris.
  4. Assess the Circulation Pump: Listen for any unusual noises from the pump that might indicate a problem. The pump may need to be repaired or replaced if it’s not circulating water effectively.
  5. Adjust Radiators: Make sure all radiators are fully open and correctly balanced. This ensures even water flow and distribution throughout the heating system.
  6. Consult a Professional: Given the complexity of diagnosing and fixing issues indicated by an A4 fault code, it’s advisable to seek help from a Gas Safe registered engineer. A professional can accurately diagnose the problem, whether it’s with the sensor, pump, or system configuration, and undertake the necessary repairs or adjustments.


The Viessmann A4 fault code highlights a significant issue with the return temperature sensor or related system components. Addressing this fault requires a careful examination of the heating system, sensor functionality, and water circulation. Professional assistance is recommended to ensure accurate diagnosis and effective resolution, maintaining the safety and efficiency of your boiler operation. Regular maintenance can also help prevent such issues by ensuring that all components of the heating system are in good working order.