Viessmann A3 Fault

Viessmann A3 Fault

The Viessmann A3 fault code indicates a problem related to the boiler’s water pressure sensor or the actual water pressure within the system. This fault is typically triggered when the sensor detects water pressure that is either too high or too low, which can affect the boiler’s operation and potentially lead to safety issues. Understanding the causes and appropriate steps for resolution is crucial for maintaining the efficiency and safety of your heating system.

Causes of the A3 Fault Code

  1. Low Water Pressure: The system’s water pressure may drop below the minimum required level, often due to leaks in the heating system, recently bled radiators, or natural pressure loss over time.
  2. High Water Pressure: Excessively high water pressure can also trigger the A3 fault. This might occur due to an issue with the filling loop or expansion vessel.
  3. Faulty Pressure Sensor: A malfunctioning water pressure sensor can provide inaccurate readings, leading to the A3 fault code even if the actual pressure levels are within the normal range.
  4. Air in the System: Air trapped in the heating system can affect water pressure readings and overall system pressure.

Troubleshooting and Resolving the A3 Fault Code

  1. Check Water Pressure: Examine the boiler’s pressure gauge to determine if the pressure is too low or too high. The normal operating pressure for most residential boilers is typically between 1.0 and 1.5 bar.
  2. Repressurize the System: If the water pressure is low, use the boiler’s filling loop to repressurize the system according to the manufacturer’s instructions. Be cautious not to overpressurize the system.
  3. Release Excess Pressure: If the pressure is too high, you may need to release some water from the system via a radiator bleed valve to bring it back to the normal range. Proceed carefully to avoid sudden drops in pressure.
  4. Bleed Radiators: To remove trapped air that might be affecting pressure readings, bleed the radiators throughout your home. This can help stabilize the pressure.
  5. Inspect for Leaks: Check the heating system for any signs of leaks, which could be responsible for a loss of pressure. If leaks are found, they should be repaired promptly.
  6. Check the Pressure Sensor: If the pressure appears normal and no leaks are present, the issue may lie with the pressure sensor itself. Sensor inspection and replacement should be conducted by a professional.
  7. Consult a Professional: Given the variety of potential causes behind the A3 fault code, consulting a Gas Safe registered engineer is advisable for a comprehensive diagnosis and appropriate repair, especially if you’re unable to resolve the issue through basic troubleshooting.


The A3 fault code on a Viessmann boiler signals an issue with the system’s water pressure that requires attention. Proper diagnosis and resolution are essential for ensuring the boiler operates safely and efficiently. Regular maintenance and prompt attention to changes in system pressure can help prevent the recurrence of such faults.