Viessmann A2 Fault

Viessmann A2 Fault

The Viessmann A2 fault code typically indicates an issue with the flue gas or exhaust system. This error can arise when the boiler detects an abnormal temperature in the flue gases, suggesting that the exhaust system is not functioning correctly. The fault may be due to several reasons, including blockages in the flue, improper flue installation, or a malfunctioning flue gas sensor. Understanding the causes and appropriate steps for resolution is crucial for ensuring the safe operation of the boiler.

Causes of the A2 Fault Code

  1. Flue Gas Sensor Fault: A malfunctioning or incorrectly positioned flue gas sensor can give false readings, leading to the A2 fault code.
  2. Blockage in the Flue System: Obstructions within the flue, such as bird nests, debris, or excessive soot buildup, can restrict the flow of exhaust gases, causing the boiler to register a fault.
  3. Improper Flue Installation: Incorrect installation or configuration of the flue system can affect the proper venting of exhaust gases, leading to abnormal temperatures and triggering the A2 code.
  4. Condensate Blockage: In condensing boilers, a blockage in the condensate pipe can lead to improper disposal of condensation, affecting flue gas temperatures.

Troubleshooting and Resolving the A2 Fault Code

  1. Inspect the Flue Gas Sensor: Check the flue gas sensor for any signs of damage or incorrect placement. If the sensor is faulty, replacing it may resolve the issue.
  2. Check the Flue for Blockages: Inspect the flue for any obstructions that could restrict the flow of exhaust gases. Removing blockages should help restore normal operation.
  3. Evaluate Flue Installation: Ensure that the flue system is installed correctly according to the manufacturer’s guidelines and local regulations. Any issues with the flue installation may require adjustment or correction by a professional.
  4. Clear Condensate Pipe Blockages: For condensing boilers, check the condensate pipe for freezing or blockages. Thawing a frozen condensate pipe or clearing blockages can help eliminate the fault.
  5. Consult a Professional: Given the potential safety risks associated with flue and exhaust system issues, it’s advisable to consult a Gas Safe registered engineer to diagnose and fix the A2 fault. A professional can safely assess the flue system, sensor, and other related components to ensure proper operation.
  6. Regular Maintenance: Prevent future occurrences of the A2 fault through regular professional maintenance. Annual servicing by a qualified technician can help identify and address issues with the flue system, sensors, and other critical components before they lead to faults.


The A2 fault code on a Viessmann boiler is a serious indication of an issue with the flue gas or exhaust system that requires immediate attention. Homeowners can perform basic checks and troubleshooting, but due to the complexity and safety implications of flue and exhaust system problems, professional diagnosis and repair are strongly recommended. Regular boiler maintenance is crucial for detecting potential issues early and ensuring the safe and efficient operation of your heating system.