Boiler shows A51; fan may run but burner does not ignite

Safety: Turn off gas and power. Do not operate the boiler with suspected flue or air path blockage.

• 1. Check flue terminal: Ensure the Buderus concentric flue terminal is clear of obstructions outside.
• 2. Inspect condensate system: A blocked condensate trap can flood the combustion chamber and affect the fan/APS. Clean the trap and ensure free drainage.
• 3. Air pressure switch tubes: Open the boiler front panel and locate the air pressure switch. Check the silicone tubes to the fan/combustion chamber for splits, kinks, or water. Drain any condensate and refit securely.
• 4. Test fan operation: When there is a heat demand, listen for the fan. If it does not start or is very noisy, the fan motor may be failing.
• 5. Switch test: A technician can test the air pressure switch contacts with a multimeter while the fan runs. If the switch does not change state, replace it.
• 6. Control board: If the fan and switch are good but A51 persists, the control board may not be reading the signal correctly and may need replacement.