The display shows A21; the burner shuts down, and the unit locks out after running for a short time.

Safety: Turn off power and gas. Flue components may be hot; allow them to cool. Working on combustion and venting should comply with local codes.

• Inspect the flue gas sensor: Open the front cover and locate the flue gas temperature sensor on the flue collar or near the combustion chamber outlet. Check for loose mounting or damaged wiring.
• Test sensor: Disconnect and measure its resistance. Compare to Buderus specifications. Replace the sensor if readings are open, shorted, or far out of range.
• Check venting system: From the heater to the termination, inspect for obstructions (debris, bird nests, ice), sagging sections, or disconnected joints. Correct any issues and ensure proper slope and support.
• Verify flue design: Confirm that the installed vent length, diameter, and fittings match Buderus installation requirements. Excessive length or wrong material can cause overheating.
• Inspect heat exchanger: If the flue path is clear, remove burner access and visually inspect the heat exchanger for heavy scale or soot. A dirty exchanger can raise flue temperatures. Clean according to Buderus service procedures.
• Reassemble and test: Re-fit all panels and flue components, restore gas and power, and run the heater while monitoring flue temperature and ensuring that the A21 code does not return.

If venting or combustion issues are suspected and you are not certified, contact a qualified gas technician.