Safety first: Switch off the boiler and allow it to cool. Do not operate with a suspected flue blockage.

Check flue for obstruction: Inspect the flue terminal and accessible sections for blockages, collapsed liners, or bird guards clogged with debris. Clear or have the flue repaired.
Clean heat exchanger: Remove the burner and access the primary heat exchanger. Brush and vacuum any soot or deposits from the heat transfer surfaces following Weishaupt instructions.
Inspect flue gas sensor: Locate the flue gas temperature sensor in the flue hood or outlet. Check wiring and mounting. Replace the sensor if damaged or reading incorrectly (compare resistance to spec).
Check burner settings: Excessive gas rate or low air can overheat flue gases. Have a technician verify combustion with a flue gas analyser and adjust the Weishaupt burner accordingly.

When to call a professional: Flue system repairs and combustion adjustments must be carried out by qualified personnel.

Boiler displays E09, flue gas temperature too high, burner shuts down.

Possible Causes