Safety first: Disconnect power and gas. Risk of flue gas spillage if airflow is inadequate.

1. Inspect condensate system: Check the condensate trap and drain hose for blockage. A full or blocked trap can restrict flue gas flow. Clean the trap and ensure the drain is free-flowing.
2. Check flue for restriction: Inspect the entire visible length of the flue for sagging, disconnections, or crushing. Ensure terminal is clear.
3. Fan cleaning: Remove the fan as per Junkers service instructions. Clean the impeller blades thoroughly. Dust buildup can reduce airflow and speed.
4. Mechanical check: Spin the fan shaft by hand. If there is resistance, noise, or wobble, replace the fan assembly.
5. Electrical verification: Confirm that the fan receives the correct supply voltage and that the feedback/speed signal wire is intact. If the fan runs but the PCB does not detect correct speed, replace the fan or PCB as recommended by Junkers.
6. Professional testing: Airflow and combustion checks should be performed by a qualified technician using proper instruments.

Error C6, furnace attempts to start but shuts down with fan-related fault

Possible Causes