Furnace shows E8 and will not start, fan may run briefly then stop

Safety first: Isolate power and gas. Flue work must comply with gas safety regulations.

• Inspect the flue terminal outside for obstructions (debris, nests, ice). Clear any blockage.
• Check the flue gas temperature sensor in the flue hood or outlet for secure mounting and intact wiring.
• Reseat the sensor connector on the PCB and inspect for corrosion.
• If the sensor body is cracked or sooted heavily, replace it with a genuine ATAG flue gas sensor.
• If E8 persists with a new sensor, a combustion or flue design issue may be causing genuine overtemperature; a certified technician must perform a combustion analysis and flue inspection.