Safety: Disconnect power and ensure no hot flue gases are present before working on the exhaust path.

Remove the rear or side panel to access the exhaust (induced draft) fan.
Check for heavy ash buildup in the fan housing and flue connection. Clean with a brush and vacuum.
Spin the fan impeller by hand. It should rotate freely without grinding. If stiff or seized, the motor may be defective.
Inspect the wiring harness from the fan to the control board for loose plugs or heat damage.
Check the flue/chimney for obstructions (bird nests, soot blockage). Only inspect from safe access points; do not climb without proper equipment.
Restore power and start the furnace while observing the fan. If it does not start or starts then stops with E04, the fan motor or speed feedback sensor is likely faulty.

Technician recommended: Replacement of the exhaust fan assembly or control board should be done by an authorized Defro service center.

E04 displayed; furnace does not start fan or ignition sequence

Possible Causes