Safety: Isolate power and gas. Do not operate the furnace with suspected flue or fan faults; risk of carbon monoxide.

User checks:

Visually inspect the flue terminal outside for obstructions (leaves, nests, snow, plastic bags). Remove any obvious debris without dismantling the flue.
Ensure any air intake grills or vents are not blocked by furniture or stored items.

Technician checks:

Confirm that the fan receives power from the PCB during start-up. If powered but not running, replace the Glow-worm compatible fan assembly.
Inspect and test the air pressure switch tubing for splits, kinks, or blockages; replace tubing if degraded.
Test the air pressure switch contacts with a multimeter to ensure they change state when the fan runs.
Check the flue system for correct installation, slope, and internal blockage; rectify any installation defects.
If fan and pressure switch test OK, check the PCB fan relay and replace the control board if faulty.

Furnace displays F4 and will not start, fan does not run

Possible Causes