Display shows F4 and heater will not fire; fan may run briefly then stop (on fan-assisted Certikin models).

Safety first: Isolate electrical power. Do not operate the heater with a known flue or fan fault; risk of flue gas spillage.

• Inspect flue terminal: Check outside that the Certikin flue terminal is clear of obstructions (leaves, plastic bags, snow, etc.).
• Check condensate drain (if applicable): On condensing models, ensure the condensate trap and drain line are not blocked or frozen.
• Examine fan area: Remove the front panel and visually inspect the combustion fan for debris, seized bearings, or signs of overheating.
• Check air pressure tubing: Verify that the silicone tubes from the fan housing to the air pressure switch are intact, not kinked, and firmly connected.
• Reset and test: Restore power and call for heat. If F4 returns, do not bypass the air pressure switch.

Further diagnosis of the fan motor, air pressure switch calibration, and flue integrity must be carried out by a qualified technician.