Furnace locks out with a flue or exhaust fault; fan may run briefly then stop.

Safety first: Switch off power and gas. Never operate the furnace with a known flue fault; risk of carbon monoxide.

• Inspect flue terminal: From outside, check the Sime concentric flue terminal for obstructions such as leaves, nests, or ice. Clear gently without damaging the terminal.
• Check air intake: Ensure the air intake path is not blocked by insulation, debris, or stored items near the furnace.
• Verify fan operation: With the casing removed and power restored, call for heat and listen for the flue fan. If it hums but does not spin, the fan motor may be seized. Turn power off again before touching any components.
• Inspect air pressure switch tubing: Locate the silicone tubes between the fan/venturi and the air pressure switch. Check for splits, kinks, or water inside. Replace damaged tubes and clear any condensate.
• Condensate drain: A blocked condensate trap can flood the fan housing. Clean the trap and ensure the drain line slopes correctly.
• If the fan does not run or the error persists: The flue fan or air pressure switch may need replacement. This should be done by a qualified Sime technician due to combustion safety checks required afterward.