Furnace locks out with E03, combustion fan may start but burner does not ignite, or unit stops during operation.

Safety: Turn off power and gas. Flue gas issues can lead to carbon monoxide hazards.

• Inspect flue and air intake: Go outside and check the Termet concentric flue terminal. Remove any ice, leaves, or obstructions. Ensure the terminal clearances meet local codes.
• Check condensate trap: On condensing Termet furnaces, a blocked condensate trap can flood the flue path. Remove the trap, flush it with water, and ensure the drain hose is not kinked.
• Inspect pressure switch tubing: Open the combustion compartment. Locate the air pressure switch and its silicone tubes. Ensure they are firmly connected, not cracked, and free of water. Blow gently through the tubes to clear any blockage.
• Test the pressure switch (basic): With power off, disconnect the wires and check continuity across the switch while gently blowing into the pressure port (for normally open types). If it does not click or change state, it may be faulty.
• Check flue fan: Spin the flue fan (exhaust fan) impeller by hand. It should rotate freely without scraping. Clean dust from the blades.

Call a technician if: E03 persists after clearing obstructions and tubing. A Termet-certified technician should verify fan speed, pressure switch calibration, and combustion safety.