Furnace indicates air pressure switch error; combustion fan runs but burner does not start

Safety: Turn off power and gas before opening the combustion compartment.

• Inspect pressure tubes: Locate the silicone tubes from the combustion fan housing to the IVT air pressure switch. Remove and check for water, debris, or splits. Clear or replace as needed.
• Check flue and intake: Ensure the flue and air intake are free of obstructions and properly connected.
• Test switch operation: With power off, disconnect the wires and use a multimeter to check continuity across the switch terminals while gently applying suction or pressure to the port. The contacts should change state. If not, replace the switch.
• Reassemble and test: Reconnect tubing and wiring, restore power, and call for heat. If the error persists, the fan or control board may not be generating or reading the correct pressure.

Do not bypass the air pressure switch; it is a critical safety device.