Furnace indicates open door or cabinet switch; unit will not run.

Safety first: Power off before adjusting cabinet panels.

Steps:

• Inspect the blower door panel and ensure it is properly seated in the lower and upper cabinet channels.
• Locate the CTC blower door safety switch, usually near the control board. Confirm that the door fully depresses the switch plunger when closed.
• Adjust the door panel or switch mounting screws slightly to ensure positive contact.
• Check wiring to the door switch for loose or broken connections.
• If the switch does not show continuity when manually depressed (tested with power OFF and a multimeter), replace it with the correct CTC door interlock switch.

Note: Never bypass the door switch for normal operation; it is a required safety device.