Furnace starts but shuts down immediately when the blower door is installed; runs with door removed (defeating switch).

Safety: Never operate the furnace with the blower door removed or the door switch bypassed except briefly for testing by a qualified person.

Steps:

• Inspect door switch: Check that the door switch actuator is properly aligned with the door. Replace the switch if it is loose, broken, or intermittent.
• Check cabinet fit: Ensure the blower door is fully seated and latched. Bent panels may need adjustment.
• Airflow: If the furnace only runs with the door off, there may be severe return air restriction causing negative pressure in the cabinet. Check filter, returns, and duct sizing.