Dettson furnace shows E20; unit may not start or may shut down unexpectedly.

Safety first: Always ensure the blower door is properly installed before operating the furnace.

• 1. Reseat blower door: Remove and reinstall the blower compartment door, making sure it engages the door switch fully. Some doors must be slid into place then pushed firmly.
• 2. Inspect door switch: Locate the door switch near the blower door edge. Check that the plunger moves freely and is not broken.
• 3. Check wiring: Ensure the wires to the door switch are firmly connected and not damaged.
• 4. Replace faulty switch: If the switch does not click or fails continuity testing when pressed, replace it with the correct Dettson-compatible door switch.
• 5. Test operation: With the door properly installed, restore power and confirm E20 is cleared.