Dettson furnace lights briefly then shuts off; control board displays E3 and retries ignition.

Safety first: Disconnect power and shut off gas before servicing. Allow components to cool.

• 1. Clean and reposition flame sensor: Remove the flame sensor and clean the metal rod with fine steel wool. Reinstall so the rod is fully engulfed by the main burner flame when lit.
• 2. Check ground connections: Verify the burner assembly and furnace cabinet have solid ground connections to the control board. Tighten any loose screws or ground lugs and clean off rust or paint that may interfere with grounding.
• 3. Inspect burners: Remove burners if accessible and check for rust, debris, or misalignment that could cause uneven flame. Clean with a soft brush and compressed air. Ensure burners are seated correctly on the manifold.
• 4. Verify gas pressure (technician recommended): If flames appear weak or lift off the burner, gas pressure may be low. This requires a manometer and should be checked by a licensed gas technician.
• 5. Test operation: Restore power and gas, call for heat, and observe several cycles. If E3 persists after cleaning and securing grounds, the control board may have a defective flame sensing circuit and should be replaced with the correct Dettson board.