Dettson furnace will not start at all; no lights, no fan, completely dead.

Safety first: Turn off the breaker before opening any electrical compartments.

• 1. Check breaker and service switch: Reset the furnace breaker and ensure the furnace disconnect or service switch is ON.
• 2. Inspect control board fuse: Remove the blower door and locate the low-voltage fuse (often 3–5A automotive style) on the control board. Replace if blown, but investigate the cause (shorted thermostat wire, etc.).
• 3. Verify line voltage: With a multimeter (if qualified), check for 120 VAC at the furnace junction box. If absent, contact an electrician.
• 4. Check door switch: Ensure the blower door is properly installed and engaging the door switch.