Furnace will not start; thermostat calling for heat but no inducer or ignitor activity; E200 displayed on Lennox control board or thermostat.

Safety first: Turn off power to the furnace at the service switch and breaker before removing panels. Do not work inside the furnace with power on.

• 1. Verify power supply: Confirm the furnace service switch is ON and the breaker is not tripped. Reset the breaker once if needed.
• 2. Check the low-voltage fuse: Remove the blower door, locate the integrated furnace control (IFC) board, and inspect the 3–5A blade fuse. If blown, replace with the same rating only. If it blows again, there is a short in the low-voltage circuit.
• 3. Inspect thermostat wiring: Look for damaged or pinched thermostat wires between R, C, W, and Y terminals on the Lennox board and at the thermostat. Repair or replace any damaged cable and ensure no bare copper is touching.
• 4. Check 24V transformer output: With power restored and extreme caution, a qualified person can measure 24VAC between R and C at the board. If no 24V is present but primary power is good, the transformer may be faulty.
• 5. Reset the control: After correcting issues, power the furnace OFF for 30 seconds, then back ON to clear the E200 lockout. Observe a full heat cycle.

Call a professional if the fuse repeatedly blows or if you are not comfortable testing live voltage.