Check thermostat and low voltage wiring

What is going on

Decides whether a no-heat or no-cool problem is in the thermostat or the wiring, or in the furnace.

Check in this order

1. Remove the blower compartment door and disconnect the thermostat low voltage wires at the furnace control terminals.
2. Jumper R to W (or W1 and W2 on two-stage models) on the integrated ignition control.
3. With power on and the door interlock closed, the induced draft motor must run and pull in the pressure switch.
4. If the hot surface ignitor heats and the gas valve opens and the burners light, the fault is in the thermostat or wiring.
5. With power off, check continuity of the thermostat and wiring. Repair or replace as needed.
6. To check cooling mode, with power off jumper R to Y (or Y1/Y2) to G, then turn power on.
7. If the blower starts and the condensing unit runs, the fault is in the thermostat or wiring.
8. Reinstall the blower compartment door and verify operation.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)