Check line voltage to the furnace

What is going on

Confirm the supply voltage at the junction box and at the blower while it runs.

Likely causes

• No reading can mean open wiring, an open fuse, no power, or a faulty door interlock switch.

Check in this order

1. Turn power off, remove the burner door, and remove the junction box cover.
2. Turn power on with the door interlock switch engaged. Measure voltage across hot and neutral with a voltmeter.
3. If there is no reading, check wiring, the fuse, power supply and the door interlock switch.
4. Jumper R to G on the integrated ignition control to run the blower.
5. With the blower running, voltage should be 115 V within plus or minus 10%.
6. If voltage is low, check line wire size. Undersized long runs can cause low voltage. If wire size is adequate, notify the power company.
7. Reinstall the junction box cover and service doors, then restore power 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)