Furnace not turning on: 7 checks before you call

Split the problem in two. If nothing happens at all, with no fan, no click and no light on the control board, the furnace is not getting power or the thermostat is not sending a signal. If the furnace tries to start and then gives up, power is fine and the fault is in the ignition sequence.

That difference matters when you call for service. Tell the technician which one you have.

A normal start has a rhythm you can learn. A small fan called the inducer starts first. A short while later the ignitor glows or clicks, the burners light with a soft whoosh, and the main blower comes on last. Noting where your furnace stops in that order is the most useful thing you can report.

• Thermostat. Dead batteries, the wrong mode, a schedule override, or a blank screen. See thermostat not working or blank.
• Power off. A tripped breaker, the furnace switch bumped off, or a door panel that is not seated.
• Clogged filter. Restricted airflow overheats the furnace. The limit switch shuts it down and, after repeated trips, the board can lock out.
• Condensate backup. High-efficiency furnaces make water. A blocked drain or a failed condensate pump trips a safety switch.
• Blocked vent or intake. The pressure switch will not close and the furnace stops before ignition.
• Ignition failure. The inducer runs, you may hear clicking, and no flame appears. That is covered in furnace not igniting.
• Failed internal part. A blown low-voltage fuse, a bad transformer, a failed inducer motor or a dead control board.

Many control boards clear a lockout when power is cycled. Turn the furnace switch off, wait about a minute, and turn it back on. Then let the thermostat call for heat and watch what happens.

Do this once. If the furnace locks out again, the cause is still there, and repeated resets on a furnace that fails to light can let unburned gas collect in the burner area. Leave it off and call.

The technician reads the stored fault code, then checks for power at the board and a heat call from the thermostat. From there the test follows the start-up sequence: inducer motor, pressure switch and vent, ignitor, gas valve, flame sensor, limit switches and blower. Each part can be measured with a meter, so a good diagnosis points to one failed part with a reading to back it up.

Ask to see that reading or the failed part, and get the price in writing before work starts.

A furnace that will not start is rarely a reason to replace it on its own. Most causes are small parts. Replacement enters the picture when the diagnosis is a failed heat exchanger or control board on an old furnace, or when this is one of several repairs in a short time. The furnace repair guide covers how to weigh a large quote.