Furnace Clicks But Will Not Light: What Is Actually Wrong
Cold morning, thermostat calls for heat, and you hear the furnace go through its routine and then give up. The sequence tells you a lot if you know what to listen for.
The normal ignition sequence
A modern gas furnace starts with the inducer motor, a small fan that clears the heat exchanger and proves draft. That takes about thirty seconds. Then the igniter glows, the gas valve opens, the burners light, and about thirty seconds later the main blower starts. Any failure in that chain stops the whole thing.
Cracked hot surface igniter
The most common single failure. The igniter is a brittle silicon carbide element that glows orange to light the gas. They fail from thermal cycling, usually after three to seven years, and they crack invisibly. Symptom: you hear the inducer, you hear the click of the gas valve trying, but nothing lights and after two or three attempts the furnace locks out.
If you can see the igniter with the door off and it never glows, that is your answer. It is an inexpensive part and we carry them.
Dirty flame sensor
This one has a signature you can spot from the hallway. The burners light, run for about three to five seconds, and then shut off abruptly. Then the whole cycle repeats, usually three times, before lockout.
The flame sensor is a thin metal rod sitting in the burner flame that proves fire is present by conducting a tiny current. Soot and oxidation build on it until the signal drops below threshold, at which point the control board assumes there is no flame and closes the gas valve for safety. Cleaning it takes minutes. It is one of the cheapest repairs in heating.
Pressure switch or blocked flue
If the inducer runs but the igniter never glows, the control board is not getting proof of draft. That can be a failed pressure switch, a cracked or disconnected hose, water sitting in the condensate trap on a high-efficiency furnace, or an actual blockage in the intake or exhaust pipe. Birds nesting in a sidewall vent is more common than you would think.
Gas supply and valve
Check that the gas is on and other gas appliances work. If the stove lights, supply is fine and the problem is at the furnace: the valve, its wiring or the board driving it.
What not to do
Do not bypass a safety. Pressure switches, limit switches and flame sensors exist to keep combustion gases out of the house. Jumping one to get heat tonight is how carbon monoxide incidents happen. If you smell gas or a CO alarm sounds, leave and call the gas utility first.
Getting it fixed
Igniters, flame sensors, pressure switches and control boards ride on our trucks, so most furnace repair calls finish in one visit. No-heat calls are dispatched 24 hours a day, 7 days a week at the same posted rates. You can book in any Washington city — Seattle, Spokane, Tacoma, Vancouver or Yakima — from our service areas page.
Also worth reading: why furnaces short cycle and emergency HVAC repair.
Need this fixed rather than explained? Call (888) 380-4427 any hour, or find your city on our service areas page.