Something is wrong6 min readOctober 20, 2026
Furnace clicks but will not light, or runs a minute and quits: what is happening
A furnace that lights and shuts off within seconds has a dirty flame sensor nine times out of ten. One that clicks and never lights has a failed ignitor or a pressure switch that is not closing. One that heats a few minutes and stops is overheating on its limit switch, almost always from a clogged filter. Check the filter and the furnace switch yourself; leave the gas side to a technician.
What a normal start looks like
Thermostat calls. The inducer fan (a small motor) starts and runs for 30 to 60 seconds. A click as the pressure switch proves the inducer is moving air. The ignitor glows orange, or a spark clicks. The gas valve opens and the burners light with a soft whoosh. A few seconds later the flame sensor confirms fire. Thirty to ninety seconds after that, the main blower starts and warm air reaches the rooms. Knowing the sequence tells you where it broke.
Lights, then dies in a few seconds: flame sensor
The flame sensor is a thin metal rod in the flame. It builds a coating over a heating season or two and stops sensing fire, so the control board shuts the gas off for safety and tries again, usually three times before locking out. Cleaning it is a five-minute job with the power off and fine abrasive pad, and many homeowners do it. We still list it as a professional job because it sits next to the burners, and a technician will check the burner alignment and the gas pressure while the panel is off.
Clicks, inducer runs, never lights: ignitor or pressure switch
Hot surface ignitors are ceramic and crack with age; when one fails you hear the inducer run, a pause where the glow should be, and a shutdown. A pressure switch that will not close is more common on high-efficiency furnaces in our area: the condensate trap or the drain line is blocked and water has backed up into the switch hose. If your furnace has a white PVC vent pipe, check that the small condensate line from the furnace is draining before calling. A blocked trap is often a homeowner fix.
Runs a few minutes, stops, starts again: limit switch
The high limit switch shuts the burners off when the furnace gets too hot inside, and it gets too hot when air cannot move through it. A filter nobody changed since spring is the usual reason. Closed registers, a blocked return and a blower motor losing speed are the others. Replace the filter and open every register; if it still short cycles, the blower or the limit itself needs a technician.
The one you must not ignore
A furnace that smells of gas, sets off a carbon monoxide alarm, or shows flames rolling out of the burner compartment is unsafe to run. Turn it off at the switch, open windows, and leave the gas side alone. Heat exchangers crack with age and corrosion, and a cracked one puts exhaust into the air you breathe. Any furnace inspection should include a look at the heat exchanger and a carbon monoxide test with the blower running.
Questions on this
The furnace blows cold air for a minute before it warms up. Is that a fault?
No. The blower starts before the heat exchanger is fully warm, and on a heat pump the first air is always cooler than furnace air. If it blows cold the entire cycle, the burners are not lighting or a heat pump is stuck in cooling.
How often should the flame sensor be cleaned?
Once a year at the fall tune-up is enough for most furnaces. In houses with pets or heavy dust it may need it twice.
Why does my furnace lock out and need the power cycled?
After three failed ignition attempts most control boards lock out for an hour or until power is cycled. The lockout is a symptom; find out which of the four parts above failed before resetting it again and again.