Stop here first
Safety — Three conditions that end the troubleshooting
A carbon monoxide alarm sounding, a smell of gas, or soot and scorch marks around the burner compartment. Any of the three: shut the gas off at the valve if you can do it safely, leave, and call the gas utility from outside. A cracked heat exchanger is not a repair you diagnose from a web page.
Everything below assumes a furnace that is failing to run or running badly, with no combustion smell and a working CO alarm.
The decision list
Blowing cold air
A furnace blowing cold has one of three stories. It never lit — walk the list above from step 3. It lit and dropped out — flame sensor, or a pressure switch opening under load. Or the blower is running on a fan call with no heat call at all, which is what a thermostat set to "fan on" rather than "auto" does.
The flame-sensor version has a signature: the burners light, run for five to ten seconds, then shut off, and the furnace tries again two or three times before locking out. That sequence is almost diagnostic on its own.
Short cycling
A furnace that runs for two or three minutes and shuts off is usually tripping the high limit, which means it is making more heat than the airflow can carry away. Restricted filter, closed registers, an undersized return, or a furnace oversized for the house so badly that no reasonable airflow could keep the rise in range.
The order to check: filter, then registers, then static pressure, then the sizing. A correctly sized 80% or 95% furnace on adequate duct should run twenty minutes or more on a cold morning. If yours never does, the furnace size calculator will tell you whether the box was ever the right size.
Sources: 10 CFR 430.32(e)(1)(iii)(A) (read 2026-09-04)