Cold air from the vents in January triggers worst-case thinking, but the causes stack neatly from free to serious. Work down the list:
1. Thermostat fan set to ON (free)
Fan ON circulates air constantly — including between heat cycles, when it feels cold. Set it to AUTO and see if the "problem" disappears.
2. Filter so dirty the furnace overheated (nearly free)
A blocked filter overheats the heat exchanger, the limit switch cuts the burners for safety, and the blower keeps running — cold air. Replace the filter; if it recurs, the airflow numbers will tell the story.
3. Pilot or ignition (cheap)
Older standing-pilot furnace: relight per the door label; a pilot that won't stay lit is usually a $10 thermocouple. Modern furnace clicking without ignition: likely the hot surface igniter — inspect for cracks (power off).
4. The dirty flame sensor (the classic)
Burners light, run three seconds, die. That's oxidation on the flame sensor — a 10-minute clean with fine sandpaper, full walkthrough in our flame sensor guide or the guided troubleshooter.
5. Condensate blockage (high-efficiency units)
A blocked drain trips the pressure or float safety and locks out heat. Clear the trap and drain line.
6. Gas supply
Valve on the gas line parallel to the pipe = open. Check that other gas appliances work; if nothing does, it's a utility conversation.
7. Control board or gas valve (call a pro)
If the sequence never completes and the basics check out, you're into board or valve territory — real diagnosis time. Snap the nameplate with our free decoder first so you walk into that conversation knowing the exact model and age of what you're fixing.
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