First: check the fan setting
On the thermostat, the fan has two settings: AUTO and ON. Set to ON, the blower runs continuously, including during the long stretches when the burners are off. You get unheated air out of the vents most of the time.
Switch it to AUTO. If the problem disappears, that was it, and nothing is broken.
If it is on AUTO, the burners are not staying lit
The pattern is recognizable: the furnace fires, runs briefly, the burners cut out, and the blower keeps going while the heat exchanger cools — pushing cold air until the cycle restarts.
The usual cause is a dirty flame sensor. It is a thin metal rod that confirms a flame is actually present; when it gets coated, it stops detecting the flame and the control board shuts the gas off as a safety measure. It is doing its job. Cleaning it is a routine part of a service visit.
- Dirty flame sensor — most common by a wide margin
- Failed igniter, so the burners never light at all
- Blocked condensate drain on a high-efficiency furnace
- Pressure switch reading a restricted or blocked flue
- Overheating from a clogged filter, tripping the high-limit switch
When it is a safety issue
Repeated lockouts are worth diagnosing rather than resetting your way through. The safety controls are shutting the gas off for a reason, and cycling the breaker to force another attempt bypasses the reason.
If you smell gas, leave the building and call your gas utility from outside. If a carbon monoxide alarm has sounded, get everyone out and call the fire department. Neither of those is a "call the HVAC company in the morning" situation.
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