What can be normal
The blower may start before the heat exchanger is fully warm or continue after the burners shut off.
A brief cool period is not necessarily a failure.
Common causes
Thermostat fan settings, ignition failure, flame-sensor problems, gas supply, pressure switches, limit trips, or control-board faults can cause cold air.
A dirty filter can overheat the furnace and shut the burners off while the blower continues.
Safety considerations
Gas odor, soot, rollout, or repeated ignition attempts require immediate shutdown and professional attention.
Do not bypass furnace safety switches.
How it is diagnosed
The technician observes the ignition sequence and checks error codes, flame signal, pressure switches, temperature rise, airflow, and gas operation.
The exact point where the sequence stops identifies the likely cause.
Repair options
Repairs may involve filters, flame sensors, igniters, pressure tubing, venting, gas controls, limits, or boards.
Heat exchanger condition should be considered when overheating has occurred.
Frequently asked questions
Can the fan setting cause it?
Yes, FAN ON can circulate cool air between cycles.
Can a dirty filter cause it?
Yes.
Is gas odor normal?
No.
Should I reset the furnace repeatedly?
No.
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