Thermostat fan set to "On"
With the fan on On, the blower runs constantly, including between heating cycles, so you'll feel cool air. Set it to Auto.
A dirty filter tripping the limit switch
When airflow is restricted, the heat exchanger overheats and the high-limit switch shuts the burners off while the blower keeps running. Replace the filter and see whether normal heat returns.
Ignition and flame sensor
Modern furnaces use a hot-surface igniter. If it glows but the burners shut off within seconds, the flame sensor probably needs cleaning. If it doesn't glow, the igniter may have failed. Both are common, affordable repairs.
Condensate problems (high-efficiency furnaces)
90%+ furnaces produce water. A clogged trap, a frozen drain line or a blocked PVC vent can trigger the pressure switch and stop heating.
Gas supply
Check that the gas valve on the furnace supply line is open and that other gas appliances work. Propane users should check the tank level.
When to call a pro
Call if the error light shows a code you can't clear, the furnace repeatedly locks out, or you see soot, scorch marks or a yellow flame. See heating repair.
Heat pump owners: lukewarm air in heating mode can be normal. See heat pumps.
