Furnace Blowing Cold Air: Thermostat, Flame Sensor, and Limit Switch Fixes

Furnace blowing cold air (fan runs but air is not warm): 1) Check thermostat fan setting - if set to ON instead of AUTO, the blower runs continuously even between heating cycles, blowing room-temperature air. Switch to AUTO. 2) Flame sensor dirty: the furnace ignites but the flame sensor cannot detect the flame, so the gas valve shuts off after 3-5 seconds. The blower continues running (blowing now-cold air). Clean the flame sensor with fine sandpaper - this is the #1 cause of 'furnace lights then shuts off.' 3) High-limit switch tripped: the furnace overheated (from a clogged filter or blocked return vent) and the limit switch shut off the burner but left the blower running to cool the heat exchanger. Replace the filter and check for blocked vents.