Furnace blowing cold air

The blower and the burner are controlled separately. A continuous-fan setting (G) runs the blower on its own, and the blower also runs for a set off-delay after a heating cycle. Cold air while the fan runs therefore does not mean the furnace is broken in the blower department; it means the heat side did not run or did not stay on.

On most furnaces the sequence is: thermostat calls for heat, inducer starts, pressure switch proves draft, ignitor heats, gas valve opens, flame sensor proves flame, and only then the blower starts after a short delay. A failure at any step produces a status code, which is the fastest way to tell what stopped.

Confirm the thermostat is set to Heat with the set point above room temperature, and that it is calling (the display usually shows a flame or heat icon).

Confirm the gas shut-off valve at the furnace is fully open in line with the pipe; if someone turned it for summer, the furnace cannot light.

Read the code on the control display or LED through the door and note the exact digits before calling.

Check the vent and intake terminations for snow, ice or blockage, and the floor for water from the condensate path.

An ignition or flame-loss code says the burner never lit or did not stay lit; causes run from gas supply and ignitors to flame sensors and venting. A pressure-switch code says draft was not proven, pointing at venting, condensate or the inducer. Look the code up on the error-code pages for your brand and family before calling.