Furnace Not Heating: Thermostat, Pilot, Filter, and Blower Diagnosis

Furnace not heating has a specific check order. 1) THERMOSTAT: set to HEAT mode, temperature above room temp, fan set to AUTO. Dead display = batteries or wiring. 2) FILTER: a severely clogged filter restricts airflow, causing the high-limit switch to trip and shut down the furnace. Replace the filter and try again. 3) GAS FURNACE: check the pilot or igniter. Standing pilot: relight per instructions. Electronic ignition: listen for the click-click-click of the igniter. If it clicks but doesn't light, the igniter has failed ($30-60). 4) BLOWER: if the furnace lights but no air comes from vents, the blower motor or capacitor has failed. Listen at the furnace for the blower 30-60 seconds after ignition. 5) LED BLINK CODE: most furnaces have a small LED on the control board visible through a peephole. Count the blinks - the furnace manual has a blink code chart.