How to Fix a Furnace That Won't Ignite

Ensure the gas valve on the line feeding the furnace is fully in the "Open" position (parallel to the pipe) and that your home’s main gas service is active.

Turn the thermostat to Heat. Look through the small observation window on the furnace door. Note if the igniter glows orange but the flame fails to light.

A dirty flame sensor often prevents a furnace from staying lit. If you are comfortable, turn off the power, remove the sensor rod, gently clean it with fine steel wool, and reinstall.