If you have a relatively new furnace it is probably equipped with a hot-surface ignitor. These ignite the gas when your furnace turns on. Electricity passes through the ignitors and causes them to glow red. These components often fail. If your furnace does not turn on, look for a glow inside the furnace just after you turn your thermostat on. If you do not see it, you likely have a faulty hot-surface ignitor. These swap out quickly so there is no need to call a heating technician.
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Turn the power off at the side of your furnace and remove the cabinet doors.
Unplug the hot-surface ignitor from the its corresponding plug. The hot-surface ignitor has a white base with silver-colored ceramic prongs protruding from the front of it. Wires will come off the back of the white base.
Unscrew the hot-surface ignitor from the furnace. Look for a 1/4-inch diameter hex screw. Remove the old ignitor.
Insert the new hot-surface ignitor and fasten with the old screw.
Plug the ignitor into the wiring harness and put the cabinet doors back on the furnace.
Turn the power back on to your furnace and check to see that it operates normally.