Most homeowners assume gas furnaces are low-maintenance because natural gas and propane burn cleaner than fuel oil. That’s partially true. They generally offer fewer operational headaches than their oil-burning counterparts. But they aren’t invincible. When things go wrong, the culprit is usually one of three things: the thermocouple, the pilot light, or a glitch in the electrical system.
Before you start poking around, let’s talk about safety. Gas furnaces have control shutoffs designed to prevent leaks. They are not fail-safe. If you smell gas, do not play hero. Do not flip a light switch on or off. Do not try to manually shut off the gas valve to the furnace. Those actions create sparks. Sparks create explosions.
Leave the house immediately. Leave a door open if you can do so safely on your way out. Then call the gas company or the fire department. Do not re-enter until professionals say it’s safe.
Accessing the Pilot Assembly
Once you’re sure the air is clear, you need to see what you’re dealing with. The access method depends on your specific unit.
On some furnaces, there’s a plug-type door covering the pilot light assembly. You can pull this door straight out of the furnace housing. On other units, you’ll need to remove the entire panel that covers the pilot and gas burners.
Once the cover is off, look at the front of the furnace. This is where the action is. The pilot light controls, reset buttons, gas valves, and thermocouple are all usually housed in a single assembly here. If you’re looking for the furnace limit switch, check the plenum (the main chamber) or the main duct junction on the upper housing.
But before you test those switches or check that thermocouple, let’s look at the most common point of failure.
The Pilot Light
The pilot light is the small, constant flame that ignites the main burner. If it’s out, your furnace won’t heat.
