How to Test a Thermostat for a Malfunctioning Gas Furnace

If you have a gas furnace that is malfunctioning or not turning on, the problem may not be the furnace itself. A faulty thermostat could be the culprit in a malfunctioning gas furnace. A thermostat acts as a switch used to turn on your heat and/or air conditioning. If that switch is bad, it could be failing to switch your furnace on. Use the instructions below to test your thermostat.

Check your thermostat to ensure it is set at a temperature higher than the recorded temperature of the room. Also, if your thermostat is powered by a battery, try changing the battery. Your thermostat could be powered and have a low battery that is causing a malfunction.

If the temperature is set correctly and batteries are not the issue, then you can cross the wires in the thermostat to check that the thermostat is good.

A screwdriver should be the only tool you need.

First, turn the power to the furnace off. This is important to avoid any injuries to yourself or damages to the equipment. Next, remove the cover of your thermostat. Most covers just pop off without the need for tools, but some may be kept in place with a screw.

Take a picture of the inside of the thermostat to ensure that any wires that are unscrewed get put back in the correct place.

Inside the thermostat, there should be either four or two wires. Locate the (R) and (W) wires. Usually, these wires are red and white. The wires should be screwed into a terminal. Use your screwdriver to unscrew the terminals. Make sure to secure any wires you unscrew to keep them from falling back into the wall.

Now, twist the (R) and (W) wires together. Crossing these two wires bypasses the thermostat and signals the furnace to turn on.
Power the furnace back on and see if the furnace starts. If it does, your thermostat is faulty and needs to be replaced. If it does not, your thermostat is not the culprit and your furnace may be bad.

Return the wires to their correct terminals and put the cover back on the thermostat. It is very important not to allow your furnace to run on the crossed wires. Since the wires bypass the thermostat, the furnace will continue to run without ever shutting off. This can cause overheating and even a fire.

This same testing method can also be done on the circuit board of the furnace.

Locate the circuit board of the furnace. This will be where the (R) and (W) wires connect to the furnace, as opposed to the ends that connect to the thermostat. Follow the same steps by crossing these two wires together.

Shut the circuit board door once the wires are crossed and turn the power to the furnace on. If it kicks on, your thermostat needs to be replaced. If it does not, your thermostat is fine and further troubleshooting should be done to diagnose the problem.

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