Furnace Troubleshooting, A Step By Step Guide

With our furnace troubleshooting guide, you can do it yourself and save a lot of money. The average service call will cost you over $100. We help you keep that money! This step by step furnace repair guide will allow you to diagnose and repair the most common problems that occur in a modern gas fired furnace. This page will help you to isolate the problem to a component. Then, you can click on the blue links to go to the page for that specific component for advanced testing/repairs.


Proper maintenance will help you avoid many common furnace problems. Before you start your furnace troubleshooting, make sure that you have performed the suggested maintenance.

This furnace troubleshooting guide will cover furnaces that burn natural gas or propane. You can use our electric furnace troubleshooting guide for a forced air electric furnace or heat pump system. You can use our oil furnace repair guide for units that burn fuel oil or waste oil.

Start at the thermostat

Begin your furnace troubleshooting at the thermostat. Ensure that the thermostat is on heat and the setpoint is above room temperature. (It is best to turn it to the highest setting possible while performing troubleshooting.) If the furnace fan is not running, place the fan in the on mode.

Is the power on to the furnace?

If the fan is not running, check the breaker, fuse, and/or the switch for the furnace. If the breaker was tripped, leave it off and check the furnace wiring for loose connections. Also, you should look for burn spots on the control board and replace the board if spots are found. After any necessary repairs, the breaker can be reset.

If the breaker or fuse was not tripped, look into the inspection window of the blower compartment on the furnace. You should see a flashing green light. If there is no light (neither red or green) then the problem could be the transformer, thermostat, furnace control board, the blower motor, or it's run capacitor.

If you have a condensate pump, ensure that it's reservoir is not full. On most units there is a float switch that will stop the furnace from running if the reservoir is full. If the reservoir is full, then check the power to the unit and replace the pump as required.

After the blower motor is running, the fan can be placed back in the automatic mode at the thermostat.

Does the furnace have a pilot light?

If the furnace uses a standing pilot, ensure that it is lit and the flame is touching the tip of the thermocouple. If it is not, you can use our thermocouple replacement guide for further troubleshooting.

If the pilot is lit but the main burners do not come on, you should feel the side of the furnace. If the side of the furnace is cool, the problem could be the thermostat, furnace control board, limit control, or the gas valve.

Feel the side of the furnace

If the side of the furnace is warm, allow the unit to cool, then continue your furnace troubleshooting.

If the furnace has an inducer motor, check to see that it is running. If it is not, the inducer motor or the furnace control board could be the problem.

Does the pilot or main burner try to ignite?

On units that use a spark ignition, you can hear the pilot or main burner try to ignite as it will have a rapid clicking noise.

If the inducer motor is running, does the pilot (or main burner) try to ignite?

If it does not, the flue could be blocked, the furnace control board could be bad, or the limit control could be the problem.

If the pilot (or main burner) tries to light but goes out, the pilot assembly or flame sensor could be dirty. Our guide will show you how to clean them.

On furnaces that do not use a spark ignitor, does the hot surface ignitor come on? (You will see a bright glow in the burner compartment if it is trying to come on) If not, the ignitor may need replacement or the furnace control board could be bad.

Do the main burners come on?

Once the pilot or ignitor is on, do the main burners light? If not, ensure that you have gas supplied to the furnace. (You can do this by checking to make sure all of the manual isolation valves are on.) If you have gas, the problem could be the gas valve or furnace control board.

If the burners come on but do not stay on for more than about 3 seconds, the flame sensor could be dirty or the furnace control board could be bad.

**NOTE** On condensing furnaces, if the burners come on for longer than 5 seconds but turn off before the thermostat is satisfied, the condensate drain could be plugged.

Most of these units have a built in trap and sediment can build up in it. This can cause the furnace to operate abnormally.

Does the blower come on now?

Once the burners stay lit, the blower motor should start after a delay of from one to three minutes. If the blower does not start, the problem could be it's run capacitor or the motor.

Furnace Troubleshooting, Final Checks

You should complete your furnace troubleshooting by observing the unit's operation through a complete heating cycle. At the end of the cycle, the fan should run for about 2-3 minutes after the burner shuts off. If the fan does not shut off after the time delay, then the fan/limit control (on older units) may be sticking and require replacement. During this time you can check the temperature rise across the furnace. Using a thermometer, check the temperature of the air coming out of the registers and compare it to the average room temperature. On the inside of the furnace, you will find a label plate that tells you the range that this the difference between the temperatures should be. (On older furnaces, it could be over 100 degrees while, on newer high efficiency units, it could be as low as 35.)