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Problems With Furnace: Before You Call A Repairman

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. It also will help you to isolate the problem to a component.

1. A proper maintenance.

Be sure that you have performed the suggested maintenance, before you start your furnace troubleshooting,
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.

Firstly, the thermostat...
Start your furnace troubleshooting at the thermostat. Make sure that the thermostat is on heat and the setpoint is above room temperature. The best way to test it - just turn it to the highest setting possible while performing troubleshooting. If the furnace fan is not running, place the fan in the on mode.

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

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, make sure that it's reservoir is not full. Mostly, on 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.

2. A pilot light

Ensure that it is lit and the flame is touching the tip of the thermocouple, if the furnace uses a standing pilot. 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...
Let the unit cool, then continue your furnace troubleshooting, if the side of the furnace is warm.

If the furnace has an inducer motor, check to see that it is running. In other case, the inducer motor or the furnace control board could be the problem.

Does the pilot or main burner try to ignite?
On units with 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 flu 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 mechanism or flame sensor could be dirty.

How to clean them?
On furnaces which do not use a spark ignitor, does the hot surface ignitor come on? If yes, 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.

What do the main burners?
When the pilot or igniter is on, do the main burners light? In case they don’t, make sure 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.

What’s up with the main burner?
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.

3. A final check

Finish your furnace troubleshooting by observing the unit's operation through a complete heating cycle. The fan should run for about 2-3 minutes after the burner shuts off, at the end of the cycle. In case the fan does not shut off after the time delay, then the fan/limit control (on older units) may be sticking and require replacement. By 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. You will find a label plate that tells you the range that this the difference between the temperatures should be on the inside of the furnace. On older furnaces, it could be over 100 degrees while, on newer high efficiency units, it could be as low as 35.