Furnace Troubleshooting, A Step By Step Guide
With our furnace troubleshooting guide, you can do it yourself and
a lot of money. The average service call will cost you over $100. We help you
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.
**NOTE** ONLY YOU CAN ASSESS YOUR ABILITY TO PERFORM THIS TASK. THIS IS
GUIDE AND CANNOT PROVIDE ALL OF THE DETAILS FOR EVERY SITUATION.
Proper maintenance will help you avoid many common furnace
Before you start your furnace troubleshooting, make sure that you have performed
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
repair guide for units that burn fuel oil or waste oil.
Start at the thermostat
Begin your furnace troubleshooting at the thermostat.
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
breaker was tripped, leave it off and check the furnace wiring for loose
you should look for burn spots on the control board and replace the board if
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
(neither red or green) then the problem could be the transformer, thermostat,
board, the blower
motor, or it's run
If you have a condensate pump, ensure that it's reservoir is not full. On most units there
a float switch that will stop the furnace from running if the reservoir is full.
reservoir is full, then check the power to the unit and replace the pump as
After the blower motor is running, the fan can be placed back in the automatic mode at the
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
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,
control, or the gas valve.
Watch the video
You can watch the step by step troubleshooting video here!
Feel the side of the furnace
If the side of the furnace is warm, allow the unit to cool, then continue your furnace
If the furnace has an inducer motor, check to see that it is running. If it is not, the inducer
motor or the furnace
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
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
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
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
see a bright glow in the burner compartment if it is trying to come on) If not,
replacement or the furnace
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
supplied to the furnace. (You can do this by checking to make sure all of the
isolation valves are on.) If you have gas, the problem could be the gas valve or
If the burners come on but do not stay on for more than about 3 seconds, the flame
could be dirty or the furnace
board could be bad.
**NOTE** On condensing furnaces, if the burners come on for longer than
seconds but turn off before the thermostat is satisfied, the condensate drain
Most of these units have a built in trap and sediment can build up in it. This can cause
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
minutes. If the blower does not start, the problem could be it's run
Furnace Troubleshooting, Final Checks
You should complete your furnace troubleshooting by observing the unit's operation through
complete heating cycle. At the end of the cycle, the fan should run for about
after the burner shuts off. If the fan does not shut off after the time delay,
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,
temperature of the air coming out of the registers and compare it to the average
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
could be over 100 degrees while, on newer high efficiency units, it could be as