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High-efficiency (92 percent and higher) condensing gas furnaces are complex. They have multiple sensors and safety systems, all run by a computer. Some of the components are complex in themselves and require special equipment to diagnose and repair. But other parts are pretty straightforward and easy to replace.
We talked to the experts at Superior Heating, Air Conditioning & Electric in Anoka, Minn., to get the lowdown on which furnace problems generate the most service calls, which of those repairs can be performed by a DIYer and which ones are “pro only.” It turns out that most of the problems are caused by a clogged furnace filter or any of three easy-to-fix parts—the igniter, the flame sensor and the high-limit switch.
If you’re willing to risk $120 and buy all three parts ahead of time, you may be able to fix your own furnace in less than an hour and save a $125 service call. If the parts don’t fix the problem, at least you’ve replaced the most likely suspects before calling the pros.
We’ll show you what the parts look like and where they’re installed on a typical furnace. But every furnace brand is different. So your best bet is to find a service manual for your particular furnace online. We think these repairs are relatively easy for a DIYer, but if you feel uncomfortable with any of the steps we show here, call a pro.
Figure A: Where The Parts Are
High-efficiency furnaces are jam-packed with valves, sensors, motors and flue pipes. We’ve removed many of those larger components from this furnace so you can see what the parts look like and where they’re located. They won’t be as easy to see in your furnace. Here are some tips to help you find the igniter, flame sensor, high-limit switch and furnace pressure switch tubing.
The igniter and flame sensor are located inside a sealed combustion chamber and aren’t usually visible. But they can be replaced without removing the combustion chamber cover. If you don’t have a service manual showing their location, consult the furnace-wiring diagram to identify the wire colors for the igniter and flame sensor. To locate the sealed combustion chamber, follow the gas piping from the gas valve to a box with a cover and an inspection window. Then look for the wires going to the igniter and flame sensor. The high-limit switch is usually located below the sealed combustion chamber. The furnace pressure switch will always have flexible tubing running to it.
Before you start
Photo 1: Find model and serial numbers
Yank off the furnace covers and inspect the side panels in the burner area to find the manufacturer’s label. Write down (or shoot a cell phone photo) of the model number, serial number and date of manufacture.
The igniter and the flame sensor are like tires on your car—they wear out every four to five years. So it makes sense to keep spares on hand. Igniters and flame sensors cost about $50 each at appliance parts stores and furnace dealers. Or you can find them for less online. But first you’ll need the make, model number and serial number of your furnace. Find all that information on the manufacturer’s label (Photo 1).
Then an authorized furnace dealer or visit a local appliance parts store and buy the parts. Some furnace dealers won’t give out parts numbers or sell parts to DIYers. And appliance parts stores don’t always stock parts for every brand. In that case, search the Internet for “furnace parts” and enter your furnace brand and model number to find the right parts ( is one source).
Shut Off The Power!
All furnaces have a power cutoff switch inside the blower compartment that disconnects power to the furnace controls when the front panel is removed. But there’s still power coming into the furnace, so always turn off the furnace power at the switch on the side of the furnace and at the circuit breaker panel before disconnecting any wires, or testing/replacing any parts. And never run the furnace with the front panel off.
Check the power, then the filter
Photo 2: Perform a furnace reset
Flip the furnace power switch to off and wait for at least five minutes. Then turn on the power and let the furnace restart.
Photo 3: Test the high-limit switch
Disconnect the wires going to the high-limit switch and connect a continuity tester to the switch terminals. If the test light doesn’t glow, the switch is bad.
Repair experts tell us that they often show up at a residence only to find that the unit isn’t getting power because of a tripped breaker or a flipped switch. So check that first. Flip the switch on the side of your furnace, and flip the breaker off and on again before you even think about replacing any parts.
Next, check the filter. A clogged air filter restricts airflow through the heat exchanger, causing the furnace to overheat. The high-limit switch detects the dangerous overheating and signals the computer to shut off the burners and run the blower fan to cool off the heat exchanger. Once the furnace cools, the computer tries to fire up again. But if the filter remains clogged and the furnace overheats four or more times (the actual number varies by manufacturer), the computer will shut down the furnace until it’s repaired.
If the filter is filthy, you’ve most likely discovered the cause of the shutdown. Replace it with a new one. Then reset the furnace (Photo 2). If the furnace won’t restart, the repeated overheating may have damaged the high-limit switch. Consult the wiring diagram on the furnace door or your service manual to find its location. Then test it (Photo 3). If the switch is bad, remove the retaining screws and pull it out of the heat exchanger, noting the position of the sensor. Install the new switch with the sensor facing as it was.
Leave These Repairs to the Pros
Replacing an inducer fan motor looks like a simple job. But the fan requires special sealants and procedures to ensure it doesn’t leak any carbon monoxide into your home. Screw this up and you could kill somebody. Leave this one to the pros.
Replacing a gas valve isn’t difficult—it just screws into place. But you can’t install a new one without calibrating it to your home’s gas pressure and your furnace specifications. And that’s going to require a service call from a trained expert with the proper equipment. Don’t think you’ll save money by slapping one in yourself and skipping the calibration, because improper calibration can burn up your heat exchanger, fill it with soot, run up your gas bill or all of the above.
Video: 6 Tips for Furnace Troubleshooting
Replacing the igniter and flame sensor
Photo 4: Install the new igniter
Slide the new igniter into position, avoiding with skin and hard surfaces. Then secure the part with a screw or snap connectors. Plug the igniter’s electrical connector into the furnace wiring harness.
Photo 5: Clean off the crud
Apply light pressure and scrub off any buildup or corrosion from the sensor with emery cloth or No. 0000 steel wool.
Most high-efficiency furnaces use a “hot surface” igniter that heats up to 1,800 degrees F to light the burners. Once lit, the burners then heat a flame sensor. The furnace’s computer uses the signal from the flame sensor to confirm a successful ignition and turn off the igniter. However, over time, the constant heat/cool cycles cause the igniter to crack and fail. And the flame sensor can develop surface corrosion, causing it to send an incorrect signal to the computer. Or it can simply wear out.
The igniter can be held in place either by screws or by a snap-clip arrangement. Use a lighted flexible mirror to discover the method used on your furnace. Then remove the screws or unsnap the retainer and remove the old igniter. Use care when you install the new igniter—it’s brittle and can crack or shatter easily (Photo 4).
Next, remove the flame sensor. If the sensor element is covered with corrosion and you don’t mind replacing the sensor later, you can try cleaning it (Photo 5). Otherwise, just replace it.
One last preventive measure
Photo 6: Clear the pressure switch tubing
Connect a compressed air gun to one end of the tubing and aim the other end toward the floor. Blow out each tube and reinstall.
Condensing gas furnaces attain their efficiency by extracting water from the exhaust gases. Sometimes condensation from that exhaust can form in the pressure switch tubing. This silicone tubing runs between the flue and the heat exchanger and the safety furnace pressure switch. Experts tell us they usually remove those tubes and blow them out with compressed air as a preventive measure (Photo 6).
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- 4-in-1 screwdriver
- Adjustable wrench
- Air compressor
- Air hose
- Voltage tester
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- Emery cloth
- Flame sensor
- Furnace filter
- High-limit switch