Furnace Problems - Diagnosing a Furnace

When it gets really cold and you need that heater to kick on, you expect it to work. I know I do. But sometimes we have a dead heater sitting in our HVAC cabinet or out in the garage in need of a furnace repair. But the question comes up, what is wrong with my furnace and how do I know exactly what it is that I need to do. First thing is first, you may be working with gas or electricity which has its own independent risks, call a professional if you are unsure about your abilities. If you are a DIYer with a high success rate, then here we go! If you are unfamiliar with heater repair, check out my article on the Anatomy of a Heater. Second, gather your tools. My typical tools are:

The first thing we need to do is verify there is power. Often times, there will be a sight glass that is part of a metala cover, which covers the blower assembly and control board. Look through that sight glass (if you have one) and verify there are LEDs on. Another way to test for power is to take your NCVT (https://amzn.to/2Lrah5J), turn it on, and verify there is power being supplied to the unit. A final way is to test the leads going to the power box for ~110-120 VAC.

Normal Operation Process

Turn the thermostat on so that it calls for heat. If you don’t know how to do this, see my All About Thermostats article to get your HVAC knowledge up. Once the thermostat clicks on to call for heat, the inducer motor should turn on and stay running for about a minute or less; this activates the pressure switch. Then, the igniter should begin to glow. When the igniter glows or sparks, the gas valve should open. This is identified by flames burning in the heat exchanger. Then the furnace blower motor kicks on after about a minute of combustion and the unit blows hot air. This is a proper cycle of a forced air heater using propane or natural gas.


Thermostat Problem

If the inducer motor does not turn on when you call for heat, it is time to remove the cover from the furnace blower cabinet and continue from there. When I remove the furnace blower cabinet door, there is a small pressure switch which disables power if the furnace blower cabinet is opened. This is a safety switch and I either disable it by placing a clip on it or by taping it closed. Once that switch is depressed, I take the white and red or R and W wires out of the control wires that come from the thermostat. Then, I take a jumper wire and jump R to W. If the inducer motor kicks on, the problem is most likely either the thermostat or the wire. If you don’t have a direct wire that goes to your thermostat, but instead a wireless thermostat, that is okay, you may just have to reset that device. See my All About Thermostats article with instructions on how to reset a transponder.



Transformer Problem

If after jumping R to W the unit is unresponsive, there is a possibility that the transformer has malfunctioned. Take your multimeter (https://amzn.to/2BtzLuG) and test the secondary windings on the transformer. You should get 24 VAC or a slightly higher voltage. If you do not get 24 VAC or get a value lower than that, you may have a transformer issue. If you get 12 VAC or less, replace the transformer. Also be sure to test the primary leads to verify there is between ~110-120 VAC feeding the transformer. If there is no, voltage, ensure that there is power to the control board of the unit. If there is power to the board, disconnect the unit from power and verify that the fuse is good. A good way to properly diagnose a fuse is switch your multimeter (https://amzn.to/2BtzLuG) to Ohms mode and ensure there is continuity on both the leads. DO NOT HANDLE THE CONTROL BOARD LIVE! This is your one and only warning. This includes changing a fuse when live. If there is power to the unit, verify that the control board is receiving primary voltage. If the board is receiving voltage, but is not energizing the transformer, you have a control board issue.


Diagnosing the Pressure Switch and Inducer

The inducer kicks on first when there is a call for heat. If this does not happen, test the leads to ensure it is cgetting ~110-120 VAC. If it is getting power, you most likely need to replace this motor or entire assembly.

To check the pressure switch, one can take a multimeter (https://amzn.to/2BtzLuG) and set it to Ohms.  We are testing for continuity. Jump R to W and the inducer motor should kick on. Touch the multimeter (https://amzn.to/2BtzLuG) to both leads of the pressure switch without the control wires connected. This should show continuity on the multimeter (https://amzn.to/2BtzLuG), my meter makes an audible beeping noise continuously. If continuity breaks even for a moment, the gas valve will NOT open. Your pressure switch is a likely culprit. To double-verify that the pressure switch is faulty, jump R to W, get the inducer running, jump across the pressure switch leads. If the system returns to normal operation and does not cut out the flame, the pressure switch is your problem.

Sometimes the pressure switch can give a false reading. It is possible for the small vacuum hose attached to the pressure switch has a hole or crack. A good way to test this hose is to place a Magnehelic (https://amzn.to/2SgptFz) pressure gauge on the end of the hose closest to the pressure switch. It should give a reading above 2 inches of water. I look for 2-4” H2O. If this is lacking or is erratic, replace the hose. If after replacing the hose the bpressure reading is below 2” H2O or erratic, you most likely have a bad inducer and or inducer housing. Replace the inducer unit to rectify. A lot of times the inducer gasket becomes damaged causing the vacuum being created by the inducer motor to be less than required. A new inducer motor is generally a good idea because it comes with a replacement gasket and a fresh motor. This ensures proper operation. Don’t be a genius and make a gasket out of gasket material, a cardboard box, insulation, or some other genius McGyver mess. These are high-temperature components that can CATCH ON FIRE if installed with improper materials! Replace the whole thing to save yourself the hassle.



Diagnosing the Igniter

If the inducer motor stays running for several minutes or turns off without producing a flame, you could have a problem with the igniter. This is often identified by not observing a warm glow in the cabinet or not hearing/seeing sparks being created. The igniter can malfunction due to a broken or cracked carbide tip, the leads could be damaged or frayed, or the pressure switch is not open. The best way to diagnose this, is to dCAREFULLY remove the igniter using a 1/4” nut driver (https://amzn.to/2Sd5Ca4). Disconnect the leads and remove. The igniter can be extremely brittle and if you tap it on anything, it could shatter! You have been forewarned!!! Visually inspect the igniter if it has even a hairline crack in it, it is no good. You must replace it. If it does not produce spark, test the leads for voltage. Jump R to W and read the voltage coming out of the leads. It should be ~110-120 VAC. Replace the igniter and verify it turns on.


Diagnosing the Flame Sensing Rod

The flame sensing rod is responsible for sending millivolt voltage back to either the board or the gas valve allowing for the gas valve to stay open. If your unit fires up only for a moment, then the flame suddenly cuts out eand the furnace blower motor of death kicks on, then you could have a flame sensing rod issue. But fear not! Take the flame sensing rod out using a ¼” nut driver (https://amzn.to/2Sd5Ca4) and visually inspect it. If it has carbon buildup form the sediment in the gas, take a piece of emery cloth (https://amzn.to/2rLvm1J) and scrub it down until it looks new again. Replace it and call for heat (jump R to W). It should stay on, and some boards even have a LED which illuminates when it is sensing flame. Call for heat and see if this light illuminates first if you have one.

Diagnosing the Gas Valve

Ah gas, the feared enemy of the faint of heart. Before you begin, make sure the gas switch on the actual valve is set to the “ON” position. I can’t tell you how many customers I have had to charge a service fee to just because this switch was off. When the system checks out (inducer energizes, pressure switch closes, igniter turns on) andvalve the gas valve is supposed to open after that igniter glows, but doesn’t is not always a bad sign. First, check to see that the gas is on. I like to visually inspect the gas line to verify the gas shutoff valve is open. I even go a step further and crack open the line to see if I can smell gas then retighten it. If this gas shutoff valve is open, then you have narrowed it down to two components possible; the control board and the gas valve. If the control board is good to go and is sending the gas valve voltage (somewhere around 24 VAC), then your gas valve is the culprit. Pull out your checkbook, because this one might be expensive.

Diagnosing the Control Board (IFC)

This is one of the trickiest components to diagnose, but I do no of some big tips. First, if the board fails to energize any of the components but has power to it, it is faulty. I like to disconnect the unit form power and remove the board screws to pop the board out without disconnecting any of the leads. Look at the back sideboard carefully. A big indicator of a bad IFC is burned board or melted components. If you see burn or melting, it is possible one of the components on the board has failed. DO NOT attempt to solder the board back together!!! Remember, the control board failed for a reason, there are other components on the PCB which could be faulty and soldering the board back together could cause a fire or a dangerous arc. Don’t do it!!!

Diagnosing the Furnace Blower Motor

Jump R to G, which enables the furnace blower motor. Verify that there is no air coming out of any register and or you cannot hear the blower motor. If you can hear the blower motor and it is making a loud hum but is not turning, disconnect power immediately. Reach in the blower area and verify that the blower wheel turns freely. If the blower wheel does turn freely, there is one more part to check. Take an insulated screwdriver (https://amzn.to/2V036Wn) and discharge the leads of the capacitor by touching them together with the screwdriver (https://amzn.to/2V036Wn). This discharges a potentially dangerous capacitor. Take your multimeter (https://amzn.to/2BtzLuG) and switch it over to capacitance. Disconnect the leads using somebloawer needle nose pliers (https://amzn.to/2BvnW78). Measure between the terminals on the capacitor and it should read about 10 uf (microfarads for you nerds). Verify your reading against what the sticker on the placard says. The tolerances the capacitor gives is what you need to be at. If this is below the tolerances, replace the capacitor and try running the motor again. If it turns and runs, you are set. If it still hums and does not turn, you need to replace the furnace blower motor.

Other Problems

Limit Switches

Limit switches can trip causing the gas valve to close and the unit to stop working. They can also prevent the gas valve from ever opening. Before blaming a limit switch, you need to make sure that:

  1. It is getting control voltage (~24 VAC)
  2. The limit switch isn’t tripping because of heat buildup (what it is designed to do)
  3. The limit switch remains closed at low temperatures.
  4. The limit isn’t tripping because of a restriction in the supply
  5. The blower is energizing after flame has been produced

If these conditions are true but the limit is still opening, then replace the limit switch with the exact replacement.other Don’t try and google it, they (the manufacturers) purposefully hide their tolerances so that they can control it by part number.

Diagnostic Code Codes

Often times control boards come with an on-board diagnostic function which helps you identify what is wrong. Check out my How to Diagnostic Codes article for more on this.


HVAC units can be tricky to diagnose and this is by no means an exhaustive list. I simply try and do my best to give you the knowledge to be informed. I might get some hatemail for giving away trade secrets blah blah, but really I just want to educate. If you have questions, shoot me a line, otherwise good luck and mahalo.


Also, if you need replacement parts, you can message me on the website messaging board with your model number and serial number. I can ship parts anywhere in the country. I do mark up my parts, but you get a whole better deal with me than with your local HVAC contractor. I also do facetime to help you install the component and return the unit back to working order for a small charge. Message me if you need help, that’s what I’m here for.