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Fuse on my control board blows constantly. How do I find a Low Voltage Short?

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Problem: The fuse on my gas furnace’s control board blows constantly. How do I find a low voltage short?

Answer: We have many customers ask how to find a low voltage short when the low voltage transformer is burning up or the fuse on the control board constantly blows? Most of the time I have found that shorts occur in the thermostat wires that go out to the outdoor condensing unit. Wear and tear of sunlight, animals or weed eaters usually cause these shorts. I would like to suggest that you inspect the thermostat wires for cracks, wear and tear. Pay close attention to where the thermostat wires go through the body of the furnace or air conditioning unit. Many times the thermostat wires will short out against the body of the furnace or air conditioner. The vibration of the furnace against the sharp metal body of the furnace can tear through the thermostat wire insulation and cause a short in the wires. We have a really good tool that you can use to save fuses when troubleshooting low voltage shorts called the Lil Popper. The Lil Popper allows you to manually reset the fuse instead of replacing a fuse every time. Please click here if you would like to see the LiL Popper tool we sell. If you find out that your low voltage transformer is bad and you need a new transformer then Please click here to see the low voltage transformers that we sell. I found three really good Youtube videos on how to find low voltage shorts by ACservicetech, Word of Advice TV, and Lex Vance.  I hope these great Youtube Videos will help you out in finding where the short is located in your HVAC system. The Lex Vance video on the bottom shows how Mr. Vance found a low voltage thermostat wire short out in the AC condensing unit’s thermostat wires. Thanks so very much to ACservicetech, Word of Advice TV, and Lex Vance for making these great videos! If you have any questions please email us anytime: or comment below in our comments section below. We Would Love to Try and Help You Out and Earn Your Business! 

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13 thoughts on “Fuse on my control board blows constantly. How do I find a Low Voltage Short?

  1. Most reply’s related to the fuse blowing are blamed on the Tstat wire shorting out however this is impossible as the Tstat simply shorts the wire or I should say completes the low voltage circuit which engages the heat cycle. If the wire short together the heater will simply never shut off. If the Tstat with shorts against the furnace frame then one may expect the fuse to blow. On my own Big Maxx 20k I found if I leave the cover off the unit still blows fuses but rarely if I install the side cover the fuse goes bad in under a minute. And no the side cover is not touching any electrical components so it has to be heat related. I installed a brand new board and it was worse so the old one is in for now.

    1. Hey friend you say short in the thermostat wires? if that is true wont the fuse blow as soon as you apply power and the Thermostat would be out as well? my unit goes thru its sequence, flame comes on then the fuse blow, my last ac guy suggested the Gas valve possibly ,any thoughts on that and a way to check before trying to change the valve??


      1. Hi Tyrone! It is true that the fuse on the control board might blow (short out) when the thermostat wire that is grounded out is energized. For example, if the yellow and common thermostat wires are touching each other then the short would not occur (fuse blows) until the Air Conditioner is turned on and the yellow wire becomes the power wire. The same could happen with your gas valve. If your gas valve is shorted out then the fuse would not blow until your thermostat calls for heat and the gas valve is energized. When I was doing service work I found the thermostat wire shorts out by the outdoor AC unit were the most common most of the time because these thermostat wires were exposed to sun, and weed eaters, but yes, another component or control like a contactor, reversing valve, furnace control board or a gas valve could have a short and cause the fuse to blow. You would need to test your gas valve with an Ohm meter. Disconnect the wires on the valve and test from each lead of the gas valve to a good ground. You should not get any resistance. If you do get resistance then the gas valve is shorting out and you need a new gas valve. An easy test would be to disconnect the gas valve and see if the fuse on your board still blows out. If the fuse stays good with the gas valve disconnected then I would most certainly say that the gas valve is the problem. Thanks for asking this question. God Bless You. Steve

  2. Hi air conditioner was running good but blower kept going on non stop. So I turned it off. Know that I want to turn furnace on it keeps blowing the 3 amp fuse when I turn the thermostat to heat.
    There was and outage in neighbor hood could that had off triggered the issue?

    1. Hi Ms. Cruz! Yes, the storm could have caused a short in your control board. Of course, as you know if the low voltage fuse on the control board blows this is a sign that you have a low voltage short somewhere in your system. As explained in this post most of the time the short is in the thermostat wires, but it could be in any of your low voltage controls like the transformer wires, thermostat, control board, or any relays. If you had a power surge in your neighborhood then it could have shorted something out in your heating system. I hope that you can find the culprit and get it fixed soon! Steve

  3. My inside unit keeps the fan keep running on my inside unit and and a blue the low voltage fuse why

    1. Hi Todd! I tried to explain in the post above what causes the fuse to blow. I really do not have anything else that I can think of if it is not covered in the post. I am sorry that I can not give you any more suggestions or help. Steve

  4. I was blowing the 5 Amp fuse on the control board for my forced air gas furnace.
    My furnace is a Concord model CG90UB.
    I removed all the wires going to my thermostat and it would not blow the fuse when powered on.
    I turned off the input power and attached terminal R (24 VAC power to thermostat) and the fuse blew.
    The wire from terminal R goes to a safety switch in the condensate pump and then to the thermostat.
    I bypassed the condensate pump and to directly feed 24 vac to my thermostat and the fuse no longer.blows. It turns out the switch inside the pump that is enabled by the float had failed. This allowed water to get to the top of the tank and short the safety switch to ground. I have a new pump on order. The pump is a Little Giant 554405 Vcma-15uls. Meanwhile I have the furnace condensation hose running to a bucket.

    1. Hi Justen! Thanks so much for taking your time to explain how your condensate pump’s safety switch caused the fuse on your control board to blow! Congratulations! So glad to hear that you were able to find and fix the problem. Thanks so very much for sharing how you fixed your furnace! I hope you have a great and blessed day! Steve

  5. I have a nutone ac/heat pump blowing the low voltage fuse. When in ac position it turns on and runs. In the heat position it turns on and after about ten minutes the fuse blows. This is a mobile home unit where everything is in the outside unit, no furnace

    1. Hi David! I am guessing that you are referring to the fuse on your control board is blowing and not the high voltage fuse blowing. If it is a low voltage fuse problem then you have a low voltage short somewhere. Most of the time low voltage shorts are in the thermostat wires. We have a post about this on the following page: Since the fuse blows while in the heating mode then this could be in any of the thermostat wires that control your heaters or any of the low voltage controls like contactors or sequencers that are in your heater’s control panel. Sorry that I can not be of much help. I hope you can easily find and fix the problem. Steve

  6. My condenser keeps shorting out low voltage wires. Yesterday I ran voltage test back n forth and found that I wasn’t getting 24 volts at the contactor but the air handler was sending it. So I just used an extra wire and swapped it on both ends. Condenser Cut right on. 6 hours later , the same thing. So I repeated the same process and used an extra thermostat wire and it fired right up again. Any thought on why it’s doing this ?

    1. Hi Daniel! I have never heard of new low voltage wires shorting out that soon unless you have a loose connection or a short to ground somewhere else in your system. I am sorry that I can not give you any advice other than check all your connections to make sure they are good and tight and you do not have any shorted thermostat wire anywhere. I hope you can find the problem soon! Steve

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