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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8283
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Goodman AC and gas heater unit and it keeps blowing

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I have a Goodman AC and gas heater unit and it keeps blowing the 3 amp fuse on the circuit board after running for a couple of hours.
Welcome to Just Answer!.

There is an electrical short circuit in one of the heating controls or the wiring... these shorts can be very hard to locate..but it can be done if we take one step at a time.

My first guess is that as the printed circuit board with the fuse in it warms up during its operation.... it is shorting two circuits together inside the card.... and blowing the fuse.

Thats an educated guess. We can test that by using a hair drier to warm the card up very slowly and gently so that it is warm to the touch (120F or so, but not hot).

If that causes the fuse to blow sooner than it usually does, then the problem is in the card and the solution is to replace it.

If warming up the control card does not make the fuse blow faster, then the short is in another component, such as the gas valve, or a thermostat or control wire rubbing on the sheet metal and sorting out.

I will keep this question open for you as long as you keep rating my assistance positively.


Phil and 2 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

Thank you. I will give the hair dryer a shot and let you know.

Customer: replied 4 years ago.

Ok, so here is where we are a little over a week later. We have replaced the circuit board, the capacitor, and thermostat in the upstairs attic portion of the unit. When none of that worked my husband focused on the outside portion of the unit. He first replaced the contactor because it had what looked like a burned spot on it and then, when that didn't work he replaced the capacitor on that portion of the unit. When that did not work he investigated the wiring further and found the thermostat wires to be fine, but the whip that ran from the side of the house to the outside portion for power was filled with water and the wires inside had some cuts in them so he thought that perhaps that was causing the short so he replaced the whip finally last night. The unit ran fine all night last night, cooled the whole upstairs to 75 degrees, it ran this morning and then we went out this afternoon and came back around three and the 3 amp fuse was blown again and as a result the unit wasn't working. He replaced the fuse again and it ran for about 15 minutes before blowing the fuse and shutting down again. Since it is in the attic and it is extremely hot up there we waited several hours and tried to replace the fuse again and now it blows instantly. So that's where we are. Any ideas?

Hello again, before I forget let me say that each time a part is replaced there is the chance of an error... its best not to replace parts before testing and finding them bad.

None of those parts would cause an actual high voltage condition... the problem is in the voltage sensor the company uses most likely 98% chance thats the case.

The 3 amp fuse is related to the 24 volt control circuit... and would normally not cause a 'high voltage' read on the hi/low voltage sensor read out.... those are two different things.

the whip that runs from the side of the house to the unit could have been the problem judging by the water in the protective sheath.. and cuts in the wire insulation would cause the 3 amp fuse to blow... the cure for that is to buy another length of thermostat wire with the same colors of conductors and replace the thermostat wire you have...BE VERY CAREFUL TO MAKE A GOOD SKETCH OF THE WIRE CONNECTIONS...turn the power off to the unit and the furnace/ air handler inside when you replace the wire.


I need more data on the other issue, the unit going off on 'high voltage'... or by that do you mean just this blow 3 amp fuse.

We can go from there. I will be in for another hour or two tonight, then back in the morning. There are no time limits here.

Thank you for the positive rating!

Customer: replied 4 years ago.

He did replace the whip outside, and like I said it ran find after that for a good 12-13 hours straight last night and this morning. Now it is just blowing the 3 amp fuse again.


Hello again, at this point we have what is called an 'intermittent problem'.. that means it works great for hours or days... then fails..

Those can be quite difficult to trouble shoot because the system works when you test it most of the time.


You have replaced all of the suspect parts and wiring except the wire that runs to the thermostat on the wall.

That wire is generally stapled to the wall studs on its way to the thermostat, over time the staple can cut the wire insulation and cause a short... and blow the fuse.

Replacing that wire inside the walls is a difficult proposition..but thats what we need to look at next.


Tell me how far the thermostat is from the furnace... if its not too far, we may be able to pull new wire.

If its a long run inside the walls, we will have to use a megohm meter to check the resistance wire to wire after disconnecting it from the thermostat and the furnace/air handler.

any damage to the wire insulation will show up as a failed megohm test.
Most electrical appliance parts stores carry megohm meters in the $90 to $100 range.

let me know how all that goes,


Customer: replied 3 years ago.

Good Morning Phil,

The thermostat is approximately 15 feet from the furnace. When my husband gets home this evening he will pull the new wire for the thermostat because he said its not that big of a deal. Its just a couple of feet up the wall and then a straight shot through the attic. He is wondering if he should replace the wire that then runs to the outside portion of the unit as well. What are your thoughts on that? Do you think he should just do the thermostat to furnace wires first?

Thank you so much for all of your help so far.


Hello again Jennifer,

Thats one hard working husband you have there!

We need to be more specific about the wire that runs to the outside unit. Earlier you said the thermostat wire (whip) had been replaced. So now I am confused.

(There is also high voltage power wiring to the outside unit... that should not be a problem or any concern relative to blowing the 3 amp fuse)

If there is an intermittent short in the *low voltage thermostat wiring* running from the thermostat to the furnace/ air handler... that will blow the 3 amp fuse, if there is a short in the *low voltage thermostat wiring* going from the furnace/air handler to the outside unit, that will blow the 3 amp fuse.

If there is a short in any of the devices that wire directly to the PCB, printed circuit board with the 3 amp fuse in it, that will blow the 3 amp fuse.

Let me know I am the same page with you on this or not, we can go from there.

Customer: replied 3 years ago.

Yes he is! And I apologize for the mixed up information. I am trying to be the go between because he works in the field and can't really stopping drilling to post. :-) The whip that was replaced outside was the high voltage whip that powers the outside portion of the unit. The protective casing was worn through and those wires were sitting in water. Even though the wires were within the rubber tube portion of the whip they had splits in their covering so he replaced it all together just to be on the safe side. Didn't really want wires with bare spots sitting in water. So, he has NOT replaced the thermostat wire leading to the outside portion of the unit. So do you think that would be his next step? He is very adept at this type of thing and usually can figure it all out on his own. Last year he replaced the motor and such in the downstairs unit when it went out and it has been fine ever since. This one, the actual making the repairs he is good to go, its figuring out the 'what needs to be repaired' that has him stumped.

Thank you so much again and I will give more and more feedback as we go through this. You have been very patient and helpful.


Hello again !

If the 3 amp fuse only blows when the system is in cooling mode, replace the *thermostat wire going from the air handler to the outside unit first....or... inspect it closely where it passes the sharp edges of the sheet metal... that is a common location for shorts.

Less likely is the thermostat wire shorted in the wall on its way to the thermostat...

Let me know how that goes,

Customer: replied 3 years ago.

I will let you know. Thanks!

Finding intermittent shorts can take a while.

Customer: replied 3 years ago.

Good morning Phil,

My husband replaced the thermostat wire running to the outside unit last night and so far so good. There was one place that he found on the wire where the protective coating had been stripped (about a half inch long) that had the copper of the wiring exposed. Hoping that this is the issue. Its been running for about 8 hours now so I guess we will see what happens when it gets really hot this afternoon. Will let you know and thank you again so much for your assistance with this.


Good work! Stay in touch as necessary.


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