I have an attic mounted Carrier Air Conditioner that is fairly new. Most of the year it works fine, but it consistently blows a 5A fuse when I first turn it on after it has been off for an extended period of time (e.g., turning it on for the first time in the spring, or turning it on after a week of cool weather).

I did not find any evidence of water accumulation in the unit, nor could I see any obvious failed capacitors.

Is it possible that the thermostat is wired incorrectly?

Context view of unit

The blown fuse is shown below (I replaced it prior to taking this photo).5A

Close up of the fuse that blows

  • Have you checked the thermostat wiring to the outdoor unit for damage? Jun 17, 2021 at 23:55
  • It's possible that the thermostat is wired incorrectly given that it enters the unit where the power is supposed to, but I doubt that's the problem if it at least works intermittently.
    – Mazura
    Jun 18, 2021 at 2:55

1 Answer 1


Fuse On the Circuit Board Blowing

Low Voltage Breaker Trip

In your case, you likely have a dead short somewhere in the wiring or a bad load that is going short to ground causing the fuse to blow. Furthermore, I would remove all the control wires from the board. Make sure you have a good fuse and then reattach them one at a time.

Furthermore, when the fuse blows you will know which wire is causing the problem. Or which wire has the dead short or bad load. Then it is a simple matter of tracing it out to make sure the wire is okay. If the wire is fine with no dead shorts, then you look at the load. An example of this would be a relay or contactor.

Relays and contactors have coils in them that are energized through the wire. If the coil gets a dead short in it then there is your problem. Replace the relay or contactor and problem solved.


If it's not the wires : "starting by checking the contactor in the condensing unit" – Circuit board fuse blows on AC switch

  • What do you mean by dead short? Is that different from a regular short?
    – T.D. Smith
    Jun 18, 2021 at 12:25

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