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I just put in a new wall AC unit this morning. My old one didn't start when I turned it on last week, a voltage sensor showed the outlet and wire was live, so I just figured the AC had finally died -- it was over 20 years old and I had been wanting to replace it anyway.

Well, the new one now also won't turn on connected to the same outlet. This outlet has worked before, I've lived here since 2019 and this AC has been turned on every summer. Since the AC is brand new, I suspect something wrong with the wiring.

Some general info:

  • I have checked the circuit breaker, it is not tripped.
  • The plug has its own breaker -- the light is not on, I can press "Reset" but it does nothing, "Test" is unpressable
  • When I flip the circuit off and then on from the breaker itself, the unit and plug light will both briefly turn on, and then about a quarter second later they will turn off
  • When unplugging and plugging back in, I get nothing -- neither the plug light nor AC turns on, even briefly.
  • The outlet is a 250V/20A outlet, and the AC unit is rated for 15A, 230/208V and 60HZ -- the exact same ratings as my last AC that worked up until this summer.
  • The outlet requires a tandem plug (I believe that's the name -- the two prongs that are typically vertical are horizontal on it) and I don't have any other devices that use that type of plug, so I haven't been able to actually plug anything else into the outlet -- but the voltage sensor does say it's hot.
  • Edit: A great suggestion was made to use an extension chord from another working 220V outlet to plug the AC in -- I unfortunately have no other receptacles of this type, the only other 220V outlet I have is a 4-prong receptacle and I haven't seen any extension chords or adapters that can convert between the two -- but I am going to do more digging to see if I can find some.
  • The voltage sensor also picks up a reading along the AC unit's power chord, all the way from the receptacle to the AC itself the wire will read as hot.

I've read something about a "loose neutral" before, but I think I've also seen that 250V outlets don't have a neutral.

I've tried swapping the outlet itself to no avail -- same behavior as described above, so it doesn't seem to be the outlet.

Is there anything else I can check myself? Or is this something I'll need to call an electrician out for?

Edit: Some great advice to get a multi-meter and I did so. Testing a working outlet, I get 120V - expected. Testing the outlet the AC is on, I get just under 100V out of an expected 250V. This feels like the issue. Also, I think it explains why it very briefly tries to start up -- it gets some power, tries to do things, then needs more and can't start up fully.

Could this be a loose wire somewhere in the box? Or is this going to be something an electrician is going to need to come out and take a look at? Really trying to save some money here if possible with a wedding coming up at the end of the summer.

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    Voltmeter readings are meaningless without a load. You'd need to measure the voltage across the wires to the AC while it is being turned on. If it drops down to 210 or lower, there is a wiring issue. Also, a way to check is with a high power extension cord to a known good 220 VAC outlet, if there is one.
    – DrMoishe Pippik
    Commented May 24 at 15:43
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    A voltage sensor is good to use before sticking fingers in. A meter will give voltage reading and a better idea of what is happening. The plug with the unworking test button might point to a defective unit.
    – crip659
    Commented May 24 at 15:47
  • the AC is brand new, just installed today, when you say defective unit do you mean that literally the whole AC could be defective? in terms of another outlet, i unfortunately only have one other 220V outlet but it uses a different receptacle type -- a 4 prong outlet rather than a 3 prong outlet. i haven't been able to find any good way to convert between these different outlet types, but haven't looked for a 4 to 3 prong conversion -- is that something you could find in an extension chord?
    – user34547
    Commented May 24 at 15:50
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    You mentioned using a voltage sensor. The non-contact sensors that you just hold near a wire and see if it is hot or not, can read hot when only one of the two wires are working (similar to the "loose neutral" you mentioned). A multimeter testing for voltage between both hots would determine if both are working. Also I'd be concerned about the non functional "test" button on your breaker.
    – izzy
    Commented May 24 at 15:58
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    When you turn the breaker off-and-on (3rd bullet) is it tripped? Feel and look at it before you turn it off, and again after you turn it on. Does it feel and look different? A tripped breaker may look "on" but might be a little wiggly, and might show an orange flag. Not every breaker is the same. If it's tripping, does it trip even if the A/C is not plugged in?
    – jay613
    Commented May 24 at 16:04

1 Answer 1

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With all your updates, this absolutely sounds like a loose connection or a bad circuit breaker. Two steps:

  1. If you're comfortable testing inside the breaker panel, remove the dead front and test the voltage at the circuit breaker. A bad circuit breaker will also be putting out low voltage at the terminals.

  2. If the voltage at the circuit breaker outputs is reasonable, work through all the connections of the circuit making sure that they are clean and tight.

Once that troubleshooting is complete, you will either have a working circuit, or a good reason to call in an electrician to run a new cable.

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  • How dangerous is testing the voltage at the breaker? I’ll also check all the connections, I’m really hopeful it’s something as simple as a loose wire.
    – user34547
    Commented May 24 at 19:34
  • There are plenty of ways to zap yourself inside the breaker panel, so it's not zero-risk. People do these sort of checks all the time, they just have to be careful.
    – KMJ
    Commented May 24 at 22:05

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