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?