I have a wall unit air conditioner that works fine when I plug it into any outlet or power strip. However, when I plug it into a timer or a thermostat timer, which is then plugged into an outlet, the unit won't work. I've tried various timers nada.

The end of the air conditioner plug has a box thing with a light that is normally green. It never turns green when in the setup I described, and pressing the test button does nothing.

Plug on end of air conditioner power cord

  • Picture of the "box thing"? Make and model of the AC unit? Are the timers grounded, or just two-prong? Commented May 30, 2016 at 16:28
  • @DanielGriscom The box thing looks like this kingersons.com/New_plm_plug.jpg, I don't have exact specs of the AC but will get that tonight, and the timers are grounded (they are meant for heavy duty appliances amazon.com/Lux-Heating-Cooling-Programmable-Thermostat/dp/…
    – Rachel S
    Commented May 30, 2016 at 16:38
  • If you plug a lamp into the timer, does that turn on and off at the expected times? Is the timer rated for the amount of power the AC needs?
    – keshlam
    Commented May 30, 2016 at 16:53
  • @keshlam yes, the timers are meant for heavy duty appliances, and they did used to work until recently.
    – Rachel S
    Commented May 30, 2016 at 19:50
  • Time to run some tests on that timer. It may be flat-out broken.
    – keshlam
    Commented Apr 11, 2023 at 23:42

1 Answer 1


It sounds like your air conditioner plug has a built in circuit breaker. If so, the breaker probably needs constant current to remain set, even when the unit is nominally off (a small trickle current is being used by the breaker and its light).

If that is so, the timer would prevent the breaker plug from getting current for its standby state. It then might have to be reset before the unit could start.

You can confirm this by plugging the unit into the wall outlet and getting it to start. Then turn off the circuit breaker at your main panel. The unit will, of course go off. When you turn the breaker back on, if the unit does not start up, you probably need to reset the plug breaker. This will mean the timer system will not work.

If a timer is critical to you, many newer AC units have a built in timer setup.

ADDENDUM As I think about it, the culprit is probably the ON/OFF switch circuitry in the unit itself. Once power is removed by the timer, the circuit goes off and will not restart until the ON/OFF button is pressed. Unlike a mechanical switch (which retains electrical contact whether or not power is present), an electronic switch is really a momentary contact switch that sends a pulse of power to the circuit that then locks itself on. It always has to be reset after loss of power. If so, the timer simply will not work.

Newer units with built in timers always have power to their timer circuits and that circuit then turns on the AC circuit at the designated time. External timers shut the whole system down, and it can't restart itself.

  • Thanks for the info. Will try that test tonight. If that's the case, how do I reset the plug breaker? The reset button on the plug doesn't actually press in (as if it's jammed). I have a thermostat timer which I really like since I can target the temperature a lot better based on times of day (cooler by day, warmer at night, etc. different for days of the week). The AC unit can't do that.
    – Rachel S
    Commented May 30, 2016 at 16:17
  • I forgot to add that this until always did work with a timer, without an issue. Only now did it suddenly stop working with a timer. It would go on and off in the timer without any problem (I did not have to press the power button on the unit each time).
    – Rachel S
    Commented May 30, 2016 at 16:35
  • I tried the test with circuit breaker. After turning the switch back on, the ac turned on along with the lights in the room and everything else.
    – Rachel S
    Commented May 31, 2016 at 3:23

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