HVAC system (we're in Houston, it's still hot here) stopped working Tuesday. A/C guy came out, determined it was tripping breaker. Would trip immediately after resetting, but after a couple of resets would stay on for a few hours. Suggested we replace breaker.

I replaced the breaker. Same problem. A/C guy some back, swears that everything is good on the A/C side. Capacitors look good. Now it trips every time and won't stay on at all.

Electrician comes out. Examines breaker and panel, says it looks OK. Replaces line from panel to A/C just in case. Still trips every time. Electrician swears it must be a problem on the A/C side.

A/C guy refuses to come out a third time.

I disconnect the fan in the A/C - breaker still trips. Then I disconnected the leads to the compressor - now the fan will come on. So it seems like the compressor, or something in that chain, is causing the issue.

We have a different A/C guy coming tomorrow to look at it. I have a feeling they are going to say we need to replace it. I'm not sure how old the unit is - probably 10+ years, which seems too soon to replace to me. But that might be the best solution.

However, I want to be sure that we aren't doing this prematurely. What else can I check, or ask the A/C guy to check, that might be a cheaper/easier fix than replacing the entire system?

FWIW, we have an upstairs and downstairs unit. The busted one is downstairs. The slightly smaller upstairs unit is OK.

2 Answers 2


The condition you describe is usually an indication that the compressor is unable to provide sufficient start torque to turn without drawing excessive amperage. This is frequently due to a weak run capacitor on the compressor. Less frequently this can be caused by a contactor in poor condition or an old compressor where the bearings have worn to the point that the unit requires higher starting torque than normal. Given what you have already eliminated, the cure to this problem is very likely to be one of, or a combination of, the following:

  1. The run capacitor for the compressor is weak (most likely). This cannot be determined by visual inspection. If not already checked with a meter, then disconnect the capacitor and measure it with a capacitance meter (the main disconnect must be OFF). If less than 90% of the labeled value, then it must be replaced. Multi-meters with a capacitance scale are less than $20 from Amazon and others.

  2. The contacts on the compressor contactor are badly pitted. This can cause a high resistance connection in the contactor. Visually inspect the compressor contactor for any signs of heat damage. If possible (with the main power off), remove the arc-shield and inspect the contacts for pitting. If there are many deep pits (greater than 1/32"), then the contactor must be replaced.

  3. The compressor is a bit worn. To provide the increased starting torque necessary, install a "hard-start" kit for the correct horsepower unit that you have (roughly, your compressor motor will be 1 HP for each 12000 BTU cooling capacity). I prefer the 3-wire type of hard-start kit because it is more stout, although these are more difficult to find online than the 2-wire type. The 3-wire kits are readily available at A/C supply houses and perhaps your next technician can help you with this. If this is the problem then your unit may be on its way out; however, compressors can run many additional years with the correct hard start kit installed.

  4. Install a 3-5 min "delay on make" on the compressor contactor coil (a $10 part). If you install a 2-wire hard start kit, it is mandatory that you also have a 5 minute delay installed. (Note that if you cycle the breaker for the compressor while unit is running, the delay on make will not be triggered and you may still trip the breaker with this action).

  • 1
    Chris, this is awesome info, thanks for taking the time to write all of that up so clearly. When I disconnected the compressor to test, I did it at the contactor. I don't know if that's significant? Hopefully I'll have time to check a couple of these myself in the morning before the A/C guy gets here.
    – Jason
    Sep 4, 2015 at 2:49
  • 1
    Disconnecting the compressor at the contactor does not add any info that would help narrow this down.
    – user39367
    Sep 4, 2015 at 15:01

I'm having the same problem on an RTU American standard unit that requires two circuit compressors.

On arrival, I noticed that the breaker was tripped, so I tried turning it back on, but it tripped again immediately. I then made sure it was completely off and went on top of the roof to diagnose the unit.

I saw that the contactor was pulled in even when the unit was off, so I shut it down at the disconnect contactor still stuck in. I checked for continuity at the compressor prongs and only got continuity from C n R but nothing from R n S Reed OL. I unplugged the compressor from the line circuit and from the contactor it plugged into.turned breaker on and the unit worked fine with just that one compressor.

At this point I'm not sure if it's the compressor or the contactor being stuck that caused the issue.I'm hoping it's the capacitor and contactor that's bad and not the compressor.

So I would try that first before you change out your unit. Always shut off power before doing any work; check it 2 to 3 times be safe.

  • 1
    Reviewers - This doesn't quite seem to be a 'me too' situation; the answerer seems to have had the same problem and is showing what worked. It's just that it was not written very clearly; hopefully my edit will help there.
    – TylerH
    Nov 28, 2020 at 17:35
  • @TylerH "I'm hoping it's the capacitor and contactor that's bad and not the compressor" doesn't sound like an answer to me...
    – FreeMan
    Nov 30, 2020 at 16:28

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