Our air conditioner recently stopped blowing out cold air. When I went to check on it, I noticed that the outside unit wasn't coming on. We had had problems with ants in the contacter in the past, and so I went to take off the lid and check it out. I noticed that the circuit breaker marked "AC" was tripped. Like a good DIYer, I turned it all the way off and back on. It immediately tripped again. In an effort to isolate the problem, I went to the outside unit and pulled the safety connection to disconnect the main power of the outside unit. When I went back and reset the circuit breaker as before, the breaker didn't trip. Reconnecting the outside safety switch caused the breaker to immediately trip again. Later, we tried turning off the air conditioning in the house, resetting the circuit breaker, and turning the AC back on. The circuit breaker didn't trip as soon as it was reset, but when the AC was turned back on inside, it immediately tripped as before.
My question is what should I check in this situation? This seems like it can't be an uncommon problem, so I'm sure HVAC professionals encounter this frequently and have a checklist of things to try. What are the possible causes, remedies, and safety precautions I should consider in this situation?
Answers to some comments: No, I didn't see any obvious damage to the wiring near the outside unit. I did look pretty carefully, since I had already taken the panel off to check for ants in the contacter. I didn't try to spin the fan with my hand, and I'm not in a place that I can try this right now. I will try it later today; perhaps in the meantime that could be a step in the diagnosis of the problem if anyone gives an answer between now and then? Likely, future readers of this question will encounter the same problem and some will have a fan that spins by hand and some will have one that is stuck.