The tenant has been living here rent free for 2 months because the eviction process is so slow in California, I turned off the breaker switch to the AC so he couldn't run up the bill anymore, but the guy used to work for a heating and AC place before he got fired. He did something to make it so the circuit breaker no longer has any control of the AC unit's power supply. How do I figure out what was done and can he get arrested for it?

It's easy to do: he either connected it to another circuit, or simply turned the circuit breaker back on.

But you are probably violating the law against ousting tenants without first getting a court order. I don't know about California, but most places have such a law and I expect California's law would be as severe (upon landlords) as any of them. He could end up getting a money judgment against you, if you continue to try to "kick out" your tenant using "self-help".

I will agree, actually you are breaking the law, by turning it off. You can not turn off utilities allowed in the rent, in attempts to force eviction..
He can not get arrested for doing this, while it may be a code violation on the house, you would most likely end up liable for it. And yes, most likely in court, if there is another eviction hearing, you turning it off, will go against you.

Good reason to remember that you get a larger deposit upfront, start eviction as soon as there is any issue, and really important, don't keep utilities in your name, make renter put in their name.

Depending on what the tenant did regarding the wiring of the A/C, OP may seek an expedited eviction, citing safety concerns. I made a guess yesterday (based upon what limited information OP gave us) as to what he did, but if OP wants to have an electrician look at it, it may be that an unsafe condition was created, which might persuade the court to speed things up.

After reviewing OP's initial post, I see that " the circuit breaker no longer has any control of the AC unit's power supply". This suggests that tenant did more than simply turn the breaker back on; he probably connected the A/C to another circuit (either by connecting the wires to another breaker or to another place in another circuit). Or, I guess, he could have connected the A/C to power "upstream" of the breaker box, which would definitely constitute an unsafe condition. But, as I suggest, OP should have an electrician look at it.

I'm going to agree with AK here. You need to get an electrician in immediately to see what was done.

But if this was central air then shutting it off could get you in hot water. So you can have the electrician fix what was done, but leave the A/C on,