I check and recharge a/c units on cars all the time. One way to check and see if it's just low on freon like others said turn on the a/c and look at the compressor. In the middle of the pulley(where the belt runs on) check the center and you should see it spinning with it. If it is spinning and stopping over and over again that means you are low on freon. If it's spins constantly and not getting cold chances are you may have a leak in the line and its out. Look around the 2 a/c lines and condenser and look for bright green fluid. That's the oil in the system. If you see it that's usually where the leak is.
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