My 97 Accord air conditioning unit works, but not all the time. At times it works well, and blows out very cold air, and at other times; when I push the A/C button, the compressor never comes on. I have even pushed the A/C button and it did not come on right at once, and after a time, it just comes on by itself. I had my mechanic check it out before I knew it had this problem, and he said the freon was a bit low, so he charged it. But now, I never know when its going to work or not. Any suggestions on what the problem may be, or something I could check myself before taking it back to the mechanic?