2000 Cavalier idle problem O2 sensor?

I have a 2000 Cavalier LS, the idle jumps up and down for 750 - 2000 RPM, if it runs very long the exhaust/manifold will get extremely hot. I was told the the air/fuel mixture is wrong and that is why the exhaust is so hot. If I let it run for about 5-7 minutes and put it in gear to move it, sometimes it will back fire and die. I have a generic OBD2 code reader and all it tells me is that the idle is too high, also a local mechanic said he got the same code. Is it an O2 or other sensor, I believe this car has a couple O2 sensors. Any tests I can do?