Originally Posted by big_news_1
MAF sensor frequency or voltage
Intake air temperature
Long term and short term fuel trims
O2 sensor voltage (and general behaviors of the O2; i.e., is it at a constant voltage or jumping around)
Knock retard and knock sensor count
Misfires or misfire counts per cylinder
% engine load
These are the important parameters. You need something that will give you live data for these sensors and record data for later referencing.
If you have a laptop, you can't go wrong with AutoTap software. It will scan every parameter you need to know, including a lot of transmission info that other scantools omit. AutoTap is available from many different sources... Google it and see what kind of prices you can find. You want the package that will scan for GM enhanced parameters.
There are also stand-alone scantools. AutoXray makes a good line... the EZ Scan 4000, 5000, or 6000 will give you what you're looking for, but I think only the 5000 and 6000 come standard with GM enhanced parameters. The 4000 must be upgraded for $50 to scan the enhanced stuff. Actron also makes scanners that will do what you're looking for. The important thing, no matter what brand you choose, is to find one that includes GM enhanced parameters and is OBD2 (which is the kind of plug you have on your car).