Problem with running to parts store to read codes is, well, your not a Hot Rodder doing it yourself! But more importantly none I know of have equiptment to read OBDI codes anymore.
So you could buy a OBDI code reader. Or invest a little more for like a Auto Xray 7000 and do all OBDI and II for about $300.
But TunerPro RT software is free to try and only $39. We encourage everyone to pay so we get more updates. The software itself is incredible. Definition files are free and communitee written and can do all GM and lots more.
TunerPro does the scan end to read codes but also record data needed to tune with an ADX file. Then with an XDF file you can make changes to your bin (file in your PCM). I wrote the $EE/LT1 XDF and ADX files along with some others.
So with TP and a cable from laptop to ALDL port, about $50. you can not only scan for codes but record data needed to tune the PCM. Open your bin file, change it.
TunerCat WinFlash is the program to read and reflash your PCM for $20. Other OBDI GM vehicles have a chip that needs to be pulled, erased and programmed with a burner which costs about $100.
SO if you have a laptop and some time, for $110 you can read codes, record data adjust bin file and reflash PCM. Tune the PCM yourself. Now that's Hot Rodding!