Picked up a great little OBDII Digital Code Reader made by Innova at Canadian Tire today.
And it was only $100.
I was amazed at how many codes the computer on this car can throw.
Over 600 Generic and 300 GM specific codes.
I needed it to diagnose and turn out the CES light on the dash. Once it tells you the code(*) you can erase them if you want.
The code turned out to be P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
A consequence of running with a drilled 180 Thermostat.
Seems that you can't do too much with these '96 and newer cars without the computer letting you (and your service manager) know.
I'll keep on playing with mods for this car until I throw a P1257 code (Supercharge System Overboost)