3.1 GM, probably the EGR valve. Actron makes a scanner that can read the obd 1.5 codes. you use their scanner with interchangeable ends. FAQ is here
ActronŽ
Any garage you take it too, make sure they understand the obd1.5 thing. Its was a very stupid idea that lasted 2 years and didn't even effect all the models in 94-95.
Good luck.