EEC-IV systems (OBD-I only) were still used in some 97 HD trucks, especially if it was an OBS with one of the pre-modular or diesel engines. If it was originally sold in CA, then it should have been a OBD-II regardless. If it was a 49-state model, it probably still had the EEC-IV system.
Consult the emissions decal under the hood if it's still present. The system is specified on the label.
By 1998, all trucks were running OBD-II hardware although they were programmed for OBD-I standards for the HD trucks.
Post your VIN and someone will decode it for you.
Last edited by projectSHO89; 10-25-2009 at 08:39 AM.