1996 GMC Jimmy Computer will not self test

Brought vehicle in to be inspected. Failed do to a faulty O2 sensor. Had garage replace all four sensors. Drove vehicle 200 miles and brought back to be reinspected. The computer said that the EGR, Catalyst and Evap systems were not ready to be tested. AFter two more attempts the garage finally said the computer was bad and needed replacing. Had that done and still have the same problem. The computer will not self test those three systems and the truck therefore cannot pass inspection. They are at a loss and told me they can no longer help. The check engine light is not on and the vehicle runs great.