1997 GMC Truck

I cannot get my truck to pass the emissions test. The test stops at the o2 sensor and the printout says unsupported. I have had the dealer install updates in the pcm and eventually replace the pcm. They told me to start the truck in the morning put it in drive with the air conditioner running for 5 minutes and this will reset the new pcm. Sounds fishy. I have had this truck inspected 5 times and am getting frustrated. Also as a side note, the service engine soon light does not work.