1995 Oldsmobile Code readers

Hi, I have a 1995 Olds Aurora and the check engine has been coming on. At first it only came on after idling for a coule of minutes but never when driving. Now it comes on almost imediately after starting the engine. I have tried to read the codes but I can not get the reader to work. I have also had the guys at the parts store try and they were also unsuccesful. When connected it sends back an error messege saying cannot read codes. Even though its a 95 I have to select 96 OBDII and use that cable to connect and it reads connected but will not scan. Is this a common problem? I think we peobably have a bad oxygen sensor as our milage has dropped a from around 19 city to 14. Also hetting more of that unleaded fuel smell and used to not smell that way.
Thanks

Mike

Update:
Just learned that 95 Aurora's were built during the change from OBDI to OBDII so it seems that they installed a OBDII port with a OBDI compatable computer so most code readers can't access the computer. Guess its time to take it to the shop.