trouble retrieving trouble codes..HELP!!!!

the ses light on my 95 gp has came on. I took it to autozone for the scan,but had the wrong aldl hook up.So he went to get the jumper key to make the ses light flash the trouble code and it stayed steady,not even the code "12". Any one have this problem before?.....I'm at a loss. Also, let me add, that it came on right after installing a junkyard transmission, but there seems to be no problems with shifting or anything. In fact the car is running beautifuly.I'm stumpt......PLEASE HELP :confused: :confused: :confused: