trouble codes on 95 chevy????

i have a 95 chevy s10 with a 2.2 and the check engine light is on. i took it down to the local autozone where they hook it up to the machine. it has a obd2 hook up but the machine keeps saying cant read code. i also have a 95 s10 blazer with a 4.3 and the check engine light is on so i took it to them and it has the obd2 hook up also so they hooked it up and it cant read the code on it either. ive tried everything i know. is there some the wrong with the code readings on the 95 year model chevys???