I have a 95 chevy 1500 5.7l v8 automatic 2wd. The check engine light is on, so I took it up to a mechanic. They hooked it up to the machine and it didnt even register that it was hooked up. About a year ago I used a jumper and counted the flashes to get my codes. I told the mechanic and so he also hooked up the jumper and still no reaction. Where do I start?