I have a 1994 chevy p/u silverado with a 5.7 engine. It has 306,800 miles on it. Recently at high speeds above 70 the truck will miss like a plug wire, fuel injectors, or an electrical problem. How do you check for codes. I know you can jump A and B on the diagnostic plug. There are no letters on the plug to show A and B. Does anyone know which pins they are? I replaced plugs about 3 months ago. I need help because I have a long drive on Monday.