It sounds like the five-pin fault code reader might be designed to work with specific systems, and the non-catalyst Pinto engine might not be compatible with it. The reader could be expecting signals from certain sensors or ECU systems that aren't present or active in the non-catalytic version. The fact that it worked on the catalytic car suggests it's reading data from a different system that the older engine just doesn't have. You might want to check the manual for the reader to see what it's specifically designed to work with, or try a different diagnostic tool that's more compatible with your engine.