Why would computer codes be unreadable on a 1995 Ford Contour?

[igglydooble]

My check-engine light was coming on intermittently over the course of a few weeks, and the car seems less powerful both accelerating and cruising. Once it got very unresponsive, backfired once or twice, and pretty much limped home. The tank was below 1/4 so I filled it and since then it's been stable (but with the same reduced power). Then it started getting louder and the exhaust got quite smelly. So I took it to a mechanic. He says the exhaust smell is due to a broken flex pipe. But he says he's unable to read the codes out of the engine computer, and that concerns him (and me). Do you know why codes would be unreadable? Could damaged wires cause the problem? -- I recently wrapped electrical tape around some wires whose insulation had cracked & fallen off. If there are others in bad shape, perhaps that is messing something up -- ? What should we do about the check-engine light if we can't read the codes to diagnose problems? Is there a way to read them that my mechanic isn't thinking of?

Any help from someone who understands these things would be greatly appreciated!