That's right! My check engine light has been on for 3 yrs! Finnally found the problem and thought I'd post it here to help someone else, perhaps.

I have a 1994 Cherokee RHD postal jeep XJ. (4.0 L inline 6cyl).
My problem was check engine light on with a code of fuel injector circuit malfunction (12-27 I believe). Causing an engine miss and errattic idling.

Narrowed the problem down after a few yrs. , to the wiring harness leading away from fuel injectors. Jeep would run better and light would go out after shaking harness. (but problem always came back in a few days).
Got about a two month fix by suspending harness from the firewall with zip-tie, at a higher than normal level. (this is the larger harness where fuel injector leads enter, up close to firewall).

When I had a week off, I finally worked up enough nerve to investigate the harness more closely. Cut off all of the factory tape and exposed the 50 or so wires inside the bundle.
Thinking it might be a broken copper wire inside the insulation, I wiggeled every wire and pulled every wire to check for streachiness in the outer covering. After two or three days of testing (not knowing exactly what I'm looking for)..... I finally found it!!

The six fuel injectors have two leads each, one of which is a ground. The six grounds are bundled together and have black tape and heat-shrink tubing on them. I cut this tape apart and found two of the wires were not soldered. They were just laying loose along side the others, not bonded.
There was no evidence of solder on the joint of the six wires. They appeared to be bonded at the factory with what looked like a crimp tool and electric weld. Needless to say, two of them must have slipped out of the tool before the electric bond took place. The factory put on the shrink tube and the wires were touching nicely for the first 11 years. O2 and heat made the wires have an intermittant connection, thus triggering the computer to store the code and put the check engine light on.