Time to time people have posted with a persistent check engine light where there wasn't an obvious cause. I had this, with the code relating to long-term fuel trim. It would not reset (using the under-hood diagnostic box which 1995 and earlier have) no matter how I tried. Previously I could easily reset any codes I got, though they came back sooner or later if something was really wrong. For instance the slow front oxygen sensor response code came back every several weeks till I replaced the sensor.

I found that the intake air hose (the flexible bellows between the air mass meter and the throttle body) had dried up some and developed several cracks that were hard to see. This wasn't bad enough to be obvious in the engine's performance.

When I replaced the hose the check engine went off by itself. I did not have to try resetting it again.

Just a hint in case others of you with the older vehicles experience a similar problem.