Mopar Madman & Dartman are right. Just a "reminder" to have the emissions systems checked.
I bought a 90 brand new, and the light came on at exactly 60,ooo miles. According to the Chilton's Manual, there was supposed to be a "reset button" next to the glove box that would turn the light off, but it wasn't there. The Chilton book covered from 89-91, so maybe 89 has that reset button. I dunno. Anyway, I took it to my brother-in-law who ran a Firestone store at the time and he hooked the scan tool up and shut the light off. After 120,ooo miles it came back on, so I pulled the face of the dash off, and took the bulb out. Done deal.
On a side note, a scanner is not needed to retrieve the trouble codes from these trucks. Simply cycle the ignition key from "off" to "on" 3 times, leaving it in the "on" position. The CHECK ENGINE light will flash any stored codes. Just count the flashes and write the numbers down. The trouble codes are listed in the Chilton Manual. Code "55" means end of codes.
Hope this helps!!
George