Service Engine Soon light question

I have a 1992 Safari 2WD V6 with 175000 miles and the check engine light will come on after I have driven it for a while. It doesn't always come on and it never comes on when I first start it up or let it sit and idle. The van runs pretty good, but shifts a little sluggish. Could it be an O2 sensor? Thanks for the help.

get the error code. any mech should have areader.
don't replace parts unnessarily.

yes it could be an o2 sensor or any one of another hundred or so possibilities.

Sorry I didn't post on this sooner. Turned out to be my VSS (Vehicle Speed Sensor). I already knew that it didn't work because when I replaced the tranny, I didn't replace the VSS correctly. Thanks for the help.