2001 Isuzu Trooper Check Trans light?

My Trooper's Check Trans light came on today, but there is no obvious problem with the transmission. It shifts fine. However, the last couple of days, on occasion when I go from park to drive, it just revs up. However, after I put it back into park and into drive it, it works. I took it back a foreign car mechanic, and he checked it for me and told me he really didn't see any thing wrong with it. I haven't had the SUV very long, and he told me the transmission had been rebuilt recently. Does anyone know what the problem may be? Could it be something electrical or maybe it just needs new fluid?