Dusty, the check engine light or o/d light never does anything strange pertaining to the tranny.
I was also wondering if while driving through town or city if it is a good idea to turn o/d off...it seems like when o/d is off the truck doesn't do so much useless shifting and it downshifts when you let off the accelerator letting the engine and transmission act almost like a brake, I kind of like it. Doesn't hurt anything does it?