The Overdrive dash light comes on as I engage the stalk switch but the automatic transmission won't shift into Overdirve, though it works fine in the first three gears, any solution, could it be the throttle position sensor?

The button on the shift lever labeled "Overdrive" cancels the overdrive when pushed in. This is for towing, going up steep inclines, and going down those same inclines.

With the switch out, the overdrive should engage once the motor is warm.