Oldsmobile/Buick Repair: service engine soon, throttle position sensor, better mileage

Question
1993 Buick Regal 3800 V-6 4spd automatic.  When light is on it won't shift into 4th.  Light goes off and it runs fine with low rpm's at cruising speed.

Answer
well jim .this is an unusual prob.but let,s give it some thought.the only connection between the service eng light and the transmission is a lock out torque switch.this part is located inside the trans on this vehicle.it,s primary function is to lock the torque converter at cruiseing speed to reduce slippage  hence better mileage.but on your car if you look down the very front of trans closest to the front bumper you will see a round valve there with a vacum line attached.follow that vacum up to the throttle valve make sure it is hooked up at both ends.if so then the throttle position sensor would be the next step but you will need to know the code stored first to pinpoint   billymac