My 2001 buick regal ls, will no longer shift into overdrive. all other gears are perfect.tune up, fluid and filter changed and still the same.

It is safe, and even recommended to use "Drive" and dis-engage the "Overdrive" when driving in the city, and definitely when towing...You can work on the problem by changing tranny fluid, and filter...or look into the Lucas Oil Transmission Stabilizer bottle treatment...Hope this helps.

You are lugging the engine. Overdrive is only for use in top gear when you get to cruising speed to reduce the engine speed.

Just having overdrive out would make it rev that high. It sounds like ur stuck in maybe 2nd gear. Did you add to much trans fluid possibly?

Alot of people have the same problem, including me. Changing the transmission fluid is not going to help this problem. THe transmission filter is not something you change as much as you clean and reinstall, its just a metal screen. Call Kia and ask for an "OVERDRIVE SHIFT SOLENOID" they will need the last 8 digits of your VIN, vehicle identification number, to locate your part. I am in the process of changing mine. The part number should be something like 46313-39051. That number is for a 2001 Kia Optima, with the 4 cylinder. There are three shift solenoids bolted to the bottom of your transmission should. I'll let you know what the difficulty level when I am finished.

throttle positioning sensor. http://troubleshootmyvehicle.com/ check this website, lots of good info

Try tuning the transmission by changing the fluid, filter and pan gasket. You might have to do it twice but in a few cases with newer vw beetles I have noticed a pattern of bad transmissions

Check the electrical connector above the modulator. If the connections are loose or the connector is damaged... similar things happened to mine.

change the tranny fluid and filter

Have you ever changed the transmission fluid flush and replaced the transmission filter?

This is a common problem on some GM cars. There is a pressure control valve in the side cover of the transmission that needs to be replaced. The transmission does not have to be rebuilt. You will need to get a side cover gasket also.