I have a 2000 buick regal, transmission won't shift into overdrive?

I have a 2000 buick regal, I first recieved a code that said the tcc was stuck. I changed the transmission filter and fluid and seemed to have fixed the stalling problem. Now I have noticed my car will not shift into overdrive. While driving down the road at a speed of 70 to 75, I can shift it into drive and let off the accelerator, put back into od and it will shift into od, but as soon as I press the accelerator it downshifts into drive and stays there. Any suggestions? Please help, can't afford a professional shop.I have a 2000 buick regal, transmission won't shift into overdrive?It could be a bad sensor. The one that detects the load of the engine vs vehicle speed for the transmission to do the shifting, that is my best guess and I could be wrong. A good diagnostic for the car computer can tell you. Do you have any Engine Service Soon warning yet?I have a 2000 buick regal, transmission won't shift into overdrive?Sorry to hear about your problems. Unless you're getting other codes now that say different, it does sound like either the TCC solenoid or TCC Pulse Width solenoid is stuck. Unfortunately, the only way to get to these parts is through the side cover of the transfer case, requiring you to remove part of the steering and suspension components to remove the side cover. The solenoid itself is easy to replace, but getting to it is a HUGE PITA. I hate to give this advice, but being a home mechanic myself, there are still just some things I'd leave to a professional...this is one of those tasks. Since I assume you're out of warranty having a 2000, I recommend finding an independent shop over taking it to a dealer. They should be cheaper and more flexible with their labor costs. Just try as much as you can to find someone who is very knowledgable with automatic transmissions, particularly this one. Shouldn't be hard since the 3800 engine/4L65e tranny is a widely used powertrain in many GM cars. Best of luck!!I have a 2000 buick regal, transmission won't shift into overdrive?Sounds like you still have not really fixed the original problem. Aamco, Atlas and Cottman's transmissions will do a free diagnostic.