I have a 2001 Buick lesabre custom automatic v6 and the transmission will shift into reverse fine but it wont shift into drive I have to shift it to 1st gear and I can drive it what could the problem be?
The thing to do is get the computer scanned for codes. It's possible you have transmission codes in the computer but the check engine light is not on. Some codes will not turn on the engine light. You may have a shift solenoid problem. But normally this will not cause the loss of drive range gear.
Most of the time the problem you are describing is an internal hard part inside the transmission. Check the fluid level and condition. If it's low and fluid if it's dirty change it. If none of this helps the transmission pan needs to be removed to check for debris in the bottom. Some dirt in the bottom of the pan will be normal on a 12-year-old car, what you are looking for is chunks or large pieces of metal. If you do find some then the transmission is going to need to be removed, repaired or replaced.