* Low transmission fluid level: When the transmission fluid is low, it can cause the transmission to overheat and not shift properly. Check the transmission fluid level and add fluid if necessary.
* Dirty transmission fluid: Dirty transmission fluid can also cause the transmission to overheat and not shift properly. Change the transmission fluid every 30,000 miles or so.
* Faulty transmission solenoid: The transmission solenoid is responsible for controlling the shifting of the transmission. If the solenoid is faulty, it can cause the transmission to not shift into overdrive. Have the solenoid checked and replaced if necessary.
* Faulty transmission valve body: The transmission valve body is responsible for directing the flow of transmission fluid. If the valve body is faulty, it can cause the transmission to not shift properly. Have the valve body checked and replaced if necessary.
If you have tried all of these things and the transmission still does not want to shift into overdrive, you may need to take it to a transmission shop for further diagnosis and repair.
