* Low transmission fluid: If the transmission fluid is low, it can cause the transmission to slip and not shift properly. Check the transmission fluid level and add more if necessary.
* Dirty transmission fluid: Dirty transmission fluid can also cause the transmission to slip and not shift properly. Have the transmission fluid changed if it is dirty.
* Faulty shift solenoid: The shift solenoid is responsible for controlling the shifting of the gears. If the shift solenoid is faulty, it can prevent the transmission from shifting out of fourth gear. Have the shift solenoid tested and replaced if necessary.
* Faulty transmission control module (TCM): The TCM is responsible for controlling the overall operation of the transmission. If the TCM is faulty, it can prevent the transmission from shifting out of fourth gear. Have the TCM tested and replaced if necessary.
If you are unable to identify and fix the problem yourself, it is best to take your vehicle to a qualified mechanic for diagnosis and repair.
