Possible causes and solutions:
1. Low transmission fluid level
* Check the transmission fluid level using the dipstick.
* Replace or add transmission fluid according to manufacturer’s recommendations.
2. Worn or damaged brake pads
* Check the brake pads for wear or damage.
* Replace brake pads if necessary.
3. Faulty transmission control valve (PCM)
* The PCM is the computer that controls the transmission.
* Diagnose the PCM using an OBD-II scanner.
* Replace the PCM if necessary.
4. Faulty transmission speed sensor
* The transmission speed sensor tells the PCM about vehicle speed.
* Diagnose the transmission speed sensor using an OBD-II scanner.
* Replace the transmission speed sensor if necessary.
5. Faulty valve body
* The valve body controls the flow of transmission fluid.
* Check the valve body for leaks or damage.
* Replace the valve body if necessary.
6. Stuck or worn pressure relief valve
* The pressure reducing valve regulates the pressure of the transmission fluid.
* Check the pressure relief valve for sticking or wear.
* If necessary, replace the pressure relief valve.
7. Worn or damaged transmission
* A severely worn or damaged transmission can cause the vehicle to roll away.
* Have the transmission inspected by a qualified mechanic.
*Repair or replace the transmission if necessary.
Warning:
Do not attempt to repair the transmission yourself unless you have experience. Contact a qualified mechanic for diagnosis and repair.