Possible reasons:
Electronic problems:
* Faulty gear selector
* Open or short circuit in wiring harness
*Faulty Transmission Control Module (TCM)
Mechanical problems:
*Front clutch pack malfunction
* Problem with pistons or solenoids
* Wear or damage to gearbox parts
Diagnostic process:
1. Check the transmission selector and wiring harness for signs of damage.
2. Use an OBD-II scanner to read transmission related trouble codes.
3. Check the level and condition of the transmission fluid.
4. Inspect the transmission for leaks or visible damage.
5. Perform a drive test to reproduce the problem and evaluate transmission behavior.
Possible solutions:
* Replace the transmission selector or wiring harness.
* TCM repair or replacement.
* Repair or replace faulty parts in the transmission, such as the clutch pack or solenoids.
* Transmission fluid replacement.