Possible reasons:
Mechanical faults:
* Gearbox malfunction (breakage of gears, wear of clutches)
* Problems with the gear shift drive (shaft rods, cables or electronic control unit)
Electrical faults:
* Malfunction of the reverse gear solenoid in the automatic transmission
*Transmission wiring harness or electronic control unit (ECU) problems
* Transmission sensor malfunction
Other reasons:
* Low fluid level in automatic transmission
* Dirty or spoiled automatic transmission fluid
* Faulty automatic transmission mode switch
* Gear selector lock
Diagnostics:
1. Check the level and condition of the automatic transmission fluid.
2. Inspect wiring and connections for damage.
3. Check trouble codes using a diagnostic scanner.
4. Carry out mechanical diagnostics of the transmission, including inspection and testing.
Solution:
1. Repair or replace the faulty transmission.
2. Replace or repair damaged gear shift drive parts.
3. Troubleshoot solenoids, wiring harnesses, or ECU.
4. Add or replace automatic transmission fluid.
5. Replace the faulty automatic transmission mode switch or gear selector.