Possible reasons:
1. Transmission problems:
* Torque converter failure: This device transmits torque from the engine to the gearbox. If it is faulty, torque will not be transmitted to the rear wheels.
*Transmission malfunction: Internal components of the transmission, such as gears or synchronizers, may be damaged or worn, preventing reverse gear from engaging.
*Low Transmission Fluid: Insufficient transmission fluid can cause slipping gears or complete transmission failure.
2. Electrical problems:
* Faulty gear selector: The gear selector may not send the correct signal to the transmission.
*Wiring Problems: An open or short circuit in the wiring can disrupt communication between the gear selector and the transmission.
* Faulty Transmission Controller (TCM): On some Passat models, the TCM controls gear shifts. A faulty TCM can cause the transmission to malfunction.
3. Mechanical problems:
* Damaged Drive Shaft: The drive shaft sends power to the rear wheels. A damaged shaft may prevent the vehicle from engaging reverse gear.
* Faulty differential: The differential distributes power to the rear wheels. A faulty differential can cause the rear wheels to not receive power.
Diagnostics and repair:
1. Check the transmission fluid level.
2. Carry out computer diagnostics.
3. Inspect the drive shaft and differential.
4. Check gear selector and wiring.
5. If necessary, replace faulty components.
Please note that transmission repairs can be complex and expensive. It is recommended that you have the problem diagnosed and repaired by a qualified mechanic.