Possible reasons:
*Transmission problems:
*Low or contaminated transmission fluid
*Hydraulic control unit malfunction
* Mechatronics malfunction (for DSG)
* Wear of friction discs
* Engine problems:
*Low compression
* Malfunction of the ignition or fuel supply system
* Air intake clogged
* Electronics problems:
* Faulty crankshaft position sensor
* Faulty transmission input shaft speed sensor
*Faulty throttle sensor
*Other reasons:
* Clogged catalytic converter
* Exhaust system malfunction
* Clutch (for manual transmission)
Diagnosis and elimination:
1. Check the level and condition of the transmission fluid. If necessary, add new fluid.
2. Scan trouble codes. This may help identify the faulty component.
3. Inspect the gearbox. Look for leaks, mechanical damage or unusual noises.
4. Check transmission pressure. Low pressure may indicate worn or failing components.
5. Carry out engine diagnostics. Make sure the engine is running normally.
6. Check the sensors. Replace faulty sensors.
7. Check the catalytic converter and exhaust system. Replace them if necessary.
8. If necessary, overhaul or replace the gearbox.
Please note: A vehicle that stalls when shifting into gear can be dangerous. Don’t try to drive it before you fix the problem. Seek professional help from a car service center.