Possible reasons:
Mechanical:
* Breakdown of gear teeth
* Sticking valves
* Wear of friction clutches
* Clogged pipelines
Electronic:
*Transmission ECU malfunction
* Speed and position sensors
* Wiring and connectors
Hydraulic:
* Low automatic transmission fluid level
* Contamination of the hydraulic system
* Valve unit malfunction
* Wear of seals
Troubleshooting steps:
1. Checking the automatic transmission fluid level:
* Check the fluid level with the engine warm and the gear selector engaged.
*Add fluid if necessary.
2. Fault diagnosis:
* Use a trouble code scanner to look for transmission related fault codes.
* Check fuses and relays.
* Inspect wiring and connectors for damage.
3. Troubleshooting mechanical problems:
* Replacement of faulty gears, valves and clutches.
* Pipeline cleaning.
4. Electronic troubleshooting:
* Replacement of transmission ECU, sensors or wiring.
* Elimination of corrosion and poor contact.
5. Troubleshooting hydraulic problems:
* Replacement of damaged seals.
* Cleaning the valve body.
* Flushing the hydraulic system.
6. Other actions:
* Reboot the transmission ECU (disconnect the battery for 10 minutes).
* Clean the battery terminals.
* Make sure the air filter is clean.
Note: Automatic transmission diagnostics and repair require special equipment and knowledge. It is recommended to contact a qualified auto repair shop for professional troubleshooting.