Possible reasons:
1. Low transmission fluid level:
*Leaks from transmission or transmission cooler
* Irregular transmission fluid changes
2. Worn or damaged gears and bearings:
* Long-term operation with low fluid levels
*Hard gear shifts
*Slippage when shifting gears
3. Faulty torque converter:
*Overheating due to wear or slipping
*Leaks from torque converter
4. Faulty transmission solenoid:
* Electrical problems
* Mechanical failures
5. Clogged transmission filter:
* Allows dirt and metal particles to enter transmission fluid
* Causes increased friction and overheating
6. Clutch slippage:
* Wear or damage to friction discs in clutch packs
* Slipping when changing gears
7. Transmission fluid is too thin:
*Using fluid that does not meet vehicle specifications
* Viscosity too low at operating temperatures
Diagnostics:
*Checking the transmission fluid level
* Gearbox diagnostics using a scanner
* Physical inspection of the transmission for leaks or damage
* Check the condition of the transmission fluid for the presence of metal particles or a burning smell
* Test drive to evaluate gear shift symptoms
Repair:
* Replacement of transmission fluid and filter
* Repair or replace damaged gears, bearings or torque converter
* Replacement of faulty solenoids
* Flushing the gearbox to remove contaminants
*Adjusting or replacing the clutch
*Using the correct transmission fluid according to vehicle specifications