Possible reasons:
Transmission problems:
* Worn or damaged friction discs
*Hydraulic unit malfunction
* Problems with automatic transmission wiring
* Insufficient transmission fluid level
Engine problems:
*Loss of power due to faulty fuel system, ignition or mechanical problems
* Timing belt or chain slippage
Differential problems:
* Damaged differential
Other reasons:
* Brake sticking
* Problems with speed sensors
* Malfunction of the engine control unit (ECU)
Diagnosis and elimination:
1. Check the level and condition of the transmission fluid.
2. Conduct a visual inspection of the automatic transmission for leaks or damage.
3. Read diagnostic trouble codes (DTCs) using an OBD-II scanner.
4. Perform a transmission slip test to check the operation of the friction discs.
5. Check the pressure in the automatic transmission using a pressure gauge.
6. Check the electrical wiring of the automatic transmission for breaks or short circuits.
7. Inspect the engine, including the fuel system, ignition and mechanical components, for any problems.
8. Check the differential for damage or leaks.
9. Check the speed sensors and ECU for malfunctions.
Note: Accurate diagnosis and troubleshooting may require specialized equipment and knowledge. It is recommended that you take it to a qualified mechanic for proper service.