Possible reasons:
Transmission fluid:
*Low fluid level
* Contaminated or old fluid
* Wrong fluid type
Switching valves:
*Dirty or worn valves
* Faulty valve control solenoids
Hydroblock:
* Internal leaks
* Worn or damaged seals
Torque converter:
* Torque converter lock slip
*Worn or damaged internal components
Electrical problems:
* Malfunction of control solenoids
* Problems with the wiring of the automatic transmission control module
Other:
* Worn or damaged clutches
* Damaged gears or bearings
* Faulty speed sensor
* Faulty automatic transmission control module
Diagnosis and elimination:
1. Check the level and condition of the transmission fluid. If necessary, add or replace fluid.
2. Check shift valves and solenoids. Use a scan tool to diagnose problems or check them mechanically if possible.
3. Inspect the valve body for leaks or damage. If necessary, replace the valve body.
4. Check the torque converter for slipping or other problems. If necessary, replace the torque converter.
5. Check electrical connections. Make sure there are no breaks or short circuits in the wiring.
6. Check for other potential problems. Replace worn clutches, gears or bearings as needed.
7. Reprogram or replace the automatic transmission control module. This can solve problems caused by faulty software or electronic components.