Possible reasons:
Mechanical:
* Wear of friction discs or gear belt
* Clogging of the valve body with chips or dirt
* Torque converter malfunction
* Wear of planetary gear bearings
* Breakage of gear teeth
* Insufficient level or poor quality of transmission fluid
Electrical:
* Malfunction of gear control solenoids
* Open or short circuit in wiring
* Malfunction of speed or gear selector position sensors
Hydraulic:
*Damage to oil pump
* Pressure regulator malfunction
* Clogged oil channels
* Transmission fluid leaks
Diagnostic steps:
* Check the level and condition of the transmission fluid. Low level or contaminated fluid can cause slipping.
* Scan your vehicle for trouble codes. The codes may indicate specific faulty components.
* Check the condition of the friction discs and transmission belt. Worn or damaged components may slip.
* Check valve body and hydraulic system. Make sure valves and passages are not clogged and are operating properly.
* Check wiring and solenoids. Make sure all components are intact and connected correctly.
Repair:
* Replace worn or damaged components.
* Clean or replace the valve body and oil filter.
* Repair or replace faulty solenoids or wiring.
*Add or replace transmission fluid.
Warning:
A slipping automatic transmission can damage the transmission. If you are experiencing this problem, take your vehicle to a repair shop as soon as possible.