A/T Upshifts Trough Gear Too Early
From stop the upshifts trough gear occurred too early this is because of the transmission have wrong fluid level, a faulty electronic sensor, throttle valve linkage or governor problem.
Wrong Fluid Level
For the automatic transmission to operate properly, the fluid level must be within the required level. The automatic transmission is very complex part and works at high precision which when the requirements is not met, like the flow of hydraulic fluid pressure, will make the transmission malfunction.
Park the car on level ground, let the engine idling at operating temperature then check the automatic transmission fluid level. Refer on the vehicle manual on how to check transmission fluid. Usually when checking the fluid level, you need to remove the transmission fluid dip stick then wipe the end of the dipstick which is submerged to the transmission fluid. Re-insert the dipstick all the way and pull it again. Now check the fluid level if there is enough fluid inside the transmission.
If the transmission lacks fluid or too much fluid then this is the cause why the automatic transmission upshifts through gear too early. Add some fluid if the fluid is low. Refer to the manual for the correct level of fluid. If the fluid is too much simply drain some fluid out until the correct level is met. Also, ask the mechanic to change the filter.
Faulty Electronic Sensor
This is only possible if your transmission is computer controlled. The automatic transmission changes to high gear when it is time to shift. This happens if the vehicle speed and the throttle position reach a certain level. Two sensors, the vehicle speed sensor and the throttle position sensor, is providing data to the transmission computer to know when to shift, because the upshifts is too early then it more likely that the sensor is faulty.
Bring the car to a shop that specialized in transmission repair and ask the mechanic to run some diagnostic test on the car. Problem in transmission is difficult to determine unless the actual transmission is check.
Throttle Valve Linkage or Governor Problem
If the car does not computer controlled transmission then problem on the throttle valve linkage is most like the reason of early upshifts. The governor purpose is to tell how fast the car is moving. When the gear upshifts are early the governor is also a possible problem.
First is adjust the throttle valve linkage, this usually fixed the problem. If the problem still occurs then a major transmission repair is necessary and it is very costly to have a transmission work on the car. Seek a several transmission repair shop to make sure about the problem before proceeding with a major transmission repair.