Hi.
The first thing to do is replacing the transmission fluid. Metal debris
inside the transmission box gets mixed with fluid and can cause similar
problems.
The automatic transmission uses throttle position
sensor to determine position and opening of
throttle.
IF throttle position sensor is defective, the car may have problems shifting into one or more gears.
The
automatic transmission is controlled by an electronic control module,
that is energizing a set of solenoids to shift into gears.
The
control module is a rectangular box, often located behind the dash and
wired to transmission. The solenoids are attached to valve body,inside
transmission.
If a solenoid is defective, or if control module is defective, the car may have problems shifting into one or more gears.
Problems
concerning transmission control module, TPS and transmission solenoid
are usually detectable reading fault codes stored on the car main
control module (ECU/ECM).
Transmission problems are often returning a
code p0700 after an OBDII scan.
Stored codes can be retrieved using an OBD scan reader.
If the car auto transmission is not shifting into gear, talk to the mechanic about the problem.
Only when the car has a really high mileage, or when there is a factory problem there is need to replace the gearbox.
A damaged gear box at low mileage is rather unusual.