Automatic transmission diagnostic problems surface in many different forms, including dashboard indicator lights, slipping or harsh shifts, poor performance, and strange noises. Scanning transmission codes is an essential first step when performing a diagnosis. The information provided aides in determining the cause of the problem. Transmission problems can be divided into two categories: electrical circuit and all other.

Transmissions no longer use vacuum modulators, governors or kick down cables to control operational attributes. The functions of these elements are now handled by electronic speed sensors, throttle position sensors, and mass air flow sensors.

A scan tool can also help you check for communication faults between the PCM and transmission controller if the vehicle has separate computers. Problems here will typically cause the transmission to go into a “limp home” mode that may lock it in 3rd gear.