The electronic throttle control is a system composed of the Throttle Position Sensor (TPS), a throttle body electronically controlled by an internal motor, two throttle position sensors used for correlation factors, and the engine management module.
When the accelerator pedal is depressed, a sensor indicates the desired amount of throttle opening. That information is sent to the Powertrain Control Module (PCM). In response, the PCM sends a voltage to the electric motor, which opens the throttle plate. Besides, two-position sensors are located in the throttle body, which converts the amount of throttle opening into a voltage signal.
The PCM constantly observes the correlation between the two voltages. When the voltages match, the system operates correctly. But if there is a difference in the voltages for a few seconds, the DTC P2138 OBDII code is set.