What does it mean?
Unlike manual transmissions, which the driver manually selects gears, automatic transmissions shift gears by switching hydraulic circuits to engage and disengage different gearsets. In fully-automatic transmissions, this can occurs without any driver input other than the accelerator pedal. On the other hand, automated-manual, and some automatic and dual-clutch transmissions enable the driver to change gears via push-button without the need for a clutch.
The hydraulic pressure is generated in the transmission pump and then distributed to the rest of the transmission via a series of valves, which can be both hydraulically and electronically controlled. Electronically-controlled valves are modulated and monitored by the transmission control module (TCM), which may be integrated into the engine control module (ECM). Depending on driver demand and engine and transmission loading conditions, hydraulic pressure needs to be adjusted.
For example, on light acceleration, one would expect the transmission to shift smoothly, perhaps imperceptibly. On the other hand, on hard acceleration, one would expect a firmer shift quality. In these two cases, pressure being lower or higher is automatically modulated by the hydraulic pressure control solenoid, also called the line pressure control valve.
The ECM monitors hydraulic pressure via the line pressure sensor (LPS), making continual adjustments in line pressure control (LPC) solenoid duty cycle as needed to achieve the pressure needed for specific driving conditions. Basic Hydraulic System Diagram – LPS and LPC Highlighted If the ECM detects a problem in the pressure sensor or the pressure sensor circuit, such as unexpected voltage, it will treat this as a fault and stop using the sensor for feedback.
The ECM will store a diagnostic trouble code (DTC) in system memory and illuminate the malfunction indicator lamp (MIL).