What does it mean?
OBD II fault code P0868 is a generic code that is defined as “Transmission fluid pressure (TFP) sensor –low”, and is set when the PCM (Powertrain Control Module) detects that the transmission fluid pressure is lower than the minimum allowable value. All automatic transmissions use pressurized transmission fluid to engage/disengage clutch packs and other components such as the lock-up torque converter clutch in order to accomplish gearshifts.
However, this pressure is not constant, and in some cases, pressure is temporarily relieved when certain transmission are moved. In a fully functional system, an internal pressure pump (that is most often driven by the engine) provides the pressure. The pressurized fluid is then directed by the PCM into various hydraulic passages via electronically controlled solenoids and other valves to cause the transmission to shift.
Thus, when the pressure falls below a limit set by the manufacturer, the PCM or TCM (Transmission Control Module) is no longer able to initiate gearshifts, since a set minimum pressure is required to initiate a gearshift on the one hand, and to maintain the gear selection on the other hand. To maintain a selected gear, the transmission fluid pressure must be sufficient to keep the currently selected clutch pack from slipping.
To monitor the transmission fluid pressure, all transmissions are fitted with pressure-sensitive sensors that transmit the actual fluid pressure to the PCM on a continuous basis. In most cases, this sensor is supplied with a 5-volt reference voltage by the PCM. As the fluid pressure changes, the resistance in the sensor also changes, with more or less current (known as the signal voltage) passed back to the PCM.
The PCM interprets the received signal voltage as pressure, and will set code P0868 and illuminate a warning light when it senses that the transmission fluid pressure has fallen below a specified value. Note that in some cases, the PCM may also initiate a fail-safe or limp mode in order to protect the transmission.