What does it mean?
OBD II fault code P0751 is a generic code that is defined as “Shift solenoid (SS) “A” -performance or stuck off”, and is set when the PCM (Powertrain Control Module) or TCM (Transmission Control Module) detects an abnormal signal voltage, or a voltage that indicates that the transmission shift solenoid labelled “A” is stuck in the “OFF” or closed position.
All automatic transmissions use electrically operated, and computer controlled solenoids to shift pressurized transmission fluid between hydraulic circuits to affect gear shifts, and regardless of the number of gear ratios any given transmission has, each gear is selected/deselected by a dedicated shift solenoid.
For instance, while some older transmissions may have only four shift solenoids labelled “A”, “B”, “C”, and “D”, newer transmissions usually have more ratios, with the additional shift solenoids labelled “F”, “G”, and “H”, for a total of seven shift solenoids.
In terms of operation, when a gear shift is initiated the solenoid that controls the next higher (or lower) gear is energized by the PCM/TCM, which redirects pressurized fluid into the hydraulic circuit(s) that controls that particular gear ratio. Thus, when a particular hydraulic circuit is pressurized, all the transmission components that select that gear are moved into position.
The pressurized fluid then maintains pressure on clutch packs, which in turn, prevents the rearranged planetary gears from disengaging. This series of events is roughly analogous to the way different combinations of gears are selected to establish different gear ratios in manual transmissions. When the PCM/TCM detects a problem with shift solenoid “A”, the controller can no longer control gear shifts efficiently, and it will set code P0751, and illuminate a warning light as a result.
The image below shows how shift solenoids are typically arranged. Note though that shift solenoids are often not accessible from outside of the transmission, and in these cases, the transmission oil pan must be removed to gain access to individual shift solenoids, internal wiring, or sometimes to a shift solenoid pack that must be replaced as a complete unit.
Note that the arrangement of shift solenoids in this image is for illustrative purposes only; refer to the manual for the affected application to determine the exact location of shift solenoid “A”.