What does it mean?
OBD II fault code is defined as “2-3 Shift Malfunction” and is set when the PCM (Powertrain Control Module) detects a malfunction or abnormality in the hydraulic (or sometimes electrical) circuit that controls gearshifts between first and second gear on fully automatic transmissions. Gearshifts on fully automatic transmissions are accomplished by a microprocessor that controls shift solenoids that manage the effective transfer of pressurized transmission fluid between various hydraulic circuits.
By switching pressurized fluid from one circuit to another, different gear ratios are established by the reconfiguration of the planetary gears in the transmission. This reconfiguration has the same effect as changing gears on a manual transmission to produce different gear ratios. Automatic transmissions use an internal pump that is driven by the engine through the torque converter to maintain the pressure required by the transmission clutch packs to transmit power through a planetary gear set.
Therefore, when a shift solenoid fails, or inhibits the free movement of pressurized fluid from one circuit to another, there is insufficient pressure in the affected circuit to engage a gear, or to maintain the selected gear ratio.
Although design specifics vary greatly from one manufacturer to the next, all applications use input data such as the engine load and speed, transmission turbine speed, throttle position, and road speed to calculate the optimum gear ratio to suit the reigning operating conditions. By always ensuring that the selected gear ratio is appropriate for current operating conditions, engine performance is maximized, and the least fuel is used.
Code setting parameters also differ between manufacturers, but in general, code P0782 will be set when the PCM detects a mismatch between the desired gear ratio, and the gear ratio that the transmission is actually in at the time. When this happens, a trouble code will be stored, but the CHECK ENGINE warning light may, or may not illuminate, since some applications require several failure cycles before any warning lights will illuminate.
The image below shows the typical location of shift solenoids in an automatic transmission. Note the healthy, bright red color of the transmission fluid.