What does it mean?
OBD II fault code P0778 is a generic code that is defined as “Pressure control solenoid B -electrical malfunction”, and is set when the PCM (Powertrain Control Module) detects that the output voltage to the pressure control solenoid labelled “B” is either stuck at 0 volts, or is shorted to battery voltage when the solenoid is operating according to valid TCM (Transmission Control Module) gear shift calculations and strategies.
All automatic transmissions use varying degrees of hydraulic pressure to accomplish gear shifts, and to engage/disengage various transmission components such as clutch packs and the torque converter lock-up clutch. To make gear shifts comfortable and efficient, the PCM/TCM controls a series of pressure control solenoids to redirect pressurized transmission fluid from one circuit to another.
This process is roughly analogous to the way different gears are decoupled and engaged in different combinations in manual transmissions to bring about different gear ratios. However, in automatic transmissions the process of establishing different gear ratios depends on how efficiently pressurized fluid is channeled between hydraulic circuits, and should one pressure control solenoid (or its control circuit) fail, pressurized fluid cannot be controlled effectively.
When this happens the PCM or TCM will set code P0778, and may also illuminate a warning light. The image below shows a typical transmission pressure control solenoid.