A shift solenoid B malfunction is the failure of shift solenoid B in an automatic transmission to reroute the transmission fluid's flow that changes the shift valves' positions in the valve body. The failure prevents the transmission shift from second to third gear. A powertrain computer displays OBD II trouble code P0755 when the number of revolutions per minute does not change because the transmission did not shift, and the vehicle's speed sensor does not show a road speed increase.
A transmission control unit use solenoids to control functions mechanically, based on preset commands from transmission and engine sensors. When a vehicle has two solenoids and four gears, its transmission may use solenoid A to drive in first gear, solenoids A and B to run in second gear, solenoid B alone in third gear, and neither solenoid for fourth gear. Indications of a shift solenoid B malfunction include reduced fuel economy, an illuminated check engine light and a vehicle not shifting from second to third gear correctly. Some drivers may experience misfiring symptoms and decreased performance. It is unusual for drivers not to notice problems.
The OBD II trouble code appears due to defects in the shift solenoid wiring, connector or the shift solenoid itself; constricted hydraulic passageways due to dirty transmission fluid; and dirty valves. Mechanics often misdiagnose the malfunction as a driveline or engine misfiring problem. The malfunction creates noxious gases, such as oxides of nitrogen and hydrocarbons, that create and add to smog, and carbon monoxide, which is odorless but deadly.