What does it mean?
OBD II fault code P055B is a generic code that is defined as, “Engine oil pressure sensor/switch B – circuit range/performance” and is set when the PCM (Powertrain Control Module) detects an abnormal or unexpected voltage or current in an electrical circuit that is associated with the oil pressure switch/sensor labeled “B”.
Note that on most vehicles that use two oil pressure switches/sensors, the switch/sensor that is labeled “B” is usually the switch/sensor that monitors the lower end of the allowable oil pressure range. NOTE #1: Note that code P055B refers specifically to an undefined or unknown failure, malfunction, or defect in one or more electrical circuits that are associated with the oil pressure switch/sensor labeled “B”, and NOT to an abnormally low engine oil pressure condition.
NOTE #2: While most modern vehicles have only one oil pressure switch/sensor that monitors the oil pressure in the engine’s lubrication system, many vehicles, such as VW and related VAG-group vehicles have two oil pressure switches/sensors. In these applications, one switch/sensor monitors the upper range of the allowable oil pressure range, while another switch/sensor monitors the lower range of the allowable oil pressure range.
NOTE #3: When the oil pressure in an engine with two oil pressure sensors falls below a minimum allowable threshold, the contacts in the switch/sensor close, which then triggers a low oil pressure warning light/message in the dashboard, as well as an audible alarm as an additional alert.
Although proper lubrication has always been important in internal combustion engines, it is greatly more so on modern engines because modern engines are not only built to closer tolerances than ever before; they also operate at higher temperatures and work at higher rotational speeds than ever before.
While many lubrication-related challenges that came with improved engine designs have been overcome with improved engine-manufacturing methods, new materials, and vastly improved engine oil formulations, the close tolerances in modern engines require relatively high oil pressures to ensure that all the moving parts in modern engines are properly lubricated.
Therefore, all modern engines are fitted with at least one dedicated sensor that monitors the pressure in the lubrication system, which information it relays to the PCM continuously. In terms of operating principles, oil pressure switches/sensors are typically either normally open or normally closed, and most are simple designs that depend on high or low oil pressure to act on an internal diaphragm to open or close a set of contact points to complete an electrical circuit.
Depending on the vehicle, though, some oil pressure switches/sensors contain pressure-sensitive elements whose electrical resistance changes in response to the pressure being applied to them. In the latter designs, the changing oil pressure produces changes in the PCM-supplied electrical current that flows through the resistive element, which changes the PCM then interprets as changes in oil pressure.
Moreover, most, if not all vehicles that are fitted with oil pressure gauges in addition to oil pressure warning lights use oil pressure switches/sensors with pressure-sensitive elements. Regardless of the design or type of oil pressure switch/sensor fitted to any given vehicle, though, the PCM on that vehicle requires valid, accurate, and plausible input data from the oil pressure monitoring system to prevent the setting of false-positive faults.
Moreover, on engines that are fitted with variable valve/camshaft timing, the PCM also requires accurate input data on the oil pressure to control these functions. Therefore, when the PCM receives implausible, unexpected, or otherwise suspect input data from the oil pressure switch/sensor, it will recognize that it cannot control one or more critically important engine control functions, it will set code P055B and illuminate a warning light as a result. Note that depending