The Transmission Fluid Pressure Switch allows the engine's control module (PCM) or Transmission Control Module (TCM) to use hydraulic fluid to engage the clutches in the transmission to allow for smoother shifting while driving. If the PCM or TCM detects an issue with the hydraulic clutch or the pressure switch, the PCM or TCM will set code P20A1
OBD P20A1 Lanchester 18 code and tailpipe testing are two different approaches to identify vehicles in need of repair. The OBD system looks for broken or malfunctioning emissions control components and P20A1, while tailpipe tests sample a vehicle's exhaust to see if it is above or below certain prescribed limits. Given the robust nature of today's emissions control components, it is entirely possible for an individual component to P20A1 OBD/2 Lanchester 18 malfunction without leading to an immediate increase in emissions at the tailpipe.
In such cases, other components (like the catalyst) can temporarily compensate for the part that is broken however, these other components can only do double duty for so long before they, too, begin to malfunction. In addition, OBD also monitors for P20A1 OBD/2 Lanchester 18 and other malfunctions in the fuel system problems that traditional tailpipe tests were not designed to identify. Most state and local areas also include a gas cap pressure test as part of an emission inspection.
OBD2 reports a sensor fault, replacement of the sensor is unlikely to resolve the underlying problem. The fault is most likely to be caused by the systems that the sensor is monitoring, but might even be caused by the wiring to the sensor itself.
As long as the throttle plate remains closed, the idling speed and quality is controlled by the PCM via the idle control valve, and as can be seen from the above it should be clear that idle control on a gasoline engine is a complex affair.
For instance, if the engine is started in sub-zero temperatures, the PCM will command the idling control valve to allow less air to enter the engine to enrich the air/fuel mixture to create a condition which is roughly analogous to how chokes worked on carburetted engines to control idling in cold weather.
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