Introduction to On Board Diagnostics (II)

Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsHowever, manufacturers choosing to allow the OBD II to better isolate the fault to specificcause would not use the general fault code/message, but would use the more specificcode/message associated with the particular circuit, component, or system.For example, in diagnosing a 5 volt reference throttle position sensor, if the input signal atthe PCM is stuck at near 0 volt, the manufacturer has the choice to select either of two codes:P0120 (general malfunction), or P0122 (specific low circuit input ), depending on themanufacturer’s diagnostic procedures. The root cause of this fault can be any one of electricalor mechanical problems. Identification of the root cause is done using the diagnosticprocedures and is not implied by the DTC message, thus allowing the manufacturer theflexibility in assigning DTCs.The powertrain control strategies in performing OBD II diagnostics depend on eachmanufacturer who has considerable flexibility as to how the diagnostics are implementedprovided the above guidelines of SAE J 2012, SAE 1979, and SAE J 1930 are complied with.A typical use of OBD II procedure is given below as a generic example:The diagnostic mode is entered by switching on the ignition and then simultaneouslydepressing the OFF and Warmer buttons on the climate control system (cadillac).