Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD II DiagnosticsPerform powertrain control functions to reduce emissionsand meet OBD II regulations.Automatic system Adjustment:: The PCM during closed-loop mode of controlchecks the open-loop calculated air/fuel ratios and compares them withclosed-loop average limit values which are the ideal values for minimumemissions. If the difference is large, the PCM corrects the open-loop lookuptable values so that the open-loop values are in close agreement with theclosed-loop values. This updated open-loop lookup table is stored innon-volatile RAM memory. When the engine is started next time the PCMuses the new lookup values which are closer to the stoichiometric ratio.This feature is important since it enables the PCM to adjust to long-term changesin engine and fuel system conditions due to wear and usage. This is similarto fuel trim algorithm for fuel injection control.These are all the PCM control functions performed to reduce emissionsand comply with OBD II requirements.