Air-fuel feed back control is the modulation which increases or decreases fuel injection quantity as determining exhaust gas rich or lean with HO2S adjust combustion is done at ideal air-fuel ratioto so as to raise the efficiency of catalytic convertor. Generally, NOx increases at lean combustion while CO, HC increases in rich combustion. Thus, in order to purge all of these gases with catalytic convertor, air-fuel ratio should be nearly at the ideal ratio. However the range in which all gases purified successfully is too narrow, it is impossible to meet ideal range with open loop control, therefore feed back control using HO2S is required. HO2S output signal changes rapidly near ideal air-fuel ratio and this characteristic is used at feed back control. comparing HO2S output signal and reference value, PCM increases fuel injection quantity at lean condition and decreases at rich condition. As deterioration of engine, due to the characteristic changes of many components of intake and fuel line or inevitable tolerance of components at production process, achieving ideal air-fuel ratio is almost impossible with the fixed fuel injection duration. Regarding the change of cross section by clogged injector, feed back correction is performed yet, it cannot cover all the ranges. It means if the correction range is massively seperated from the ideal value or if the mean of feed back control range too inclines to lean or rich condition, feed back correction does not work efficiently. Therefore processing correction value throughout long time statistically during driving, PCM controlls fuel injection duration as fitting the mean to be the ideal air-fuel ratio value. Through adaptive control, accuracy of the control could be improved and the value of adaptive control varies countiuously during driving and is always up-dated.

Checking air-fuel ratio correction value every 0.75 sec. at idle, if the value within the detecting condition for more than 0.3 sec., PCM sets P2188. MIL(Malfunction Indication Lamp) turns on when the malfunction lasts till consecutive 2 driving cycle.