keep in mind that the oxygen sensor is a "trusted" sensor (as is the tps/maf/map/ect/iat). that means that the pcm ASSumes that its fine, even when it may not be.

if your o2 sensor has over 50k or youve burned oil/coolant then the sensor is likely skewed and no longer switching at stoich.
stay away from bosch, ive had way too many odd issues with em...denso/ngk/delphi sensors only.