the most drastic fueling affecting tuneup item is the upstream O2 sensor., its the primary device used to determine a/f ratio, they skew as they get older and no longer switch at stoichiometric. if your sensor has over 50k on it, then yes its likely that its skewed and not reading properly.
the O2 sensor is one of the "trusted" sensors, which means the pcm assumes its perfect, even when it isnt, the vehicle would have to be equipped with a wideband or another NB sensor for the pcm to have any diagnostic functions for the upstream O2....so they just assume its perfect
dont buy bosch, get denso/NGK/delphi/ac delco
