Credit: Eddy

The Nissan Maxima was made in FEDERAL and CALIFORNIA specs all the way through 2001. However all Maxima’s starting in 2001 were built to California specs. This was to align to the latest National Low Emission Vehicle (NLEV) EPA regulations.

This just basically means that all 2001 Maxima’s has 4 O2 sensors.  Two upstream that monitor A/F ratio and two downstream that sit behind the pre-cats to monitor them.