I am a bit confused, but it's not all my fault I swear

I have an 01 maxima and am trying to replace at least one of my o2 sensors. I'm aware of this posting here:

http://forums.maxima.org/5th-generat...on-issues.html

which shows a map of 02 sensors. It's also titled cali spec. I'm 98% sure that my car is a fed spec. I checked the serial number and the 12th digit seems to indicate U which I understand means the car was intended for United States, not California. So this becomes the question....how many o2 sensors do I have.

I've called 4 dealerships. Three of them say there are 4 o2 sensors, One of them says there are 3 o2 sensors.

Anyone have any idea who is right? Are the o2 sensors the same or different for the fed spec/cali spec vehicles? Is it just that the cali spec one's have 4?

Any help appreciated. I figure I am just going to go OEM rather than worry about bosch/NGK quality.

 

Originally there was a difference between the Cali spec and Federal spec. Cali spec had 4 O2 sensors and Fed spec had 3 sensors. As the government cracked down harder on polution, all cars became Cali spec using 4 O2 sensors and swirl valves. According to the 5th gen link, it says all cars starting with the 2001 are Cali spec.