V6 with 4-O2 sensors question
 

Quick question:
Generally speaking, on a V6 with 2 upstream and 2 downstream O2 sensors, does the PCM adjust the AFR of each bank independently based on values from their respective upstream 02 sensor, or does the PCM adjust the AFR of both banks together with combined values of the two upstream O2 sensors?
Curious is all.

Depends. Most systems I know of adjust each bank (somewhat) independently. But I can imagine a low-end system that doesn't. I'd think that a 4 sensor system uses all 4 sensors; otherwise why have the expense of 4 when 2 will suffice?

There is a long term fuel trim that can adjust the banks or even individual cyls differently but the O2 sensor is involved only if the exhaust pipes are completely separated before the O2 sensors and cats. On most V engines there is a cross over point before the cats.

My 3.0L Mazda Tribute has cats inside the manifolds with 2-O2 sensors on each manifold. It also has a third downstream unmonitored (no sensors) cat.

yes - Bank 1 and Bank 2 are independant
 

yes bank 1 and bank 2 are independant
they share a common load sensor and fuel supply
but fuel trim correction per bank is unique to the bank

the system will trim on the rear 02 sensors as well

if both banks have long term fuel trim corrections that exceed +/- 10% and are the same
the problem is in load calculation or fuel supply or air supply

if only one bank has such an error , the problem only affects that bank