Originally Posted by ShadowTek
So if the purpose of the upstream sensor is to determine the "correct" air/fuel ratio, then the downstream sensor will force a different AFR to get the desired emmisions effect?
What happens if you unplug the downstream sensors? Can the upstream sensors not control the poper AFR without the downstream sensors?
This just seems like needless complexity.
The upstream O2 sensors control the AFR, the downstream ones are only there to detect a faulty cat, if you disconnect them it will throw a code.
The first thing I would do is swap both upstream sensors over to see if the problem moves to the other bank. If it does then your O2 sensor is faulty.
Without proper diagnostic equipment you have to use a process of elimination, not guesswork.
One thing at a time