As you are facing the front of the engine,your left side or the back bank is bank one(cylinders 1,2,3),your right or the front bank is bank 2(cylinders 4,5,6).
You have 4 O2 sensors, bank 1 the front or upper O2 is 11, the rear O2 is 12 on bank 2 the front O2 is 21 and the rear is 22. The front O2's are actual O2 sensors they are measuring the oxygen content in the exhaust after combustion. The two rear O2 sensors are monitors they measure the ability of the catalytic converter to do its job. They do not switch as much as the front they usually switch about once compare to the fronts switching about 9 times or 9:1
Your code is for bank 2 lower O2 or the monitor which will be mounted either in the cat or just after.
Here is a pic of the O2 locations
This will help.
Thanks.
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