Originally Posted by
Ste
What has been said above is not right.
Bank 1 and bank 2 is defining which bank of cylinders, not which O2 sensor.
If you have an inline 4 cylinder, then you only have Bank 1. Bank 2 comes into play for the V6 engine.
Bank 1 Sensor 1 is before cat and Bank 1 sensor 2 is after cat.
Low cat performance is measured by both O2 sensors and looking at the difference between the 2, to give effective cat performance.
If you don't also have lambda trim faults, then either the second O2 sensor is duff (stuck in range), or the cat is duff, or you have a leak in the exhaust pipe.
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