My ML55 has the "check engine" light on and when I plug in my code reader, the code "P0130" "O2 sensor circuit malfunction" "bank 1" "sensor 1" appears.

I have installed a K&N air filter and cleaned the Mass Air Sensor since. No luck! Perhaps I have misunderstood which "bank" or "sensor" entirely??

I'm hoping someone out there has had some experience with diaganosing this. Thanks in advance for your help.
Aram