I was just told that I need to replace my bank2 sensor1 O2 Sensor on my 99 CLK430 and I was wondering where this O2 sensor is located. I have replaced an O2 sensor on my 88 300E but it only had one O2 sensor. If anyone could tell me where its located and maybe how to get to it I'd appreciate it, thanks.

If i remember correctly it would be the right front o2 sensor

Bank one is the right bank (on the V engines, right front cylinder is #1, then count back on that bank, depending on number of cylinders, 2, 3, (for V6), 4 (for V8), 5 and 6 for V12,
then the left side is bank 2, starting with cylinder 4 for V6, 5 for V8, or 7 for V12.
So it is correct that Bank 2 O2S 1 is the one on the left side in FRONT of the cat. This is the "important" one, the "control" O2 sensor. #2 is the "diagnostic" sensor, it's only real purpose is to make sure the cat is working properly.

Gilly

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So it is the left front NOT the right front (I do have a V8)