Thank you for this very helpful post. I am a little confused about bank 1 sensor 1 versus bank 1 sensor 2. I have attached a photo from my C300, from the top. I was wondering if you could help me figure out. The two images on the top are from back of the engine, and the three in the bottom is from the left side of the engine passenger side. I was not sure which one is sensor 1 of the bank 1. Thank you again.