Thanks! This has been puzzling to say the least... First the car was not running right but without CEL, but one of my catalytic converter had burnt. SO, I decided to replace all o2 sensors (160kmi... I know, lesson learnt). First I replaced the two rear sensors only and started getting fault codes for the front sensors - strange. I called Bosch and they said that can happen with old sensors not working together well. So, I replaced the front ones a few days later. The car runs much better but I continue to get codes, now often with P0100 as well.

So, I went from a car that was not running well without CEL, to one that is running much better with CEL. I can't wait to get my new MAF.

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)