Don't know if this is common to all "C" class but here goes. Had the same issue. Best I can tell the fault code is in memory until cleared/reset by the tech. CE light has several items that can trigger it. Do a search on CE and you'll see. My C280 has two O2 sensors. The first one (the expensive one 164.00 for me in Houston)senses the exhaust and gives feed back to the computer, the second one is a comparator, it compare what it senses down stream with what the first one senses and the computer makes the corrections accordingly. My MB tech was very encouraging to get it taken cared of soon, which I did and the engine light did clear. A hard lesson learned about this 'C' class also is do not refuel with the engine running. Reason am told by Houston MB dealer and my MB tech is that the computer senses a delta P in fuel pressure. It is sensing the expected fuel pressure and when the fuel cap is removed the then internal fuel pressure is changed thus giving a delta P and the computer registers it as a fault. So now with that lesson learned I will always cut the engine off when refueling. Hope this helps out a bit.
regards, LH