Last night, I replaced the two o2 sensors BEHIND the catalytic converters. The car was running sluggish and I have 160kmi on my 97 E420. I had no O2 related codes before. Now, the car runs much better but I got P0130 and P0150 faults. They seems to correspond to the front sensors that I have not yet replaced. Any thought?

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

Rear O2 sensors are there ONLY to monitor catalyst operation & should NOT have any effect on how the engine performs!

Yes, those are front O2 codes. Are you sure you replaced the REAR ones?

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Yes, I replaced only the rear ones and then got front sensor codes, very strange... Any guesses?

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