After refastening the the connectors the code 12 is gone, but I am still not trusting the sensor because the car feels wrong at idle and on acceleration -- as if the fan clutch is engaged, even though it is not.

Could the O2 sensor still be bad, even if not throwing a code?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black