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Check Engine light on, changed oxygen sensor before cat conv on 2004 Mercedes-Benz C230

And the light always comes on when I'm doing line for at least 20 min, but the car doesn't stalls or anything, fuel consumption is normal, cleaned the MAF and it pops up again. Any ideas

what are the eng codes?
P0171
thats a lean code for bank 1 look for vac leak causing the problem
where are the vaccum lines?
seek a diag and est from a good repair shop in your area.
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1 more answer , 4 more comments
you need to have someone look for a vacuum leak...
how many vaccuum lines could be involved?
or wich is the most common
a few vacuum lines COULD be involved...but it usually is not as easy as a vacuum line...most likely it is some gasket somewhere in the intake portion of the engine.
Gaskets sound expensive
Gaskets themselves are not expensive....what may be expensive is getting them to their place!