Nick, I did not replace the O2 sensors when I did the exhaust. I am more concerned with a larger issue going on with the engine that is causing the O2 sensor to initiate a code. The car runs great, but the left exhaust will soot and not the right. Is this a sign of something going on in the left bank?

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The soot you see could be a product of a faulty o2 sensor or a larger fuel trim problem. I would need to see fuel trim at idle, 2500 rpm and at a steady cruise.

- Nick