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Downstream sensor doesn't affect operation. It's an indirect indicator of whether the catcon is working properly, in that the downstream sensor should measure noticeably leaner exhaust than the upstream one, if the HCs are being broken down by the catcon. A catcon failure, an exhaust leak or other even might show similar readings for upstream and downstream, which should trip a fault code.
Any chance that the overheat damaged the head gasket? A small coolant leak into the cylinders might not be noticed yet, but could poison the O2 sensor (upstream would affect performance). Is it using any coolant at all?
Any chance that the overheat damaged the head gasket? A small coolant leak into the cylinders might not be noticed yet, but could poison the O2 sensor (upstream would affect performance). Is it using any coolant at all?