O2 sensor replacement can't locate it on the downstream 2001 murcury villager

The downstream O2 sensor will be after the catalytic converter. The cat is typically near the engine in the exhaust. It will be much larger in diameter than the rest of the exhaust.

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The downstream sensor will be AFTER the cat. It may be on the top of the exhaust pipe and hard to see. As a rule it is close to the convertor.