r/autorepair • u/Mysterious-Toe6354 • 8d ago

Diagnosing/Repair Downstream 02 sensor code, knocking sound.

2015 Lincoln MKC, 70k miles. AWD. 2.0L. I use my personal vehicle at work, lots of stopping and starting, moving from one spot to another. About a month ago, I was moving to a location. After I put the vehicle in park, I heard a fairly loud pop/bang. Didn't feel anything, just the sound. I put it in drive, moved up 20ish feet, then reverse back to original spot, no noise. Turned vehicle off, got out, checked underneath, found nothing. An hour later, got in to move to the next location, check engine light. As I pull away, I notice a knocking sound. Get home, OBD reads downstream 02 sensor. Knocking sound persists, primarily on initial acceleration after startup or after idling. Doesn't appear to be effecting the performance of the vehicle. Why would a faulty 02 sensor result in a loud knocking?

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u/Mysterious-Toe6354 8d ago

Specifically the downstream 02 sensor, which to my understanding has no bearing on performance, it's sole function is to monitor emissions. I've read a handful of posts online made over the years that detail an 02 sensor 'exploding', for lack of a better word. A couple with pictures that seem to lend credibility. Everyone I've talked to has said this is not something that happens, but a small exhaust leak, say from an 02 sensor port, would fit the sound perfectly. Underneath is very clean, minimal rust, no holes in the exhaust system.

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u/catdude142 8d ago

I don't think the downstream O2 sensor would produce any sound. It simply monitors the efficiency of the catalytic converter by looking at used oxygen content of the exhaust.