Q: The check engine light came o and the code is P0420 and I can't figure what exactly to replace

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I replaced the downstream o2 sensor and I the light went out for a while but came back on should I replace the CAT or he upstream o2 sensor

My car has 150000 miles.
My car has a manual transmission.

As you probably know, the catalytic converter reduces emissions by injecting oxygen into the exhaust stream, mixing oxygen with the emission gases and essentially incinerating the pollutants before they can exit the tailpipe.

The converter works with two oxygen sensors, one upstream near the exhaust manifold and one downstream on the other side of the converter. If the readings between the two sensors are too similar, it indicates that the converter isn’t working properly. Your guess that the upstream O2 sensor or catalytic converter could be at fault are likely correct. The good news is that the catalytic converter may be covered under warranty. Be sure to also check the wiring and connectors to the upstream O2 sensor before replacing the unit. If you need to have this serviced, a certified professional from YourMechanic can come to your location to diagnose the Check Engine Light and can move forward with the appropriate repair.

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