Q: PO420 Efficiency below threshold bank 1. Catalytic converter or O2 Sensor?

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My car only has the trouble code PO420 Efficiency below threshold bank 1. I am wondering if my catalytic converter is faulty and read somewhere that looking at the fuel trim can help determine that. With the car on and running, I read my car using an OBD2 scanner and it says "ST FTRM1 (%) 0.0", "LT FTRM1 (%) 8.6". Upon initial starting the ignition, the ST fuel trim was reading around -4.7 and leveled out after a couple minutes of running at idle to get to 0.0 but the LT fuel trim maintained 8.6 the entire time. I am wondering if this will tell me anything about how my catalytic converter is running or if my O2 sensor may be bad. For the O2 sensors, it read, "O2S12 (V) .415" and "O2S11 (V) 3.231".

My car has 164000 miles.
My car has an automatic transmission.

Hi Justin. Thanks for contacting us today. At first glance, the numbers you listed indicate that the catalytic converter is likely clogged. You might want to read this article about the OBD-II trouble code you’ve listed, as it will give you some good ideas on correctly identifying the source of the P-0420 code.

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