Code P0421?

Which engine do you have, are you sure you have the right sensor? With the four cylinder there is no question but the v6 has two secondary sensors. I don't think it is the sensor if you have changed it twice and it is not a circuit failure code, so I don't think it is a connection problem. Here are the main reasons for the code:

Faulty Catalytic Converter: The most common cause of a P0421 code is a failing catalytic converter.

Exhaust Leaks: Exhaust leaks before the catalytic converter can allow oxygen to enter the exhaust stream and affect the readings of the downstream oxygen sensor.

Can I ask if you have an exhaust leak? This will cause the problem, if there are no exhaust leaks, I would replace the catalytic converter. Here is a generic guide to help you step by step with instructions in the diagrams below to show you how on your car:

https://www.2carpros.com/articles/catalytic-converter-replacement

Check out the images (below). Please let us know what happens.

Here is how to check for an exhaust leak:

https://www.2carpros.com/articles/step-by-step-guide-how-to-check-for-an-automotive-exhaust-leak

Does the bank 1 sensor 2 lay flat as well? Here is a guide to help with the wiring testing and the engine wiring diagrams so you can see how the system works and which wires to test:

https://www.2carpros.com/articles/how-to-check-wiring

Check out the images (below). Please let us know what happens.

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