Code P0421?

CHOOME

MEMBER
2006 MITSUBISHI ECLIPSE
110,000 MILES

I'm getting that code, but I do know that the bank 2 sensor 2 is bad. I have changed it 2 times and it still shows that it's not working by my ODB11 reader. Could it be that I got 2 bad 02 sensors in a row? Or is my harness connection bad?

5 Replies

KEN L

MASTER CERTIFIED MECHANIC
47,256 POSTS

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.

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.

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Wednesday, September 13th, 2023 AT 12:08 PM

CHOOME

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I just saw this to reply, but thank you so much for your response, I love this site. I do have V6, but I was wondering how to check for an exhaust leak it just seems strange. That the Bank 2 Sensor 2 in the live graph is flat No movement. That's why I was thinking it was a connection. Any more ideas? Thank you

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Friday, September 15th, 2023 AT 12:53 AM

KEN L

MASTER CERTIFIED MECHANIC
47,256 POSTS

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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Friday, September 15th, 2023 AT 11:17 AM

CHOOME

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7 POSTS

Thank you much for the diagram but I can't understand. Not all the Sensors are good except Bank2 Sensor2. I'm bumming it's the Converter. But thank you for all your info.

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Saturday, September 16th, 2023 AT 7:35 PM

KEN L