Code p0421

Good afternoon,

There is no 421 code. There is a 420 which I assume is the code. It is for the front cat converter. When you said you replaced the rear cat, what exactly did you replace?

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

I attached the diagram of the system and also the front or only cat converter for you.

If you have no exhaust leaks, it is the only possible failure. I attached some possibilities for you.

Roy

here is a list of everything I have had replaced: catalytic converter, MAP sensor, all exhaust leaks, and downstream o2 sensor. I do still have a valve gasket cover leak and still getting that p0421 code on my car listed above coupe.

Can you please verify the code? There is no 421 code listed.

Roy

Code P0421 – Catalytic Converter Oxygen Sensor Below.

What is the voltage reading on the downstream O2 sensor?

Is it steady or does it very?

Roy