Hello. I have an 03 Honda Accord Ex 4 cylinder. I had a p0420 code and replaced both O2 sensors. Now the code is back, so I'm thinking it's my catalytic converter. I got under the car today and noticed what appears to be a leak/Crack in my exhaust. I was wondering if that could be contributing to the code? And if there was anything else I could do to clear my code before replacing my cat? Thank you.

 

Where's the exhaust leak?

The oxygen sensors actually measure the oxygen DIFFERENCE inside vs. outside. So if the exhaust leak is blowing onto the outside of the sensor (either one) it'll throw off the measurement.

 

​​​​​​The leak is located about a foot behind the actual catalytic converter.

 

A mechanic told me that the rear O2 sensor actually wants to make sure the reading there is less than the front O2 sensor - whatever it's reading (pollutant levels).
I thought it was more specific than that. That's seems quite odd you have a leak after the Cat, and also after the rear O2 sensor I assume. I don't know how that would affect those readings.
(And you can get that light to go off another way.)
I had that code thrown on my TSX. It's the rear O2 sensor that is throwing that code, not the front O2 sensor.