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Bad Catalytic Converter?

The dealer just told me that my catalytic converter was bad on my 2006 Chrysler 300 2.7L engine with 93k miles. The code that was pulled was P0420. I took the car to my mechanic and he said that it was unlikely that the CC was bad. He said he checked the exhaust for any restrictions and found it to be ok. He said that he did find 2 coils that were bad from arching and decided to change the plugs as well. The plugs were also changed at 60k because the engine light was on but the code pointed to the EGR that time. Within 500 miles of recent maintenance the check engine light illuminated(again)with the P0420 code. Could it still be unlikely that the CC is bad? How can I diagnose the CC? If the CC is bad could another problem have caused this to happen and if I have to get a new CC what would I have to do to prevent this CC from the same fate?

The thing tripping the code is the downstream oxygen sensor. The ECU is comparing the signal from that to the signal from the upstream oxygen sensor. If the difference is not sufficient, the ECU thinks the cat converter is not doing its job completely.

The only way to definnitively find out of it’s the sensor or the converter is to put both the upstream and downstream sensor signals on a scope and compare them. A good shop should be able to do this.

Thanks for the reply. I will be sure to give this a try.

mountainbike is right on as usual. It is also a good idea to check for exhaust leaks. Here is a pretty good description of the code just FYI: