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Got the ol’ P0420 code recently in a 2006 2.4l. Jeep has new oxygen sensors (previous owner) so I did a few tests on the Cat.
First, the OBD showed the second O2S sensor fluctuating between .300 & .700. Jeep was warmed up and rpm’s at 2,000.

Also the exhaust temp exiting the cat was significantly lower than the temp entering it (which was pretty hot).

Is it safe to go ahead with the replacement or should I try more tests? One of the reasons I’m hesitant is that the previous owner gave me the original Cat along with some extra parts. Can’t see a second cat going bad so fast. Maybe he used a junk yard replacement? Thanks!
 

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Got the ol’ P0420 code recently in a 2006 2.4l. Jeep has new oxygen sensors (previous owner) so I did a few tests on the Cat.
First, the OBD showed the second O2S sensor fluctuating between .300 & .700. Jeep was warmed up and rpm’s at 2,000.

Also the exhaust temp exiting the cat was significantly lower than the temp entering it (which was pretty hot).

Is it safe to go ahead with the replacement or should I try more tests? One of the reasons I’m hesitant is that the previous owner gave me the original Cat along with some extra parts. Can’t see a second cat going bad so fast. Maybe he used a junk yard replacement? Thanks!
I was always taught that P0420 are pretty much always bad cats. There are a few tests you want to do ahead of time and it's basically making sure the O2 sensors are working properly.

When checking these, you look at the o2 sensors (live data through a scan tool) and see if they report a lean condition when pulling a vacuum tube or a rich condition when doing a snap throttle. Then you look for to see if downstream mirrors the upstream sensors too closely. If all that checks out, it's time for new Cats.
 
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