P0402

I've been struggling with O2 sensors codes on our 2000 XJ while trying to get the truck to pass NY emission inspection. Found a blown fuse in PDC under the hood which seemed to eliminate the trouble. We now have a P0402 fault code referring to cat efficiency.

The exhaust was upgraded and rear cat removed as a result of major power loss due to a well placed strike by a tree stump, but I believe the O2 issues at least may have existed prior. 2/4 sensors were replaced a few months ago and the pre cats looked good (visual inspection during engine swap).

Question: Is it possible that the P0420 code will work itself out (I've read it may be a "soft" code)? And, could the efficiency issue be related to the exhaust work?

Am I to assume that the first two sensors have been replaced, but not the two after the cat?

I believe it was the two after the cats that were replaced.

If that is the case, then they are probably telling you that the two in front of the cats are worn out. They do wear out over time like any other device that passively generates its own electricity, like solar panels. If you replace the two in front of the cat, clear the code, then you will probably fix that problem. Milage may also improve