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?