P0420 - simply install the $5 fix and be done with it. it's too much of a debacle to otherwise worry about.
if you really want to put some effort into - first simply seal up any leaks in the exhaust system.
ABS sensors may just need cleaning, sometimes they can have debris stuck all over them.
Otherwise they're attached by like one bolt and then another bolt or two for a bracket just to hold the wire and super easy to replace. 3 bolts, a few minutes, very easy job.
If any recent work was done, make sure it's plugged in and fully seated (unplug it and plug it back in).