My P0420 started at 98k miles. TCM had been replaced by Ford at 75k. I replaced the purge valve first and the P0420 kept coming back every 5k-8k miles. I finally replaced the downstream o2 sensor and added an o2 sensor extender and this fixed the problem. My cat isn't clogged or anything and I now have nearly 146k miles on it with no trouble codes and has passed emission testing.
see here: https://www.focusfanatics.com/forum/...20-solved.html