As the title suggests, I got the P0420 code (Catalyst System Efficiency Below Threshold Bank 1) after filling up on my way home from wheeling in TN . While wheeling, I had some hits and caused a leak at the joint between the 2 cats on the driver's side (bank 1). The next day it went out so i thought nothing of it and I replaced the gasket along with a new stud then tightened it all down eliminating the leak. The code came back and keeps coming back after 20 miles or so. Could this be a coincidence? I ordered a new downstream O2 that will arrive tomorrow but I am not sure if this will be the fix or if I will be spending my birthday/bumper $$ on a new manifold/cat. Thoughts?