So, a friend of mine, who is a very non DIYer asked me to look at his P0420 code two years or so ago (CE light ON). This after he took the car to the dealer and they said 'catalytic converter', and that it would be $2500. I've never seen a CC go bad before, even though I'm sure they do from time to time. So, I changed his upstream O2 and the code went away for about 6 to 9 months. Then, it came back, but with a bank 2 issue. So, I changed the downstream O2. And, again, the code went away for about 6 months. Now, it's back. So, the story is the code comes and goes with the hot and cold weather. The car has always run great with no running issues even with the CE light on. And, I would hate to advise him to go ahead with the CC change, and then that not fix the issue. Does anybody think there may be some carbon buildup or other possible issue causing the intermittent code? Anybody else had something like this happen? Thanks.