I have been dealing with a P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)) for the past two years, having episodic success with getting it to stay off for a while. Here is the deal... Back in 2007 I got the aging, stock muffler replaced with a cat-back system and a new high-flow cat. Six months or so after this change I got a P0420 light. No driveability issues, just the light. I disconnected/reconnected the battery and put some BG44K in the tank and the light stayed off for ~1000 miles of combined city and highway driving before coming back on. I then replaced the gasket on the rear O2 sensor and tried to clean it. Again, the light stayed off for ~1000 miles. With the light on for the third time, I replaced the rear 02 sensor and again, the light stayed off for ~1000 miles of combined city/highway driving and is now back on for the fourth time.

The truck actually passed emissions a couple of months back, and again, I don't see any MPG or driveability issues, and I don't hear or see any exhaust leaks. Basically, what I am thinking at this point is that I need a new CA-compliant cat to better match the stock cat's performance. Unfortunately I was an idiot and let the muffler shop who did the work keep the stock cat. If any one has any ideas on how to proceed, I am all ears!