left catalyst below tolerance

Just wondering if anyone has any insight on this. The engine code is a 420. I believe the left back O2 sensor was changed a few months ago and the check engine light came on again shortly after. It is a 99 Tahoe with 84K on it. I am starting to think it may be the left cat converter. Thanks.

It most likely is a bad cat. I had a P0430(Catalyst System efficiency Below Threshold Bank 2) a while back. Your P0420 is for the bank 1 side. It was explained to me that when these codes are thrown that there really is no other possibility that its anything other than a bad cat. Good thing for me was my truck only had about 65,000 miles on it at the time and GMMerlin brought to my attention that catalytic converters are covered under a 8 year/80000 mile warranty. I got both of mine replaced for free at my Chevy dealer. Since you have 84,000 on yours, its looking like you will probably have to foot the bill.

Would bank two be the right or left cat since it comes off the left side of the engine but that cat it feeds is on the right side of the truck.

Bank 1 is the left side and Bank 2 is the right.
A P0420 or 430 indicated that the converter cataylist can no longer do its job...as long as there is no misfire condition present, replaceing the converter will take care of the problem