It said left bank catylist below threshold. I have no idea what that means.. Here is my car. I have a 1999 Camro SS with a six speed, 14,800 miles on it. It sat for most of last summer because 4.30 a gallon for premium was a lot to handle. I did clear the code and it has not come back on after driving about 200 miles this weekend. The car runs fine, I can't tell any difference and the only modification I have made is and SLP air lid and K&N filter, past that it is 100% stock. Thanks.

 

P0420 is most likely the drivers side cat going bad. I wouldn't worry about it unless it keeps coming back.

 

It hasn't come back, it could be the old gas (stabil was used) that was in the tank from last summer, that is burnned through. It the cat is going bad looks like it will be time to put on headers, high flow cats etc etc. It would be cheaper, or at least that is what I will tell my wife.