I have a 2008 Altima and the service engine soon light come on and the diagnostic tool says code PO420 is showing and its probably the CatConv. Does anyone know if I should change the 02 sensors first, new gas cap, add fuel additive to help clean CC, change the CC, which is the most expensive step. I'm just looking for someone who has encountered this problem and who have some valuable information to help me save money and time in diagnosing this issue. Car seems to be running fine but something minor is starting to happen to cause a code to pop up. Thanks for any information you may have to help me in this issue.