Any Mazda 3 owners had any codes come up related to your catalytic converter? I've had a recurring P0421 code coming up on my 2010 Mazda 3 S Sport sedan that causes the Check Engine light to come on. I've had a mechanic check the spark plugs and he said they're good and don't need to be replaced. I had one of the O2 sensors replaced recently and that also did not solve the problem. It looks like I need to replace the converter that's attached to the exhaust manifold, which is not a cheap repair.
My car is also designed to meet California emissions standards, so I'm wondering if I could even use a converter that is Federal EPA compliant or it that would just continue to cause the CEL to go on. Anyone have any experience with this?

 

P0421

POSSIBLE CAUSE
• Leakage exhaust system
• A/F sensor looseness
• HO2S looseness
• WU-TWC deterioration or malfunction ( Catalytic convertor )
• PCM malfunction