Q: Code p0421

Are there other codes set? This code, by itself, P0421 indicates that the engine management system detected the catalytic converter is performing below expected standards. This means the engine is sending contaminants (unburned fuel, oil fumes, etc.) down the exhaust, and these contaminants are preventing the cat from working as expected.

Using a cat cleaner may "help" short term, but your basic problem is unresolved. Replacing the pre-cat O2 sensors may help if they are old and not providing good signals to engine management. But again - any other codes will guide you to the source of the real problem.

I would recommend having one of our mobile technicians come to you to perform a Check Engine Light inspection to properly diagnose this problem, get an accurate assessment of damage, and a cost estimate for repairs. YourMechanic.com can dispatch a mobile, professional mechanic to your location for this repair.