Hopefully folks are still reading/responding to this post. I was getting both P0421 and P0171 codes on my 2000 Mazda Protege ES (1.8 L, manual tranny) recently. Sensors are good. Had a major leak in intake so I had the plenum, manifold, and throttle body gaskets replaced. Still getting the P0421 code. I guess I have to take it to Mazda. My local mechanic says he he's having a hard time finding aftermarket cats that meet EPA standards here in Los Angeles, CA so he recommended I call the Mazda dealer for this job.
Any other (and more cost effective) options out there? Is this something I can order and do myself? Hopefully I can just get away with just the front cat.