P0421 (sometimes P0300 and P0660)

Hello all.

Please help me out! :-(
I love my car!

I have a stock 2002 Mazda Protege5. There's no shortage of P0421 threads on this forum but I've yet to find someone who has actually FIXED the issue once and for all.

I've had a P0421 + CEL for at LEAST 3-4 years. Yep, years. But the gas mileage and general performance of the car has remained stable for all this time, despite the CEL and the P0421. Ultimately, it was a horrible idea not to tend to it right away-- but I didn't expect to keep the car this long.

Thing being, I now have to get it smogged, the CEL is getting in the way of doing so. I have a gut feeling that it would pass but you can't smog with a CEL.

Aside from some pinging at 2000RPMs and a bit of struggle on VERY steep hills (pedal all the way down and car has difficulties going up, especially if it's also hot out), the car runs great and idles quite nicely (even from a cold start). It has 255K miles on it and runs very well considering. (Sidebar: I'd love to resolve the pinging noise; tried higher octane already. No additives yet and probably won't go there.)

I just ordered the OEM precat and before I install it, I want to be sure that I've covered all my bases and won't ruin the new precat. I've also heard horror stories about people replacing the precat, O2 sensors, EGRs, etc.--And the CEL and P0421 issues persist. I simply can't afford to have that happen. Not financially or otherwise.

Here's what I've completed thus far:
- Spark Plugs / 9-Jun-2012
- Wires / 9-Jun-2012
- Fuel System Cleaning (mechanic cleaned all lines, no additives) / 9-Jun-2012
- Coils (2) / 17-Sep-2012
- O2 Sensor, Downstream only / 19-Sep-2012
- PCV Valve / 22-Sep-2012
- Battery Checked, 12.9V = OKAY / 22-Sep-2012
- MAF cleaning (only) / 24-Sep-2012

I've reset/deleted the codes every time I've done a job and the CEL comes back on after some amount of driving (all monitors cleared). Here's a curveball, after resetting, I do get P0300 and P0660 as well but they eventually subside (intermittent, I suppose?). I haven't checked for these codes recently and they MAY have been resolved by the new coils.

I wanted to check and clean the EGR but it's way beyond my abilities. I'm not trained and most of what I've done thus far has been very easy and I've relied on the suggestions of others and knowledge I've gained from this forum.

I paid a pretty penny for the precat and I'm happy to make the investment on other parts, too-- especially if that means I can get more life out of the car and pass CA smog.

What else would compromise the (new) precat? If the EGR was "bad", wouldn't I be getting EGR specific codes?

Has anyone fully resolved this code/associated issues? Again, passing smog is my immediate goal but if I can prolong the life of this great car by doing some other repairs, I'll do it.

Thanks everyone.
NM