I have a '02 Mazda Protege 5 with a similar problem (diagnostic code P0300 and P0421). Originally I had a problem with the catalyst converter (P0421) and I had it changed. A couple of weeks later the car started to sputter while in low idle and eventually I had to turn the engine off and start it back up to get rid of it. The check engine light turned back on, this time marking P0300 and P0421. I also noticed that while driving on the highway (60-70mph) the car would do a quick sputter and then it wouldn’t accelerate past 50mph. Again I had to come to complete stop and turn the engine off completely (at this time the car was sputtering on low idle) and turn it back on for it to go away and drive at a speed of about 55-60mph so it wouldn’t happen again. I took it to the mechanic again to check the catalyst converter again and if this was causing the random cylinder misfire as well. I just got off the phone with the mechanic and he says the problem seemed to be the oxygen sensor which they’ve changed and apparently all is well again. Does this make sense that an oxygen sensor triggers both P0421 and P0300? I will keep you posted once I drive the car for a while.

 

well one thing that can cause the missfire ,is a very simple thing which a lot of shops overlook is the air hose from the mass airflow sensor to the intake manifold. it usually cracks at the fexible bellows and allows air into the circuit,,, most times it is this it is about 2 and five eights inches diameter put a new one on,,
hope this solves ypur problem

 

I realize it has been several years since your original reply but my sons 02 is doing everything you described yours was doing. What you described your mechanic doing did it fix the problem with your car that you can remember? Thank you for your time.