I bought a 02 mazda protege5. When i bought the car 7 months back it had a code for a catalyst converter (i cant remember code)... after doing some work i found out it was a o2 sensor. Replaced that, and get a whole lot of hesitation plus lack of power felt like i was riding a bull lol.. had alot of carbon build up, cleaned it out... got a cylinder 4misfire swapped the spark plugs. And still cylinder 4misfire......

Drove 1200miles fl-ny on cylinder 4 misfire..

noticed oil seeping in the #4 spark plug. Replaced the valve cover gasket and now im good but NOW.... its giving me p0300.. any ideas??

 

You may have a coil going bad and inadvertently swapped them while doing the plugs. If it has 2 cylinders per coil like my 01 2.0L you lose half the engine with a bad coil.