codes

A few weeks ago, the CEL came on. I took it to Advanced Auto, they gave me the code, went to the mechanic, and he said that it was one of the oxygen sensors. I paid for that repair, and by the time I came back home, the CEL was back on. I took it back, they said nothing was wrong with the...

I drive an 03 Mazda Protege5, who has given me no problems. Last night, my check engine light started to flash and my engine had no power. I quickly got the codes (thank goodness for my Bluetooth OBDII reader) and got two codes - P0421 (I knew about that one) and P0304. From what I've looked up...