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Thread: Help! P0304 code

  1. #1
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    Help! P0304 code

    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, I'm going to replace my wires, spark plugs and ignition coils. Should I check/ clean my EGR valve as well? If anyone has pics or schematics, I'd GREATLY appreciate it!

  2. #2
    The Diagram Dude pcb's Avatar

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    Replacing your plugs, wires and coils will probably fix your problem.

    Cleaning your EGR may not be necessary.



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  3. #3
    The Diagram Dude pcb's Avatar

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    Quote Originally Posted by FoxyRoxy05 View Post
    ...- P0421 (I knew about that one)
    Sounds like your pre-cat is shot.
    You could replace it or do the "non-fouler trick" to get rid of that code.
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    Thank you for your advice! I replaced the plugs, wires and ignition coils, as well as clean the MAF sensor. I also cleared the codes and reset the ECU. The car seems to be running fine but it was a bit of a rough idle when I stopped at a light. Is it true the ECU might take some time to be back at 100%?

  5. #5
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    "Non-fouler trick"? Not familiar with that one ....

  6. #6
    The Diagram Dude pcb's Avatar

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    Quote Originally Posted by FoxyRoxy05 View Post
    ...Is it true the ECU might take some time to be back at 100%?
    Yes, it's true. The ECU needs to relearn how to idle your car.
    It takes about 100 miles with plenty of times to idle your car.
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  7. #7
    The Diagram Dude pcb's Avatar

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    Quote Originally Posted by FoxyRoxy05 View Post
    "Non-fouler trick"? Not familiar with that one ....
    The "non-fouler trick" makes the ecu think you have a happy and healthy pre-cat, even if it's shot, or not there at all like if you have a header.





    Drilling out the first one is a PITA !!...
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    The Diagram Dude pcb's Avatar

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    A new pre-cat is about $130 US ... As long as you don't live in California or New York...

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  9. #9
    Registered Member Riot_Polizei's Avatar

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    Quote Originally Posted by pcb View Post
    The "non-fouler trick" makes the ecu think you have a happy and healthy pre-cat, even if it's shot, or not there at all like if you have a header....
    ...Drilling out the first one is a PITA !!...
    Hey, apparently there's an easier way of doing this!

    If you follow the link in the above comment, one of the top reviews specifically mentions using this in an aftermarket header (not sure if OBX or not) to perform the non-fouler trick.
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  10. #10
    The Diagram Dude pcb's Avatar

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    Quote Originally Posted by Riot_Polizei View Post
    Cool,.. I didn't know there was something like that...
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