Well at first when I purchased the car and checked my computer I had P0301, P0303, and P0304. The P0301 and 303 were easy to fix, all I did was replace the spark plugs and they never came back. Nevertheless now I keep receiving P0304 code, but it does not turn on the CEL. What can be done to remove/fix the P0304 code? Btw I have an '04 WRX.