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misfire on hwy

justin10morrissey

New member
have an 04 gtp it throws codes p0420 and p0300. 0420 is from the hi flo cat dont know why p0300 but when its getting on hwy its misfiring and runs very poorly. need help or suggestions
 


recently changed the plugs and wires cant remember if it was doing it before i did this but it is still doing it. havnt replaced coils yet thats next on my list. plugs are autolite 606's and wires are napa beldan 7mm car has the zzp downpipe and throws a p0420 code bc of the highflo cat. when it misfires the mil flashes and pulls code p0300 random multiple misfire. maf is newer had on the car for about 7months. the car has 135,xxx miles. i believe i answered all of your questions. also the mil only flashes on highway it seems
 




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