My 2004 GTP miss fire at 2800 and up. It runs and starts fine up to 2800. Had it scanned and code po455, and po300 showed up. Checked out fuel pressrue, fuel regulator, sparks plugs and wire, and air filter. All of tose are good. So what is the problem? Any suggestion?

http://www.obd-codes.com/p0300
A code P0300 may mean that one or more of the following has happened:

Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked EGR valve / passages
Faulty camshaft position sensor
Defective computer

http://www.obd-codes.com/p0455
With a p0455, the most common repair is to:

Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back.
Otherwise, replace the gas cap, and/or
Inspect the EVAP system for cuts/holes in tubes/hoses (you may hear a vacuum noise or smell fuel), repair if necessary

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01 GTP SE Sold. 89 Mustang Vert 331 T-trim

I bet one of your coil packs are bad. Also check your spark plugs to make sure the ceramic is not cracked. I have seen both of these happen many times.

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Thanks,
TDCRacing

I would bet it is your spark plug wires. Replace them with stock or PRJ's.

Been there, done that.

I just had a bad coil pack on my 04 L26.

Pull each wire off the coils one at a time and start the car. It will run and the coil will be arcing. The one that went bad on mine was a little brownish instead of the clean silver look.

I would check it out before spending money. I already had MSD wires and bought the ZZP 10.3's thinking that was the problem, but it was not.

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04 GTT - Retired
Best ET 13.016
Best Trap 115.56 MPH

I would also check the MAF. What type of conditions cause the misfire? Wide open? Gentle uphill cruise? Any fuel trim data?

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Six-speed top-swapped GT, sold.
2006 F-150 4x4 for a daily driver.

Im gonna ask if there was any work done to it recently so that we can narrow down what the problem may be.

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2000 Pontiac Grand Prix GT