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I drive a 2007 Pontiac G5 2.2L. It has a cylinder 3 misfire, and whenever it does have a misfire it shifts hard between 1-2 and 2-3. When you cold start it it'll idle fine for a few minutes and then you can hear the misfire start, it'll start making weird noises and idling really rough. I replaced the plugs a couple months ago (they were cheap plugs.) I also got the code ran again and it had a O2 sensor code, coolant Sensor code and the misfire. I replaced the coolant sensor and replaced the cylinder 3 coil. And it was fine until about 3 hours later i got in it and it started missing again and shifting hard. I'm really confused as to what it could be, I do know that it needs a catalytic converter as well. Any help will be greatly appreciated!
 

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Could be an injector as well.
 

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P0303 is not necessarily a cylinder 3 misfire. Are you getting a ''U'' code? GM Techs have been chasing the P0303 gremlin for a long time. The ECM for some reason puts out a P0303 when it doesn't know what to put out. Put a scanner on the car that can count misfires and at which cylinder to be sure you have a misfire. Clean the MAF and throttle body, make sure your air filter is clean enough. Reseat connectors on ECM, TCM and BCM. Disconnect battery while doing all this and make sure key isn't in ignition while cleaning throttle body.
 
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