I'm the original owner of a 2003 G35 Coupe 6MT. It's been about 12 years and almost 250K miles. Lately, when I drive my car agressively, I got a O2 sensor code, then a few months later, a misfire code, P0303. I changed the spark plugs, but the car feels like there's hesitation at higher RPMs (3,500 or above). After a few weeks of NORMAL driving, the car seems to be running better. Does anyone else have any recommendations on where to look for additional problems? I'm hoping I didn't miss anything and cause more problems in the future. Thanks for everyone's help.

PS: I have not changed any of the oxygen sensors and they are all OE. I do notice that the exaust tips are more darker than usual after driving for a week.

 

I just googled P0303 and it sounds like cylinder 3 has a misfire. I'm betting that its your coilpack on that particular cylinder. #3 is going to be the middle coil on the passenger side. With the amount of miles on your car, you might as well replace all of them just to be on the safe side. As far as the o2 sensors, again with the amount of miles on your car, you might as well replace those since it will affect response. I know it sounds like i'm telling you to throw money at your car, but wow 250k miles!? I thought I was getting old with 115k on my original owned 04 coupe

Hope this helps.

 

^^ true - most likely the coil pack for #3