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I have recently purchased an 03 SER spec v. To replace my beater mazda. Since purchasing the car it started running the p0300 random misfire code and the p0303 cylinder 3 misfire code. I have replaced the coils, plugs, injectors, fuel pump, Cam and crank sensors to try and fix the problem. Then after some more research I learned about the computer controlled throttle body that this car has. (Not impressed with that design nissan). So suspecting the throttle body I took it into a local mechanic who confirmed that I had a bad TB as well as fixing a vacuum leak I had. My mechanic was cool and advised me to replace the TB myself to save money and bring it back in later for reprogramming. So finally after getting all that done. The car definitely drives better. No hard starts anymore. And the idle doesn't surge up and down anymore. But I'm still getting a p0303 code. It still idles a little funny but way better and the car runs and drives well. Sometimes when you start to idle it will rev up to 2000rpm but then go down to 1000rpm and stay there. I don't know why I'm still running a code thought. Since literally replacing everything. I really just think it's a computer isue and nothing really wrong with the car. But I worry if I ever have to smog the car again that it won't pass.
 

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Man, oh man. I had similar issues with my 04. Drove me insane for years, never did figure it out definitively. The only way I got it 'fixed' was with my UpRev tune. I should add that my head gasket needed replacing maybe 12K miles down the road-- so it may have been the head gasket with a slow leak all along, who knows for sure.

I would try cleaning the MAF (make sure it's completely dry before reassembly). It's cheap and easy. https://www.autozone.com/miscellaneous-cleaners-and-degreasers/electrical-parts-cleaner/crc-mass-air-flow-sensor-cleaner-11oz/36011_0_0

Also some have claimed success by running a fuel system cleaner (Seafoam or a complete fuel system cleaner during the next fill up) in case the injector isn't providing enough fuel to properly combust.

Others had success by relocating or wrapping their knock sensor-- I never did this but I just remembered reading about it. Maybe something to research.

Lastly, have you checked the Head Gasket? A mechanic can perform a leakdown test to confirm.
 

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I figured it out. I did the SES reset trick and it worked no more service engine soon light for me. I knew it had to just be the computer since I already had it checked out by a mechanic and the TB got replaced. But I thought I could just clear the code with my scanner but of course nissan can't make it that simple.
 
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