I've had this problem for a while now and the mechanics are having a hard time figuring it out as well.
My 2003 350z w/ 10000k mileage started misfiring pretty badly.
We changed the sparkplugs.
There was still misfire.
I took it to a nissan dealership to diagnose the problem. They said the cats were bad, so I bought brand new ones. But after installing them, they started to burn red hot. Also the car keeps making a popping sound. We decided to keep the car off so the new cats don't toast.
The only code that is coming up is a random misfire.
Could it be the sparkplugs? coils? Intake Leak? ECU?
I've been trying to figure out this problem for a while now and money is becoming a big issue. Those new cats burned my pocket and it's still not fixed. I'm pretty pissed off. Can someone help me out?

 

I'm having a similar issue. any updates?

 

Could check your injectors man. Sometimes a bad injector could cause a misfire code... as ive read.

 

Same here bro. Just got new plugs and no results. I'm gonna move the coils and see if it changes. Wish me luck

 

Any luck with the misfires? Sorry to bring up an old thread but the same code caused me to change the spark plugs on my 03 Z with 68k miles and I still have a misfire. Any ideas on what it could be? Thanks fellas

 

Sorry to revive this thread... but any luck?? I'm having the same exact issue...

 

Every car will need to be checked for MULTIPLE potential reasons for a random P0300 misfire code. There is no magical fix. If you don't have the equipment to do a long-term glitch test, then take it to a diagnostic specialist who can put it through a drive cycle to see if they can get the problem to repeat. If a tech can't replicate a random misfire, then they won't be able to diagnose the problem. The problem with the P0300 code is that it will trigger the CEL after so many misfires are detected and this can take some time. If you're not getting it with a cylinder-specific code as well, then it would indicate that you should look for a more generic cause for a misfire (mechanical, spark or fuel).

Don't throw parts at it or you'll be spending money until you're blue in the face.

 

In my case the P0300 code was due to a leaky valve cover gasket. You see from other posts that oil drips down into the spark plug tubes causing a short in the coil packs. Solution: new valve cover gaskets, new plugs and new coil packs. Its possible I could have gotten away with out replacing everything but the car had 70K at the time and this is a common problem so might as well replace all the bits at once.