It gives me great pleasure to report to all of you that I finally found out what the hell is wrong with my Xterra. if any of you remember I complained about losing mass horsepower after 2000 RPM and continually coming up with a p0 300 code. After throwing at least $1,000 into the motor. New plugs and coil packs valve cover, crankshaft position sensors camshaft position sensors. 3 hour diagnostic at Nissan. only for them to tell me on the fourth hour they're getting ready to pull apart catalytic converters to see if they were bad.I told him no that's not necessary cuz I had already figured out that's probably what it was going to be. At a shop had catalytic converter changed out on the driver side. Only for them to report that there was no change. The mechanic not knowing much at all stated that he thought one of the cylinders was dead. I thought that was kind of weird seeing eye when you start the truck up it sounds perfectly fine.so the next thing I did was disconnect the pipe from the second catalytic converter to the muffler, and the second catalytic converter was as plugged as the first. The truck being a little louder than normal it ran great. Drove 40 miles up and back on the expressway between 70 80 miles an hour. And not one single hiccup .if anybody has similar problems like this around a hundred and fifty to hundred sixty thousand Miles please look into the catalytic converter before doing anything.