Will not start

Hello, sorry to hear your trouble with a new vehicle. If the actual miles are really 300,000, then it's sounding like the catalytic converter might be clogged up. Seems like the engine is having a hard time what we call "breathing". Same as if you had an extremely dirty air filter. Not much air would be able to get in. In the case with a catalytic converter, the engine can't push it's exhaust out or take in air. What you could do, this is kind of a bad yard test, but take out one of the oxygen sensors up before the catalytic converter and see how it runs. I'm assuming this is a V6 engine but if the converter is that bad, you should notice a difference by taking one oxygen sensor out up front. It will be loud, but you won't be running it like this for too long. Let us know what you find.

https://www.2carpros.com/articles/engine-has-low-power-output

https://www.2carpros.com/articles/symptoms-of-low-engine-power

Oh, I didn't realize it wouldn't start at all now, I thought you were just losing power. You're mentioning you were losing coolant, did you notice if the vehicle had actually overheated by the gauge in the instrument cluster? If it's a no start now, then you need to start with basic diagnostics. If this started with a lose of power, then to a no start, I would check the fuel pressure to start. When you turn the key to the start position, not run, just one turn on key, you should hear the fuel pump prime for 2 seconds. Do this with the door open so you can hear if the fuel pump is priming. It will be a lite buzz type noise coming from the rear of the vehicle. You may have been losing fuel pressure while driving, then the fuel pump may have just failed. I'm having trouble finding information on this vehicle. Does this have a 2.5 liter, 4 cylinder engine, a 2 wheel drive? And are you in the U.S? This looks like a Nissan Xterra. If you can give me the VIN number it will help finding the correct information? I want to make sure I give you the correct information.