07 Xterra, starts and runs fine mechanically, no codes thrown, only idles?

It could be a lot of things hard to guess without being able to do some tests first hand. Air flow meters used to give problems but would set a lean code. Could the catalytic converter be plugged up? Dropping the exhaust at the manifold or front of cat and then revving the engine would prove or disprove this. I would attach a vacuum gauge to the engine manifold and see what intake manifold vacuum is should be approx 21 in HG. Lack of fuel pressure or delivery volume? A professional grade scan tool reads stored trouble codes, pending trouble codes, looks at live sensor data so you can look at a sensors performance and output signals without having to do intensive testing at each sensor, (cheaper scan tools have limited capabilities and may only read the trouble code stored). As with most things automotive it's best to check the basics first, check for vacuum leaks, fuel pressure and delivery volume. Verify engine mechanical integrity before looking for complex problems check that the basics are sound.