I have a 2001 Xterra XE that won't start. All lights work, clicks when ignition is turned. Son is ASE mechanic. Replaced battery and starter (actually twice, thinking the first one he bought was bad). Car won't start! Don't have the tools to check computer. Thoughts?

It sounds like the starter relay is working since you here the clicking but you should double check. Also, check the input signal to the starter. See if there is a voltage there when you turn the key.

Check fuses both under the dash and in the engine compartment. Maybe one is blown?

Is it a manual or automatic