2017 Rubicon Wont Crank
 

Trouble shooting my buddy's Rubicon. Turn the key and it wont crank, lights work, gauge, winch but it just clicks once. Battery was tested and is good and full charge. If I jump the starter solenoid it will spark and crank but not start. Any help on what to check next?

Does the solenoid click when you press the starter button? Does the EFI make a buzzing noise as the fuel pump primes and the servo motor spins up on the throttle body? Check all the fuses. Modern Hondas have a ridiculously complex wiring system, (Compare it to a Suzuki 500KQ, which also has EFI) involving a relay that fires another relay, both wired through the ignition switch, which then runs most of the "live" side of the wiring loom. You will need the wiring diagram to make any sense of the system, and be at least competent with reading electrical diagrams and testing circuits.

Fuel pump primes. Everything is working like it usually would just wont even try to crank. Theres only about 80 miles on the machine. Barely used. Dealership thinks it is the starter and wants him to bring it in and they will replace starter. I disagree, I jumped the solenoid and it cranks but wont start, that tells me the starter isnt shot. All fuses check out good. Battery is 100% and lights, winch and gauge work. Neutral lights up.

First of all, don't do anything to void the warranty.
Then my first stupid question...is the switch in the run position when you jumper the solenoid?
Second stupid question since I'm not familiar with your machine... Is there maybe a permissive switch that has to be satisfied before starting (like the seat switch that makes you have your ass in the seat of a lawnmower before you can start it) that still isn't satisfied? Normally they prevent the solenoid from being picked up. If you're arbitrarily bypassing them by jumping the solenoid, they may still not be satisfied and prevent starting even though it's cranking (maybe something that grounds the ignition system???). Pull the plug and see if you're getting a spark.
You can also check the solenoid contacts by putting a voltmeter across them. It should read 12 volts open and drop to zero if it makes up properly.

The starter lock-outs are, as Yoakum wrote on the kill switch, it won't operate the starter if the kill switch is in the Kill position on most Hondas. The other lock-out is the neutral light has to be on for the starter to work. On older Hondas this also gives a better spark, you often can't see one if spinning the engine over in gear with the solenoid shorted. If the Honda dealership are not complete Morons, the first thing they will do is check power is getting to the starter when the button is pressed, our standard test is to put a high wattage 12v test lamp between starter live post and earth. Press button, if the lamp lights but starter doesn't spin, faulty starter, if the lamp doesn't light but solenoid clicks once, faulty solenoid. If solenoid buzzes, flat battery or bad heavy duty cable connection (including earths). If solenoid doesn't click, still could be solenoid, but could be a lock-out, or start button, or any wiring in these circuits, so start checking the thin wires to solenoid for power with start button pressed and work back until you find the fault.

Sort of off-topic, but the little red multi-meters that Harbor Freight gives away for free (with purchase) are well worth having in your tool box.