So here is a weird problem ..i can start my2015  Z6 and it starts fine ..shut it off and try to start it again and everything works electrically , light, dash fuel pump 4x4 switch all. except the start/solenoid circuit then after leaving the Z6 to sit for 20 Min. it starts right up ....shut it off and hit the key again and it rolls the starter maybe 2 times then nothing ,no start circuit and no sound from the solenoid ...wait 20 min. and it fires right up again ...so after a lot of testing and studying the wiring diagram i see there are 2 relays under the pass side dash that send the signal to the starter solenoid under the seat so i found the relays on Amazon 3 for 14.99 so since the Z6 takes 6 i ordered them... fought to get the complete relay wiring harness down to see what i was doing and replace all 6 relays and presto problem solved ...to test the old relays is found the 4x4 switch circuit and tested each old relay and found 1 bad  relay and on questionable so that's why i replaced them all and kept the 4 relays that worked for on the trail in the emergency tool kit ..
I then pushed the relay harness into a piece of 2" abs pipe cut to fit and zip tied it to the cross bar under the pass. side dash for easy access on the trail . guess the Chinese ones don't last ..my Z6 only has 750 Km on it ..

Hope this helps a no start Z force 600 EFI