x15 and the dreaded 559 code.

[marek4792]

buddy has an 2018 x15 performance stock file 565hp 2000tq 500k miles. It is intermittently down on power, did the leak down rail doesn't hold pressure.
-fuel filters replaced with new fleetguard. bypassed for testing purposes with bucket of fresh fuel.
-pressure relief valve and sensor replaced torqued to spec.
-high pressure fuel pump replaced with new unit, core will be checked for failure, maybe not even the culprit.
-when doing the injector cut out engine runs consistently on 1 injector across all 6
-capped each injector and did the leakdown on all 6 test still dropped rail pressure.
-injector performance tests fails at point 3 for all 6 at one point wasn't letting us do it due to "low rpm"
All that's left is injectors and their quill tubes and harness? Guy doesn't want to fire the parts cannon but he's running out of options? any insight thoughts?