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Problem solved ! Corsair support asked me to check the pins of the 24pin connector, and it turned out that pin 4-5 were slightly "opened" … I closed the female pins a little more, and now it works fine :) (see 24pin.jpg) This could have been a result of using the paperclip method to start up the PSU to test my water cooling loop without connecting anything else to the PSU. So next time, I will be a little more careful about the pins :)

Hi, a little late as you already could RMA it, but this sounds similar to what i'm now experiencing with a (also brand new sealed) RM850: https://forum.corsair.com/v3/showthread.php?t=189694 my PSU only succeeds in starting up the motherboard if I leave in the paperclip used for the self test … like you said, it could very well be "cable has built in capacitor that could be faulty" (or some motherboards not reacting to it as expected)

Hi all, There seems to be an issue with my new RM850 (2019) I just got for a new computer: When connecting the 24pin ATX to a new Asus Crosshair VIII Hero motherboard, the motherboard's RGB LEDs light up, and also the start button on the motherboard. But when trying to start up (by pressing this integrated start button on the motherboard, or the case button) nothing happens … Here are some things I tried: - Paperclip+fan method test is OK > so the PSU delivers power (also the LEDs light up). - When connecting the 24pin ATX to my older Asus Crosshair VI Hero, I have the same effect (and it works fine with a HX750i). - When connecting the 24pin ATX of my HX750i to my new Asus Crosshair VIII Hero, it starts without any problem. There seems to be something wrong the RM850 receiving the 'start signal' from the motherboard ? By using the paperclip method and leaving the paperclip in the 24pin ATX connector (from the backside), I've been able to start the motherboard, boot and install windows ... But I have a feeling that this is not how it should be ;) Any idea's why this is happening, is there something I can do about it ?