After confirming that the quadro functioned fine, I decided that was a good excuse to completely re-run every cable in the system, and work my way back up, and this time, after reconnecting everything, it works exactly as expected. I'm truly baffled where the problem was and hate not discovering the root cause, but re-connecting everything from the ground up solved the problem.

Some additional information: I used a bridging clip on a second power supply to enable it to be used without a mainboard, and then a molex connector to the pump and Quadro, and it works perfectly. I can even connect the USB back to the mainboard and it works completely. However, if I connect power to the quadro from the same PSU as the motherboard, it does not work.

I've tried two different power supplies and two different cables. I don't think it's that side.

Confirmed that with only the molex power plugged in and no sensors, fans or usb, it still does not power on the computer. Ohms across the 5v and ground next to it (right side of the power port with mitered corners up) reads ~12k ohms which is the same as the bottom of the circuit board solder locations. I don't know if that's good or bad.

New 2020 build: 3900XT, Asus ROG Strix x570-e Everything went well installing the Quadro and fans originally. 2 ambient sensors, 5 fans split across 2 radiators and connected to Fan headers 1 and 2. EKWB D5 PWM connected to header 3. Had everything tuned the way I liked it. Was seeing sporadic hard-resets and situations where the computer would refuse to power on. There was a brief flash of power but then nothing other than ambient Mainboard lights. I decided to try a different PSU (brand new Se...

I'm having an almost identical problem with a Quadro controller. If I plug in the 5v molex to the PSU, with USB connected to a MB header, it won't power on. If I remove the 5v Power, it boots fine. It previously worked fine. I've tried two different power supplies (same problem), I've tried multiple different cables between the Quadro and the PSU, different ports on both PSUs, and different USB headers. The only thing I can't change easily is the Quadro. Did you ever figure your problem out?