Question Is there a BIOS setting for a PCIe extender on the z390?

[XLer8]

I have a Asrock Z390 Taichi Ultimate and a PCIe extender.

All works on other slots but main. GPU, tuner and nic tested. Works on older non-igp system.

 

[PingSpike]

You could try turning the PCI-e link speed down to 2.0 or even 1.0. Those extenders seem to vary wildly in quality and ability to support full speeds.