Just got the Z590 Dark and swapped out my Z490.  System has a 3090 KPE and an EVGA 1600 T2 PSU.  When I start the system I get stuck on POST code 97.  This appears to be related to the GPU.  I've tried a few things listed below:
 
- Updated the Z590 Dark BIOS to 1.03 (most recent) - No change
- Tried different BIOSes on the Z590 and the 3090 - No change
- Disable PCIe 1 (Turning off the 3090 effectively) - System boots fine with iGPU
- Disable iGPU - No Change
- Reseated the 3090 - No change
- Swapped out the 3090 for a GTX 750 - System boots fine with the GTX 750
- Enabled ReBAR - No change
 
So the system works fine with other another card and the iGPU.  Any other suggestions for things to try? 
 
As a last resort I could try the 3090 with the old Z490 board again, but the 3090 has the HydroCopper block on and is in a loop, so I'd had to drain the loop and pull it to test (and do a quick test to POST)

Hello, I’m new to liquid cooling and I’ll be doing my first loop soon so I can’t have much say.
But if you google “Post code 97”
There are forums that cover this issue already and some are even related to EVGA forums.

If you haven't, try disabling CSM and Fast Boot in the BIOS, and see if you can get a good POST with the card in that way on either of your motherboards. If that doesn't do the trick, then the 3090 probably needs replaced. 

Sorry to hear about the RTX 3090 fault. Hopefully the RMA will be quick.