No display output on Mu

Hi,

I've designed my own carrier board and am currently commissioning it (needs a custom BIOS that I'm checking out)

The unit has previously worked but I had wiring issues on my USB ports. I fixed these but have parked the project for a bit while I worked on other things. I've returned to the project but cannot get my Mu to boot up. I just get a black screen on the HDMI output. I've also tried plugging in a screen to the eDP but this also shows nothing.

When I put the Mu into my Lite Carrier it works perfectly so the issue is with my carrier board.

I am running with stock BIOS at the moment. The only difference for the custom BIOS is to split the x4 PCIe into separate lanes. This doesn't matter for now as I have nothing plugged in. Here's what I've found so far:

- The power and reset button seem to work correctly.

- S0 and S3 are high showing the unit is running.

- The CPU fan is running at normal speed (not fast like it does when the unit sleeps)

- All voltage rails are correct and stable (5V, 3V3, 3V3_Standby, 15V_USB). I even checked where I could on the Mu and saw 3V3 and 1V8 are good.

- It draws the same power (~600mA) as the Lite Carrier.

- When I scope the BIOS signals (you can just get to the CS and MISO pins with the heatsink fitted), I see data reads for around 10s after power on and then no more.

- If I switch to external BIOS (currently unprogrammed), the unit does absolutely nothing, suggesting the on-board BIOS is working correctly.

- USB_OC is high so no issue there.

- No data appears on the Debug UART.

I have nothing plugged into USB, Ethernet, or M.2 so it should be booting from the eMMC into Windows.

I don't know what else to check. Are there any issues on a carrier board that would stop a Mu running? If it's an issue with my HDMI port, I should still see output on eDP?

Does anyone have any ideas how I can debug this?

Thanks