Xbox wont display and powers off

I have a xbox one that doesn’t display and powers off after a minute.

The front light comes on and fan powers up, but no signal to TV.

I’ve checked the following:

• Cable (different cable used)
• HDMI port (Cleaned, looks fine - no broken pins)
• Different button combos used to force low res setting - no joy
• Removal of harddrive and power up - no difference
• Checked for obvious component overheating
• No signs of water damage, although bottom of board does have a white residue in distinct shapes, which I’m assuming is part of the production process.

I plan to remove the CPU heatsink next to check the components underneath, so want to be sure to test everything I can before I take the heatsink off.

Can anyone recommend anything else to test?