Xbox 360 light starts flashing red when hdmi is plugged in

This is an RROD that indicates GPU failure. The system is no longer repaired or supported by Microsoft, you'll have to buy a replacement

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today despite this Garry

Sorry to hear that, just to cover everything from your side.
Did you tried using a different HDMI cable, a different TV or using a different power brick to see how it goes?

When you say "the light starts flashing red" are you seeing like a "red ring" around your Xbox 360 power button?

Hi,

It's an XBOX 360 Pro, and old style one with HDMI, The games work on other consoles and there's one red light flashing. I have the xobox upright and it's the bottom right light that flashes

Web translation:

I was playing and the red lights appeared

I was playing and the red lights appeared

Response:

Hi there,

for any red light issues please check the steps on this link:
https://support.xbox.com/xbox-360/console/flashing-lights-solution

or
https://support.xbox.com/xbox-360/flashing-lights/power-button-blinks-red

Those links should have all steps to try when you encounter red lights.

Let us know the results

Try this it might help

support.xbox.com/.../flashing-red-light-on-the-xbox-360-s-console-power-button-2122791