Within 10 seconds of turning on my Xbox 360 S, even after not playing it for days, the light goes from green to red dot. No screen image or error message. Here's what I've tried to fix it but nothing has worked yet:
Checking power cord and box, and using a brand new one,
Checked all inner and outer cables/connectors,
Pulling out 250 gb hard drive, shaking and putting back in,
Taking Xbox apart and cleaning all dust from it,
Taking out hard drive and trying to boot,
Taking out CD drive and booting,
Replaced the fan with a Whisper quiet,
Unhooked the fan and let Xbox overheat so as to reset it (did exactly as DB said in his excellent video).
I've now got my heat sync and chip apart and will buy Arctic Silver 5 today and re thermal paste the chip and reassemble. I hope it works but don't expect it will because the red light comes on so fast, there's no time for it to be a heating problem.
If any serious tech out there reads this and can offer something likely to work, please do. I have no warranty and the box is about 3 years old (but has been well taken care of, well vented, cleaned, etc.
Okay, I did the Arctic Silver thermal clean and replacement idea and still a red dot. Now I'm trying the DB overheat reset idea again.