Xbox 360 0020 error code

Many people have. You need to replace the xclamp and make sure there's extra pressure (less than half a turn) on the front right corner of the GPU. You can read more about xclamp replacement here.

what i did was took my xbox a part readying on eople makeing it over heat whit a towle i use a heter i disconted to fans at the back and i made it over heat with it switch on till the 3 lights when to 2 red light the cooled it down but it back to gever and it worked

here is a site i found that shows detailed fixes for this error with your 360's - here it is

First, my xbox would have one red light and E 74. Then when I opened it up and booted it I got three red lights and 0020. I took out the motherboard, replaced the heatsink compound and did the x-clamp fix, but I couldn't get nylon washers in the right size so i just used no washers at all. Varying the tightness of various x-clamp bolts gave me 0020, 0003, 0002, 0001, or it would boot up to the black screen and I'd get E 74 again. Other times the xbox would almost immediately overheat. Most of the time I would get 0020, though. At that point, I gave up and I just tightened up the bolts replacing the x-clamp a certain way (nothing seemed special about it) and put the motherboard back in the case without testing it. I screwed everything back together, and since the heads of the bolts were thicker than they should have been, the motherboard didn't really sit flat in the metal cage. But i just screwed the motherboard back into the cage not to tight with the 9 small silver screws, which I would imagine put some stress on the mobo. Then I pu the rest of the case on and the 6 long screws, and once I had snapped everything back together I tried booting it up. To my great surprise it booted up fine! I played some halo 3 and it has booted up fine every time (3 times) since then