1)If you worked on the mainboard recently(soldered or so) remove these wires because you might be drawing too much power from the Southbridge
Also look for missing/bridged components like capacitors/resistors
2) X-Clamp replacement
3) Southbridge X-Clamp-Fix
4) Reflow the Southbridge/Ethernet chip area
5) Reball the Southbridge - bridged solder
6) if you work in dumping lite-on 8xxxx and 9xxxx with jumper, Check your dvd drive pcb (board) again, if there's still some shorting.
7) Wrong GPU model (e.g. Zephyr/Falcon GPU reballed onto a Xenon board).

0021 error code meaning.

CustomRGH wrote 1)If you worked on the mainboard recently(soldered or so) remove these wires because you might be drawing too much power from the Southbridge
Also look for missing/bridged components like capacitors/resistors
2) X-Clamp replacement
3) Southbridge X-Clamp-Fix
4) Reflow the Southbridge/Ethernet chip area
5) Reball the Southbridge - bridged solder
6) if you work in dumping lite-on 8xxxx and 9xxxx with jumper, Check your dvd drive pcb (board) again, if there's still some shorting.
7) Wrong GPU model (e.g. Zephyr/Falcon GPU reballed onto a Xenon board).

0021 error code meaning.

He asked for help. Not google. If you flashed your console just recently, you may have a bad flash. If you can't boot XeLL and can't solder/don't have a JRP or Nand X, send it to someone who can flash you back to stock and start over.