Here is how you interpret the LEDs to get the code number:
All four lights flashing - 0
One light flashing - 1
Two Lights flashing - 2
Three lights flashing - 3

0001 power supply problem
0002 Network Interface problem
0003 Power problem could be the PSU could be the GPU/CPU, somehow the console isn't getting clean power from the power supply.
0010 over heating
0011 over heating - If you are receiving this error after disassembling your console make sure to all 8 of the heatsync screws are tightened securely to the board/heatsink holes.
0012 over heating
0013 over heating
0020 (Not yet known, possibly overheating)
0021 DVD Drive Time out - Can be caused by problems with a firmware flash. This is also speculated to sometime be caused by a problem with the southbridge chipset on the motherboard.
0022 GPU Error / GPU Overheating
0031 Screws where X-Clamps were are to tight.
0103 CPU Error/ CPU Overheating
0110 Memory Error / Memory Overheating
1003 Hard Drive Error... It could be a problem with the Hard Drive itself or a problem with the internal connection to the hard drive, Try removing the hard drive and playing without it
1010 Hard Drive Error, Can be caused buy a corrupt or missing eProm.
1022 AV cable error... There is a problem with the AV cable, try using a different AV cable. (could also be a problem with the encoder chip)
1023 DVD drive not connected, connect DVD drive to boot
1444 and up There is no "4" in the error codes four lights is a "0" go back and check your code again.