The specific type of hardware failure can be determined by a "hidden" error code
Turn the xbox 360 on, and wait till the 3 red lights are flashing.
Press and hold the sync up button (the small white one), while holding that button press the eject button.
The LEDs will now blink the first number in the code (as described below).
Release the eject button and press it again.
The LEDs will now blink the second number of the code.
Release the eject button and press it again.
The LEDs will now blink the third number of the code.
Release the eject button and press it again.
The LEDs will now blink the forth number of the code.
Release the eject button and press it again.
The LEDs will go back to the 3 red flashing lights.
You should be able to determine the difference between the 3 flashing lights and the error code lights by the rate in which they flash.

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