How to find your error code:

The specific type of hardware failure can be determined by a "hidden" error code

• Turn the xbox 360 on, and wait till the red lights are flashing.

• Press and hold the sync up button, 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 1/2/3 red flashing lights.

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

For example you error code could show 4 4 2 4 which interprets to 0020

--==ERROR Codes==--
If there is a problem with the Xbox360 The LEDs in the ring of light will flash in a particular pattern. The pattern itself leads some insight to the problem. Certain patterns will lead to numbered error codes which will provide further insight to the problem. The numbered error codes are described below their corrosponding LED code.

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Sections 1, 2, 3, and 4 are flashing red
- The AV cable cannot be detected

THINGS TO TRY
Make sure that the AV cable is correctly connected to the Xbox 360 console.
Disconnect the AV cable from the Xbox 360 console, and then reconnect the AV cable to the Xbox 360 console.
If the four flashing red lights continue to flash, try wiping the metal area of the AV pack with a dry cloth. The metal area is the end that plugs into the console. Wipe the metal area thoroughly, and then try the AV Pack again.
If the AV cable is correctly connected but the four red lights are still flashing, substitute a different AV cable if you have one available.
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Sections 1, 3, and 4 are flashing red
- General Hardware Failure
 

 Check out this tutorial! If you are getting the 3 Red Light Fix (AKA Three Red Lights of Death, 3RLOD, Red Ring of Death, X-Clamp Fix) This link has the information you need!!

Check the secondary Error Code per the instructions in the section below

THINGS TO TRY

Try restarting the console.
If restarting the console does not resolve the behavior, follow these steps:
Turn the console off.
Unplug all the power and AV cables from the console.
Unplug the power cord from the wall socket.
Firmly reconnect all the cables.
Turn on the console.
If these steps do not resolve the behavior, turn the console off, remove the hard drive, and then turn on the console. If the 3x Red LED error light is no longer displayed, turn the console off, re-attach the hard drive, and then turn on the console.
Also examine the lights on the power supply. When you turn on the console, the power supply light should illuminate green even if the three lights on the RoL flash red.
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Sections 1, and 3 are flashing red
- Overheating

Alternativly Overheating can be cause by the console locking up or "freezing" after a set amount of time. Lockups that occur at a specific point in a game are generally problems with the game itself and not caused by overheating.

THINGS TO TRY
Let the Xbox 360 console cool.
Note You may have to wait several hours for the console to cool enough. Do not turn on the console when the console is hot.
Verify that the console has sufficient ventilation and that the fan is operating. For more information about ventilation, see the "More Information" section.
PREVENTION SUGGESTIONS
To try to prevent this problem, use the following precautions:
Do not block any ventilation openings on the Xbox 360 console.
Do not put the Xbox 360 console on a bed, on a sofa, or on any other soft surface that may block ventilation openings.
Do not put the Xbox 360 console in a confined space, such as a bookcase, a rack, or a stereo cabinet, unless the space is well-ventilated.
Do not put the Xbox 360 console near any heat source, such as a radiator, a heat register, a stove, or an amplifier.
DO NOT EVER PUT YOUR CONSOLE IN THE FREEZER OR OUT IN COLD WEATHER
Some people think that doing this might help "fix" an overheating console, in fact doing so could cause even bigger problems. Electronic devices like your console are made to function at and around room temperature, extreme cold temperatures can often cause just as many problems as extreme warm temperatures (such as overheating). Also if the console is located in a dramatically cold area while running, the difference in temperature between the hot console and cold air can create condensation which in turn will short out your console making the situation far worse. Condensation will happen more quickly if your console is overheating due to the greater difference in temperature. In short... DON'T DO IT.

A more agressive Solution to overheating is to improve the cooling system of the console. The best solution is to replace the thermal compound. If you attempt this you do so at your own risk. If you're console is still underwaranty it is recomended that you contact MS for a replacement before attempting to fix it yourself.

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Section 4 is flashing red
- Hardware Failure

The Specific Type of hardware failure can be determined by the error code displayed on the screen.
If the screen is blank or you would like additional information follow the instructions for determining the secondary error code in the section below
E45: Unknown (possibly dashboard update related)

E64: DVD Drive Error.... DVD Timeout, Wrong firmware, dvd is without f/w chip, etc.

E65: DVD Drive Error.... DVD Timeout, Wrong firmware, dvd is without f/w chip, etc. This can also be caused by the tray not being fully closed on boot.

E66: DVD Drive Error: DVD model, or version does not match that of the version expected by the dashboard. OR the firmware version on the drive is older then the firmware version expected by the dashboard. Make sure the DVD drive is of the same version originally included with the console and that it is using either the original firmware included with the console or newer.

E67: 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

E68: 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, This can also be caused by a Hard Drive Eprom Error. Some also believe this might be caused by a problem with the fans.

E69: 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

E71: possibly a dashboard update error, Check below in the "Console Reset Codes" for instructions. If that does not work there is no other solution and the console must be sent back to MS for repair.

E72: (not yet known)

E73: General Hardware Error: Ethernet port... this error is caused by a problem with the Ethernet port.

E74: Same as 0102 below --- in rare cases there is a problem with the AV cable, try using a different AV cable. If the cable is known to be working then there is a 90% chance it's a scaler chip problem (the "ANA" or "HANA" chip connected directly to the AV cable)

E76: (not yet known)

E79: 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

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Secondary Error codes

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
0023 (not yet known)
...
0101 (not yet known)
0102 unknown error
though it's most likely GPU related literally means the console does not know what is wrong. possibly a short or cold solder joint somewhere. So far there is 1 major theory for what causes most of these errors is that the Ram, CPU, or GPU are not soldered properly to the mother board due either to poor manufacturing or excessive heat up and cool down cycles that stress, weaken and eventually break the soldered connection.
There are two theories to fixing this one deals with the "X" clamps that hold down the chips. The other involves re-Heating the chips. DO NOT attempt either of these if your console is still under warranty. If your console is still under warranty return it to the store where it was purchased or call MS to have it replaced. I have also heard of limited success by simply switching the AV cable from "HDTV" mode to "TV" mode.
0103 CPU Error/ CPU Overheating - see solution for error 0102
0110 Memory Error / Memory Overheating - see solution for error 0102
...
0200 (not yet known)
...
1000 (not yet known)
1001 (not yet known)
1002 (not yet known)
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.
1011 (not yet known)
1012 (not yet known)
1013 (not yet known) possibly a dashboard update error
1020 (not yet known)
1021 (not yet known)
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
1030 (not yet known)
1031 (not yet known)
1032 (not yet known)
1033 (not yet known)
...
1444 and up There is no "4" in the error codes four lights is a "0" go back and check your code again.

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--==Console Reset Codes==--
Clear All Installed Game Updates and Console Cache
Go to the "system" blade
Select "memory"
Press Y on the HD symbol
Press X,X, Left Bumper, Right Bumper, X,X
A message will appear saying: "Do you want to perform maintenance on your Xbox 360 storage devices?"
Select Yes
Clear Any Failed system updates that cause the console to error.
With the console off, press and hold the sync up button (the small white one)
While holding the sync button press the power button to turn on the console
Continue to hold the sync button until the Console has booted up completely.
During the boot process the console should clear any failed updates, allowing you to use it normally.