When you start up your Xbox 360, you get this error:
System Error: Contact Xbox Customer Support
Error code E12
To fix Error code E12 on Xbox 360 E or Xbox 360 S:
1. Disconnect and reconnect all accessories.
2. Remove the hard drive.
3. Remove any USB flash drives. Here’s how:
a. Turn off your Xbox 360 console.
b. Unplug any USB flash drive attached to your console.
c. Turn on the console.
If this step resolves the problem, reattach the USB flash drive to make sure that the error occurs only when it is connected. If this is true, use another USB flash drive.
4. Roll back the current software updates on your console and apply new ones. Here’s how:
a. Press the Guide button on your controller, go to Settings and select System Settings.
b. Select Storage.
c. Highlight (but do not select) Memory Unit.
d. On your controller, press the following buttons in the following order:
I. Left bumper
II. Right bumper
III. The X button
e. Repeat step D.
f. When you see the following message, select Yes:
Delete System Update
If any storage device contains a system update, the update and access to Xbox Live will be removed. To use these features again, sign in to Xbox Live and download the update.
Your console will restart after the operation is complete. Do you want to continue? Yes | No.
g. Your Xbox 360 console will reboot. Apply the latest software updates and E12 should be fixed.
If you have an Original Xbox 360, look at the flashing light and look up the code on the Xbox website.
If the above doesn’t fix error code E12, then you’ll need to contact Xbox support and organise a repair.