Error E66 occurs and one red light flashes when you use an Xbox 360 console

You see error code E66 on your screen and one flashing red light on your Xbox 360 console, The E66 error contains the following error message.

System error. Contact Xbox Customer Support.
Status code: E66

When this issue occurs, the console is no longer able to connect to Xbox Live, download system updates, or play new games that require system updates:

This may mean that the console has been modified or tampered with.

You can still play your existing game discs in the console in offline mode. To do this, you have to disable automatic sign-in. Here’s how:

1. Restart your console.
2. If you are prompted to install a system update, select No.
3. Press the Guide button on your controller.
4. Go to Settings.
5. Select Profile.
6. Select Sign-in Preferences.
7. Set Profile Auto Sign-in to “Off.”
8. Press the Guide button again to return to the Dashboard.

Your Xbox 360 console can now play your existing game discs offline.