Sometimes an Xbox 360 console might display an E18 error. This is similar to the RROD error in that it probably deals with much the same problem as the RROD. However, in this case, an E18 would be displayed on the screen, whereas with RROD, no error is generally displayed, the system simply does not boot. Accompanying an E18 in that case would be a single red blinking light in section 4 of the ring of light on the console. The on-screen message instructs the user to contact Microsoft Customer Support.
The E18 error indicates that there is an error in the Digital Backbone, which includes the CPU, GPU and RAM. The culprit is usually a cold solder joint between the GPU and the motherboard which disrupts communication with other components. There are many theories as to what causes this problem, but overheating is usually at the center of them. The X-Clamps on the underside of the motherboard that hold the heatsinks in place are often blamed as they can bend and cause the board to warp when exposed to heat. This could cause a problem with the connection between the GPU and the motherboard.
This problem "may" be covered by Microsoft's extended warranty (three years) for all consoles affected by RROD and E74-related problems. To find that out, contact Microsoft Customer Support as directed on screen. For those without warranty, a fix for this problem may be to try an X-Clamp Replacement, where the X-Clamps are removed from the underside of the board, and then replaced with bolts and washers to hold down the GPU heatsink. Another solution is to carefully heat the affected area with a heatgun, although more information on this should be sought with Google as doing this could damage the board beyond repair if components aren't insulated correctly.
The error code associated with E18 is 0102 (this is the same error code often gotten from RROD-affected consoles). To get an error code from an Xbox 360 console that is displaying an E18 error (or 3ROL) you simply press and hold the "sync up" button (used to connect wireless controllers) and then press the Eject button for the DVD drive. This will bring up the first digit of the error code (usually it is all four lights blinking red, which represents 0, not 4). With the sync up button still held, press the Eject button again and you get the second digit. Do this twice more to get the full 4 digits of the error code.
For more information on the error code you receive, or on Xbox 360 hardware problems in general, read this Xbox-Scene forum thread.