What are the 3 rings of death on an xbox360?

'Three Rings of Death' (or 'Red Ring of Death', often abbreviated to 'RROD') refers to the way that three of the four light quadrants on an Xbox 360 light up red when a specific problem occurs with the hardware. Although there are several proposed methods of solving the issue once an Xbox has RROD'd, most require the owner to open their Xbox 360, thus voiding their warranty.