Fail-Safe Detonation is a term that is used to explain the self-destruct system of a WIA or KIA SPARTAN-II's armor. Once a SPARTAN-II is killed or wounded fatally, the protocol will have to be enacted by either themselves or a nearby Spartan. This is done to avoid capture of the suit and/or the wearer for identification or technological study. It is unknown if the Mark VI has this capability, since the Mark V is said to have one.
The tiny access panel at the back of the suit is opened to activate the protocol, it is activated by typing several digits only Spartans know. Once activated, a fifteen-second countdown timer starts. Once it hits zero, the suit's fusion reactor overloads and burns everything in its path in a ten-meter radius with a follow-up explosion to finish anything alive.
- John-117 is the only known Spartan to activate the fail-safe detonation (on Grace-093's suit after she was killed by a Jiralhanae in the Covenant city of Unyielding Hierophant).
- It is known that activation of the armor's fusion reactor is part of the Cole Protocol and failure to activate the core when possible will result in court martial and/or stripping of rank.