A system and method is disclosed by which an implantable cardiac device may deliver tachyarrhythmia therapy in the event of a system fault. A hardware-based safety core provides the logic circuitry for detecting tachyarrhythmias and delivering shock therapy in the event of a fault which disables operation of the device's primary control circuitry. The safety core defibrillator eliminates common mode failure of the primary control circuits used in the primary defibrillator system. Failures in the primary controller memory or execution will activate the safety core defibrillator.