An implantable device, such as a pacer, defibrillator, or other cardiac rhythm management device, can include a failsafe backup, such as a separate and independent safety core that can assume control over operation of the implantable device from a primary controller. In an example, the safety core can include a normal first safety core operating mode and a magnetic resonance imaging (MRI) second safety core operating mode that can provide different functionality from the normal first safety core operating mode.