Regeneration unit
From Tardis Wiki, the free Doctor Who reference
The Regeneration circuit was a component of K-9 Mark I that enabled him to rebuild his body after death, but this could also scramble his memdrive. Seeing as other K-9s, or at least Mark III aren't fitted with a regeneration circuit, it can be assumed he was fitted with one on Gallifrey. (K9TV: Regeneration)
