Regeneration unit

The Regeneration circuit was a component of K-9 Mark I that enabled him to rebuild his body after death, an act that could also scramble his memdrive. Seeing as at least Mark III was not fitted with a regeneration circuit, it is possible he was fitted with one on Gallifrey. Following regeneration, K-9 remembers his name but cannot recall additional details of his past; memories of his adventures with the Doctor and Leela, his two former masters, appear to have been at least temporarily forgotten. (K9TV: Regeneration)

K-9's regeneration circuit