Regeneration unit

The Regeneration unit (also referred to as the Regeneration tank and Regen Disc by Thorne and Starkey respectively) was a maintenance device implanted in robots such as K9 Mark I.

Properties
Although designed to repair small fault, such as a faulty wire or a clogged gear, the device could build a new robot from the wreckage of the original. The unit's specialty was critical energy levels. Backup power was stored in the device to recharge the robot's powercore. Under certain circumstances, it could reconstitute a lost accessory with the excess power. If a robot were only an inch from death and the regeneration unit worked, the robot would be resuscitated. The reconstitution of lost accessories was a result of the default setting. The robot would be returned to its default state, as good as new. One was built into K9 Mark I.

History
K9 used the circuit on the Bi-Al Foundation when infected by the swarm. He was able to regain his mind and help Leela. (DW: The Invisible Enemy)

After K9 self-destructed to defeat four Jixen warriors, Starkey retrieved the unit and K9 regenerated into a new body. Following regeneration, K9 remembered his name but couldn't recall additional details of his past; memories of his adventures with the Doctor and Leela, his two former masters, appeared to be at least temporarily lost. (K9TV: Regeneration)

Thorne and Lomax knew of the Regeneration unit and decided to acquire it starting with Thorne's failed plot to cause K9 to regenerate with the use of a suicide bomber. (K9TV: Robot Gladiators) They eventually succeeded, Thorne coming to Darius Pike with a proposal, not giving the requested component's name and calling it "the cybernetic equivalent of an appendix". He had him try to convince K9 to agree into surrendering the device in return for his missing memory chip, promising Darius his father would be released from his imprisonment if he complied. K9 thought it a trick, although he couldn't resist the opportunity to discover his lost memories and accepted; it turned out to be a trick. Thorne gave the unit to an unknown force. (K9TV: Hound of the Korven)

The unit was contained inside the Trojan, a behemoth created by mixing together DNA from aliens encountered by K9. After K9 depleted his energy banks to defeat the Trojan, the absence of his regeneration unit signalled his demise. However it returned to him just in time, recharging his powercore and giving him a new lease on life, reconstituting him completely. (K9TV: The Eclipse of the Korven)

Behind the scenes

 * The regeneration unit prop had writing on it which resembled the Greek-and-mathematics text used to represent Old High Gallifreyan in the classic series of Doctor Who. Specifically, it contained the phrase "δ³Σx²", which was used to represent the Doctor's name in the first edition of the 1972 behind-the-scenes book The Making of Doctor Who by Terrance Dicks and Malcolm Hulke. This would seem to suggest that K9's regeneration unit was installed by the Doctor.
 * Despite this, K9 used the circuit in The Invisible Enemy, meaning he had it long before he had encountered the Doctor.