Automated Protocol Epsilon Delta Rho

Automated Protocol Epsilon Delta Rho was a feature of the Doctor's TARDIS designed to allow the TARDIS to automatically respond to distress signals without the Doctor's input. When activating, it would be signalled by a siren noise and the letters "ΔΣΡ" appearing on the TARDIS scanners before it dematerialised on its own.

The protocol activated immediately after the newly-regenerated Fourteenth Doctor entered the TARDIS for the first time after regenerating. After being initially confused, he recognised the protocol and allowed it to take him to Wembley Stadium at the 1966 World Cup Final. The distress signal it was following was later discovered to be a trap set by the Daleks. (COMIC: Liberation of the Daleks)

Behind the scenes
When Automated Protocol Epsilon Delta Rho activates in Liberation of the Daleks, the TARDIS' screen shows the Greek letters "ΔΣΡ", meaning "Delta Sigma Rho", instead of the correct "ΕΔΡ".