Emotional inhibitor

An emotional inhibitor was a device fitted into all Cybermen. It allowed a human brain to exist without suffering a "meltdown" of overwhelming emotions at what it had become.

History
At the Battle of Battersea Power Station, Mickey Smith hacked into Cybus Industries' mainframe from Pete's World and located the code which all of the Cybermen's emotional inhibitors used, 6879760 (which was encrypted under "binary9"), and texted it to Rose Tyler's mobile phone. The Tenth Doctor loaded it to the computer system. The code was shut down and the inhibitors turned off. All the Cybermen realised what they had become and died from the emotions they felt. (TV: The Age of Steel)

Soon after, an airship was sent to the Cybus factory in Paris where it transmitted the same code. It disabled that factory's emotional inhibitor. (GAME: Airships) According to another account, a Preacher was sent into the factory to destroy the inhibitor with his firearm. (GAME: Cyber Assault)

The Cybermen of the Cyber Legions, who placed people into suits, also used emotional inhibitors. When Craig Owens fought back an attempt to convert him using the love for his son, this overloaded the emotional inhibitors of the six Cybermen trying to convert him, resulting in their destruction. (TV: Closing Time)

When Danny Pink was converted into one of 's Cybermen, his emotional inhibitor was off as he chose not to delete his emotions in the Nethersphere. This allowed Danny to retain control of himself, but in order to access the Cyberiad, he needed it turned on. Clara Oswald used the Twelfth Doctor's sonic screwdriver to turn it on, but Danny still retained control as his love for Clara overrode the inhibitor. Later, the Brigadier appeared and like Danny, he still retained control even with the inhibitor due to his love for his daughter. (TV: Death in Heaven)

When Bill Potts was mortally wounded on a Mondasian colony ship orbiting a black hole, she was converted into a Cyberman on Floor 1056. Her emotional inhibitor was the actual handlebars of the headset, the surgeon telling her it would make her "not care" about the pain she would experience. The other prototypes in the hospital lacked this feature, many constantly using their talk boxes to cry out about such, though the nurse would turn the volume down so as to make it inaudible. (TV: World Enough and Time) Despite this, due to her strength of mind, Bill retained her personality and emotions. (TV: The Doctor Falls)

Having been only partially converted, Ashad lacked an emotional inhibitor. Being a willing recruit to the Cyber-Empire, he mostly retained control of himself, claiming to the Thirteenth Doctor that he did not need to be stabilised, though he was prone to emotional outbursts. (TV: The Haunting of Villa Diodati, Ascension of the Cybermen)