The central locking pump is a vacuum pump that is located in the door behind the door trim, and is housed in a rubber housing to protect it. It is used to open and close the doors on a central locking system when the button on the key or fob is pressed. If a central locking pump fails, it will result in the loss of central locking to the door it controls. This means that the door will be unable to be locked and unlocked, creating a potential security issue.
A vacuum-powered central locking system is virtually defunct in modern vehicles now due to the creation of electrical central locking systems. However some older cars, particularly older Mercedes Benz cars will use this system.
Failure of a central locking pump will break the vacuum that is used to power the locking and unlocking mechanism, so the door will not open or close when the key button is pressed. It is possible that a failure of the central locking system is caused by a leak elsewhere in the vacuum, so it is important to check other components in the locking assembly to establish what has caused the failure