Caterpillar's Machine Security System (MSS) won't allow a machine to start with just any universal key. MSS keys are embedded with an electronic chip, each with a unique digital identification. They fit all existing Cat machines and will start them.but on MSS-equipped machines, the electronic controller checks the key being inserted against its pre-approved list of authorized keys. If the key is on the list, the machine will start. MSS is available as a factory-installed option or retrofit for most small and medium Cat equipment with 12- or 24-volt electrical systems.