Originally Posted by Ranchorossa
And how does one disable them? I am not familiar with them.
Typically the exit doors have barrel locks on them and when they're closed (i.e. when the door is barrel-locked and can't be opened) the starter interlock circuit is broken and the bus will not start (this is so the driver remembers to unlock the doors before starting off).
If you happen to have inadvertently cut or disconnected the wire for this, that will also break the circuit and prevent the bus from starting.