A system includes a control computer 160 that is programmed to perform an authentication based on an encryption key, upon being connected to a vehicle communication network 190. The computer is programmed to control vehicle operation including at least one of propulsion, braking, and steering, upon authentication by a vehicle computer that is physically attached to the communication network. The authentication may include sending a message between the computers, including authentication signatures based on the encryption key. The authentication signature may be based on a vehicle or computer identifier number or a time. A request for operating the vehicle in a manual mode may be made at a user interface 140. The request may be accepted if a correct access code is provided. Manual or non-autonomous control of an autonomous vehicle may be necessary in cases of repair or maintenance.