A vehicle computer communicatively connected to a portable calculating device and is programmed to determine in a lead vehicle that one or more conditions used for a diagnostic test (for example, an on-board diagnosis (OBD) test) are satisfied, and to send messages between vehicles to one or more following vehicles at specified time. The sent messages provide data indicating each following vehicle to execute testing at specified time. A vehicle computer is also programmed to execute testing in the lead vehicle at specified time.