The Toyota Prius is a gasoline/electric hybrid, and one of its operational advantages is that it shuts down the gas engine to save fuel and avoid idling when the vehicle is stopped. The car's popularity is growing faster than some of the technical knowledge required to service, inspect, and sometimes even just drive the cars. This page teaches one of the many tricks that can help the service technician work with these vehicles.
Engine shutdown will also happen in an inspection bay, which may cause OBD-II based testing equipment to be unable to complete its emissions-monitor tests without seeing a non-zero RPM. The engine must be idling for some test equipment to get valid readings.
Here is the easiest way to make the Prius engine start and stay idling for the purpose of inspection, diagnosis, or whatever. Assuming the car is already powered up and the "READY" light is lit:
The engine should remain running at about 1000 RPM. When finished testing, shift the vehicle back into Park to shut down the engine.
The hybrid Camry, Highlander, Lexus, and Ford Escape vehicles all require a similar "dance" to make their engines idle. Honda and other "assist" type hybrids in general work differently and do not need this special treatment.
The 2004 and later Prius is CANbus-based, and requires a tool that understands CAN to access the OBD-II data.