If a toyota prius red triangle warning light check engine light comes on, what must i do?

If You happen to see a red triangle light come on at Your dash in Your Toyota Prius that is its version of a standard check engine light. You will want to use an OBD II scan tool to check the ECU for the fault code that caused it to illuminate. Being a hybrid vehicle the Toyota Prius is a bit more complex than a standard internal combustion engine. In general it will have all the same error codes that a regular car would have, but will also have extra ones for the high voltage electric motor system. If Your fault code was caused by an electrical problem I recommend takeng it to a dealership or authorized service center as working with that can be dangerous.