I have sent my car to my mechanic to get it.

I have sent my car to my mechanic to get it checked out but he cannot get into the engine to check into his computer to check out the sort of warning light that appears on the car beside the clock. It looks like the shape of tap! He told me to get to a holden mechanic to get a dianogstic > Can u please tell me how much that would be? My car is still under warranty? Hope to hear from u. Thank you.