When you first put the key in the ignition and turn the key so that the ignition lights are on, if there is a problem the speedo will point at one of the 10MPH markers, this can then be translated to a fault.
if it points at 180MPH, this means you cant start the engine due to a major failure. I had this when the water pump sheared off, and loss all the water. Because the engine temp, and oil temp were high the ECU would not allow the engine to be started again.
Sometimes when I get in the car mine points at the 40MPH marker which must mean something. Other times it doesnt move.