When your Hakko 472D Desoldering Station displays '---' on the LED display, it is the result of a software error.  You can clear this error by performing a reset of the system.  In order to reset the system, insert the temperature lockout card and hold down the UP, DOWN, and * keys simultaneously when turning on the power.  The display will read '400' at which time you can release the buttons, the factory reset is complete, and the error has been cleared.