The interface can fail. First, make sure you have 120 volts AC between pins 1 and 2 on P4. To diagnose whether it's the interface or main control, disconnect power, unplug the interface from the main control. Reapply power and measure for 5 volts DC at P13, between pins 2 and 4. If you have 5 volts DC, replace the user interface. If not, replace the main control. In most cases, this is accurate since the interface is powered by the 5 volt DC supply from the main control board.