Customers can test the health of the power system by pressing the test button. The button is lit by a signal in the powersupply called Power_Good (PG). When the +3.3v, +5v, and +12v rails are up and within specification, the PG signal turnson, lighting the self-test LED. By removing the cable harness in units that do not pass this test, you can isolate thefailure's cause to an attached device of the power supply. For example, if the self-test LED does not light when you pressthe button, you can remove the connectors from the devices and tests again. If the PG LED now lights up, one of theattached devices is shorted and holding down the power supply. If the LED does not light up, the power supply isdefective.68
