Dell OptiPlex 9010 All In One (Mid 2012) - Manual (Page 68)

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