IBM 3590 - Manual (Page 28)

Enter
5
A push button that allows the operator or the service representative to
select the menu option at the location of the cursor arrow.
Processor Check
6
An indicator LED that switches on for 10 to 20 seconds during a
normal power-on or a power-on reset. If no fault is detected, the LED
switches off. If the LED remains on, the 3590 requires service.
8 3590 Operator Guide