IBM TotalStorage 3590 Operator's Manual page 28

Enter 5
Processor Check 6
8
3590 Operator Guide
A push button that allows the operator or the service representative to
select the menu option at the location of the cursor arrow.
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.