Dell OptiPlex 990 (Early 2011) - Manual (Page 75)

– If the LED still does not illuminate, the problem is with the
power supply.
LED
Power Button
Problem
Description Memory modules are detected, but a memory power failure has
occurred.
Troubleshooting
Steps
– If two or more memory modules are installed, remove the
modules, then re-install one module and re-start the
computer. If the computer starts normally, continue to
install additional memory modules (one at a time) until
you have identified a faulty module or reinstalled all
modules without error. If only one memory module is
installed, try moving it to a different DIMM connector and
re-start the computer.
– If available, install verified working memory of the same
type into your computer.
LED
Power Button
Problem Description BIOS may be corrupt or missing.
Troubleshooting Steps The computer hardware is operating normally but the BIOS
may be corrupt or missing.
LED
Power Button
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