OptiPlex 960 temporary 3 diagnostic LED

Does it boot??

Version of Windows?

According to the manual:

downloads.dell.com/.../optiplex-960_Service Manual_en-us.pdf

#3 = A possible motherboard failure has occurred.
Unplug the computer. Allow one minute for the power to drain. Plug the computer into a working electrical outlet and press the power button.

If that doesn't help, drain power again and open case. Remove motherboard battery and press/hold power button for ~30 sec. Carefully reseat RAM modules and PCI cards in their slots. Use canned air to clean out the dust. Then reinstall battery (use fresh battery if this one is old) and see if it boots now with only mouse, monitor and keyboard connected.

Does not boot. Operating OS is Windows 7.

I had seen the Unplug the computer. Allow one minute for the power to drain. Plug the computer into a working electrical outlet and press the power button. That was tried.

I next used compressed air and a vacuum to remove dust. No canned air available. The battery is probably the original. I did pull out the video card because it was in the way of reaching the battery. Replaced the battery. Reinstalled the video card.

Plugged in the computer with mouse and keyboard. Now the power button is blinking yellow.

Unplugged the computer and reseated RAM and PCI cards. Power button is blinking yellow.

Realizing I had skipped a step, I took out the battery and held the power button for 30 seconds. Still get same blinking yellow power indicator

If I press the power supply diagnostic button. The power supply diagnostic LED goes to green and the power switch goes to solid yellow. The DVD/CDROM leds go green momentarily.  I take this to mean the power supply is OK.

Which form factor is this 960?

Are any diagnostic LEDs on when you see the solid amber power button?

Did the fan(s) spin up when you pressed the power supply test button?

Visually inspect the motherboard for bad capacitors. They look like little cans and should be perfectly round with perfectly flat tops, not bulging on sides or top, and no signs of leaking.

Also visually inspect all USB ports from outside of the case to see if one is bent or shorted. Use a flashlight to see into them, but do not stick anything metal into the ports.

If capacitors and USB look OK, you may want to strip it down to bare essentials. Disconnect all internal drives except boot hard drive and remove all PCI cards except video card, assuming you have an add-in video card. Remove all RAM modules except the one in slot 1. Clear BIOS by removing the battery and pressing/holding power button for ~30 sec and then reinstall battery and see if it boots now.

If still no go, remove the video card, if you have one, and clear BIOS again. Then connect the monitor to one of the onboard video ports and see if it boots. And last, disconnect the boot hard drive, clear BIOS and test...

This 960 is a tower.

Pressing the power supply diagnostics does turn on processor and power supply fans. I had seen the video fan on when I started but not this time.

There are no diagnostic LEDs since the battery was replaced.

I had looked at the motherboard and USB port and do not see obvious problems. The computer went out on a power failure so I not expecting USB to be a problem.

I disconnected the two DVD drives and removed the video (not PCI) and firewire PCI cards, DIMM 2 RAM. Removed battery, connect monitor to VGA, held Power button for 30 seconds. Still blinking amber power light

Removed battery, held power button for 30 seconds. Disconnect hard drive. Inserted battery. Plugged in power cord. Still blinking amber power light.

If you remove ALL the RAM and do the battery thing, do you get any beeps from the tower when you try to reboot with mouse, monitor and keyboard connected?

If not, you've narrowed it down to the motherboard or the CPU, especially after it suffered a power failure...

In that case, only you can decide how to proceed. It's your time and money...