aux power means it is in AUX_PWR on the system board. the manual isn't clear what to do next...
searched ona few dell forums (and man lots of people are complaining about the 8500 model
have you tried the following suggestions;
Did you try clearing BIOS?
Power off and unplug
Press/hold power button for ~15 sec
Open case and remove motherboard battery
Press/hold power button for ~30 sec
Reinstall battery (right-side-up!)
Close the case
Try booting with only mouse, monitor and keyboard connected
most posts on the del web site people ended getting their motherboard replaced yes.http://en.community.dell.com/support-forums/desktop/f/3514/t/19452875