All of diagnostic lights on the front panel are off . The system light turns solid amber. The system does not start yet. the fan always starts noisely. How can I fix the problem and start the computer normally? Thanks a lot.

getnet,
What do the diagnostic led lights show. The diagnostic LEDs are located on the front of the chassis next to the power button. These diagnostic LEDs are only active and visible during the POST process. Once the operating system starts to load, they turn off and are no longer visible. Below is the manual that provides information on what the led pattern shows to help resolve the issue.
Optiplex 780 manual
http://dell.to/1HqoNmD
Thank You,
Dell-Jesse L
Dell Social Media and Communities

1) Switch the system on. Can you tell me if the power button is flashing amber or a solid amber? Please also tell me if you notice any diagnostics LEDs staying lit - these are labelled 1,2,3 and 4 and are near the front headphone sockets
2) Please remove all peripherals at the back of the system and switch the system on. Does this affect the power button LED or diagnostic LEDs
3) Have you added any components to the system e.g. video cards, sound cards etc...
4) Please open the system and remove the following components:
a) Power and data cables from hard drives and optical drivesb) Any PCI/PCI-Express expansion cardsc) All memory from the systemd) The system fan
5) With the above removed, please reconnect the power cord to the back of the system and switch the system on. Please make a note of what happens with the power button LED and the diagnostic LEDs
6) If there are no changes to the power button LED or diagnostic LEDs, please remove the power cord from the back of the system and remove the following components:
a) I/O panel cableb) CMOS battery
7) With the I/O panel cable and CMOS battery removed, please connect the power cord to the back of the system. The system should automatically try to switch on. Do you hear any beeps from the system? If so, can you tell us exactly what you hear?
If you need any assistance with removing these components, please see the User Manual here: http://support.euro.dell.com/support/edocs/systems/op380/en/index.htm
Please keep us updated on how you get on!

I think your problem is your memory or video card.

Below are some steps that I recommend.

When you plug in the power cord is there any lights whatsoever on the computer?

When you press the power button do you hear fans running, hard drive spinning, lights on the computer, hear any beeps or lights constantly blinking, hear the DvD/CD drive click and the light on the drawer of the DvD/CD turn on for a second or two? If so the laptop is trying to boot or may have booted, but the screen is not showing for some reason.

Beep tones and blinking lights tells the technicians what is going on with the computer and why it is not booting. Most times it's a motherboard issue when you hear beeps or blinking lights continuous. Most times the problem is with the memory or Video Card. Count the Blinking Lights or Beeps and take note of their pattern (1 Long or 1 Long followed by 2 Short).

Maybe your power cord is bad if you don't get anything to happen whatsoever when you press the power button.

Maybe you have a Static Charge Buildup. Remove the Power Cord, then Press and Hold the power button for 30 seconds. After 30 seconds plug the power cord back in and hit the power button to see if it boots.

Maybe your memory is bad or the memory slot is bad.

Remove the Power Cord, Open the Case and remove one of the memory chips. Set the chip aside then try rebooting the computer. If it boots, then you know the memory chip you set aside is bad so mark an x on it with an ink pen. If it fails to boot then move the chip to the next memory slot and try rebooting. If it fails take the chip out and set it aside, then put the other chip in and try booting it in both slots. If it boots, just to verify that the chip set aside is bad, mark an x on it and put it into the open memory chip slot and try booting.

Maybe your power supply is bad. Sometimes there is enough power to make the power light or some other lights come on but not enough power to make the fan turn on or hard drive spin.

Below are links and info I found from Dell's Website based on your model.


http://support.dell.com/support/edocs/systems/dime521/en/SM_EN/specs.htm

Controls and Lights Front of computer:

Power button - push button

Power light
green light - Blinking green in sleep state; solid green for power-on state.
amber light - Blinking amber indicates a problem with the power supply inside the computer. If the system cannot boot and there is a solid amber light, this indicates a problem with the system board (see "Power Problems" in your Owner's Manual).

Diagnostic lights
four lights on the front panel (see Diagnostic Lights.)

Standby power light
AUX_PWR_LED on the system board

Hard-drive activity light
green light - When the computer is reading data from or writing data to the hard drive.

LAN indicator light

green light - Solid green indicates that a LAN (local area network) connection is established.

http://support.dell.com/support/edocs/systems/dime521/en/SM_EN/adtshoot.htm#wp1056411

Sorry, the northbridge on the motherboard got fried - the motherboard is dead, replace it.

test the PSU yet,
the jumper clip tests?
are the fans dead?
http://www.pcdied.com/power.html

bad PSU
PSU overloaded
or worse. ask

a solid amber light means that the Dell Diagnostics is running a test, or a device on the system board may be faulty or incorrectly installed. HERE is a link to the diagnostic lights, what they mean and possible solutions.

I had the same issue with my Dell Dimension 5150 and I replaced the front I/O USB Audio Panel and it booted right up.

Unplug all the Harddrives, Cd Drives attached to the motherborad. Try turning up the system. If it boots, you may to have to change the I/O cables.
Still if it is amber, then your motherboard is having the problem with. As long as the LED glows PCU gotta be okay.
Kindly get back for futher help

jethro..
http://support.dell.com/support/edocs/systems/dimC521/en/SM_EN/tshoot.htm#wp1064555
check on system lights... what are the diagnostic lights telling you??
if the 4 lights are off you may have a problem with the led itself or the powersupply is just messing with your head.

Hi

First disconnecting the power cord and re-seat all connections (Keyboard, Mouse espercially). Connect up the power cord and try powering the PC.

To help you troubleshoot a problem, Dell Dimension 5100 has four diagnostic lights labeled "1," "2," "3," and "4" on the front panel. When the computer starts normally, the lights flash. After the computer starts, all four lights display solid green. If the computer malfunctions, the color and sequence of the lights identify the problem.

If the computer failed to start up check the diagnostic lights color and sequence and Check the link below for more info (Assuming you have another system that can access internet)
http://support.dell.com/support/edocs/systems/dim5100/en/sm/tshoot1.htm#wp1120147

Alternatively, contact Dell Support with the diagnostic lights color and sequence to raise a service call if the system if under warranty.

Mike