Optiplex 755 freezes during start-up with numbers 3 and 4 lit

Memory modules are detected, but a memory failure has occurred.

• If two or more memory modules are installed, remove the modules (see Removing Memory), <ADMIN NOTE: Broken link has been removed from this post by Dell> then reinstall one module (see Installing Memory) <ADMIN NOTE: Broken link has been removed from this post by Dell> and restart 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 available, install working memory of the same type into your computer (see Installing Memory). <ADMIN NOTE: Broken link has been removed from this post by Dell>
• If the problem persists, contact Dell (see Contacting Dell).

Solid green

Power is on, and the computer is operating normally. On the desktop computer, a solid green light indicates a network connection.

No corrective action is required.

Blinking green

The computer is in a power-saving mode.

Press the power button, move the mouse, or press a key on the keyboard to wake the computer.

Blinks green several times and then turns off

A configuration error exists.

Check the diagnostic lights to see if the specific problem is identified (see Diagnostic Lights).

Solid yellow

The Dell Diagnostics is running a test, or a device on the system board may be faulty or incorrectly installed.

If the Dell Diagnostics is running, allow the testing to complete.

If the computer does not boot, contact Dell for technical assistance (see Contacting Dell).

Blinking yellow

A power supply or system board failure has occurred.

See Power Problems.

Solid green and a beep code during POST

A problem was detected while the BIOS was executing.

See Beep Codes <ADMIN NOTE: Broken link has been removed from this post by Dell> for instructions on diagnosing the beep code. Also, check the diagnostic lights to see if the specific problem is identified (see Diagnostic Lights).

Solid green power light and no beep code and no video during POST

The monitor or the graphics card may be faulty or incorrectly installed.

Check the diagnostic lights to see if the specific problem is identified (see Diagnostic Lights).

See Video and Monitor Problems. <ADMIN NOTE: Broken link has been removed from this post by Dell>

Solid green power light and no beep code but the computer locks up during POST

An integrated system board device may be faulty.

Check the diagnostic lights to see if the specific problem is identified (see Diagnostic Lights). If the problem is not identified, contact Dell for technical assistance (see Contacting Dell).

The computer is in a normal off condition or a possible pre-BIOS failure has occurred.

The diagnostic lights are not lit after the computer successfully boots to the operating system.

• Plug the computer into a working electrical outlet (see Power Problems).
• If the problem persists, contact Dell (see Contacting Dell).

A possible processor failure has occurred.

• Reseat the processor (see Processor information for your computer).
• If the problem persists, contact Dell (see Contacting Dell).

Memory modules are detected, but a memory failure has occurred.

• If two or more memory modules are installed, remove the modules (see Removing Memory), <ADMIN NOTE: Broken link has been removed from this post by Dell> then reinstall one module (see Installing Memory) <ADMIN NOTE: Broken link has been removed from this post by Dell> and restart 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 available, install working memory of the same type into your computer (see Installing Memory). <ADMIN NOTE: Broken link has been removed from this post by Dell>
• If the problem persists, contact Dell (see Contacting Dell).

A possible graphics card failure has occurred.

• Reseat any installed graphics cards (see the "Cards" section for your computer).
• If available, install a working graphics card into your computer.
• If the problem persists, contact Dell (see Contacting Dell).

A possible floppy drive or hard drive failure has occurred.

Reseat all power and data cables.

A possible USB failure has occurred.

Reinstall all USB devices and check all cable connections.

No memory modules are detected.

• If two or more memory modules are installed, remove the modules (see Removing Memory), <ADMIN NOTE: Broken link has been removed from this post by Dell> then reinstall one module (see Installing Memory) <ADMIN NOTE: Broken link has been removed from this post by Dell> and restart 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 available, install working memory of the same type into your computer (see Installing Memory). <ADMIN NOTE: Broken link has been removed from this post by Dell>
• If the problem persists, contact Dell (see Contacting Dell).

Memory modules are detected, but a memory configuration or compatibility error has occurred.

• Ensure that no special requirements for memory module/connector placement exist (see Memory).
• Ensure that the memory you are using is supported by your computer (see the "Specifications" section for your computer).
• If the problem persists, contact Dell (see Contacting Dell).

A possible expansion card failure has occurred.

1. Determine if a conflict exists by removing an expansion card (not a graphics card) and restarting the computer (see the "Cards" section for your computer).
2. If the problem persists, reinstall the card you removed, then remove a different card and restart the computer.
3. Repeat this process for each expansion card installed. If the computer starts normally, troubleshoot the last card removed from the computer for resource conflicts (see Troubleshooting Software and Hardware Problems in the Microsoft® Windows® XP and Microsoft Windows Vista® Operating Systems). 
4. If the problem persists, contact Dell (see Contacting Dell).

Another failure has occurred.

• Ensure that all hard drive and optical drive cables are properly connected to the system board (see the "System Board Components" section for your computer).
• If there is an error message on the screen identifying a problem with a device (such as the floppy drive or hard drive), check the device to make sure it is functioning properly.
• If the operating system is attempting to boot from a device (such as the floppy drive or optical drive), check system setup (see System Setup<ADMIN NOTE: Broken link has been removed from this post by Dell>
  )  to ensure the boot sequence is correct for the devices installed on your computer.
• If the problem persists, contact Dell (see Contacting Dell).

1-3-1 through 2-4-4

Memory not being properly identified or used

4-3-1

Memory failure above address 0FFFFh

1-1-2

Microprocessor register failure

1-1-3

NVRAM read/write failure

1-1-4

ROM BIOS checksum failure

1-2-1

Programmable interval timer failure

1-2-2

DMA initialization failure

1-2-3

DMA page register read/write failure

1-3

Video Memory Test failure

1-3-1 through 2-4-4

Memory not being properly identified or used

3-1-1

Slave DMA register failure

3-1-2

Master DMA register failure

3-1-3

Master interrupt mask register failure

3-1-4

Slave interrupt mask register failure

3-2-2

Interrupt vector loading failure

3-2-4

Keyboard Controller Test failure

3-3-1

NVRAM power loss

3-3-2

Invalid NVRAM configuration

3-3-4

Video Memory Test failure

3-4-1

Screen initialization failure

3-4-2

Screen retrace failure

3-4-3

Search for video ROM failure

4-2-1

No timer tick

4-2-2

Shutdown failure

4-2-3

Gate A20 failure

4-2-4

Unexpected interrupt in protected mode

4-3-1

Memory failure above address 0FFFFh

4-3-3

Timer-chip counter 2 failure

4-3-4

Time-of-day clock stopped

4-4-1

Serial or parallel port test failure

4-4-2

Failure to decompress code to shadowed memory

4-4-3

Math-coprocessor test failure

4-4-4

Cache test failure