Here is some information that will help you in your quest to solving your issue with your Dell Dimension 8400
https://www.dell.com/support/home/us/en/19/product-support/product/dimension-8400/diagnose
Dell Beep Code Cause:
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
1-2-2
DMA initialization failure
1-2-3
DMA page register read/write failure
1-3
Video memory failure
1-3-1
through
2-4-4
Memory modules 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 failure
3-3-1
NVRAM power loss
3-3-2
NVRAM configuration
3-3-3
Real-time clock (RTC) or keyboard controller not found
3-3-4
Video memory failure
3-4-1
Video initialization failure
3-4-2
Video retrace failure
3-4-3
Search for video ROM failure
4-2-1
Timer tick failure
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 channel 2 failure
4-3-4
Time-of-day clock stopped
4-4-1
Serial or parallel connector failure
5-1-2-3
Memory read/write failure
5-2-2-1
Mismatch or unsupported memory modules
5-2-2-2
Mismatch memory module pair
5-2-2-3
Unable to initialize memory modules
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