See Using the Server Fault Remind Button.
Failed DIMMs are identified with a corresponding amber LED on the motherboard.
If the DIMM Fault LED is off, then the DIMM is operating properly.
If the DIMM Fault LED is on (amber), then the DIMM is failed and should be replaced [1].
The DIMM is partially ejected from the slot.
For DIMM replacement instructions, see Install a DIMM.