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I'm having troubles with my DELL T3500.  It has:

• a w5590 3.33 GHz 4c/8t


• 12 GB (3 × 4 GB) RAM DDR3 1333 MHz

I get the message Memory failure on DIMM2 Dell T3500 when I turn on my computer and it starts to boot up. 
I continuously hit F1 to continue so I could use my PC but now it shows in the Task Manager only 8 GB available. 

How can I fix it? I already switched the RAM between slots something like 5 or 6 times. It always says DIMM2 regardless what I change.

You have a Xeon W5590 CPU which uses triple channel memory. That means it can utilize 3 banks of memory at the same time for higher performance. For this reason your system also comes equipped with 3 memory modules (DDR3, 667MHz)

When your system powers on its firmware (either BIOS or UEFI in a PC) determines how much memory is present. Usually it also checks the memory.

During this check it discovered errors when accessing the DIMM in bank2. This can have multiple causes, but the most likely ones are:

• A broken memory controller (which is part of the CPU since the Nehelam series, and your CPU is a Nehelam based Xeon),


• Broken path between the memory controller and the DIMM socket (which can include dirt between the contacts)


• A broken DIMM connector (motherboard side)


• Or a broken DIMM.

Your firmware detected three 4GB DIMMs. One faulty. It made the choice of using both working DIMMs and ignoring the third, which is why you now have 2x4GB to work with.

Now to fixing this:

Since you already moved the DIMMs around and always got the same error we can assume that it is not the DIMM itself. That leaves the memory controller and the physical path between the memory controller and the DIMM.

If it is dirt then you are lucky, Cleaning the contacts might help.

If it is the motherboard then best you can do is try different configurations. Most Nehelam boards come with 6 DIMM sockets, and their layout might be described as:

Dimm 1 on channel 1A

Dimm 2 on channel 1B

Dimm 3 on channel 1C

Dimm 4 on channel 2A

Dimm 5 on channel 2B

Dimm 6 on channel 3C

Memory will usually be in the socket corresponding to 1A,1B and 1C, alloing all the memory parts to be used simultaneously. But 1A, 1C, 2B etc also may work.

May since the only Nehelam boards I owned only supported very specific memory fill sequences.

If it is the memory controller then there is nothing you can do short of replacing it. And since it is part of the CPU that means replacing the whole CPU.

Dell
specifies
that for 12 GB the 4GB sticks should be in DIMM1, DIMM2 and DIMM3.

However, according to the Crucial site article
Dell Precision Workstation T3500 compatible upgrades,
it only proposes upgrades in single 4 GB or two 4 GB,
so the manual might be misleading.

I suggest running the Crucial memory scanner
to see what it suggests as memory upgrades for your computer.
Check well the specifications of the sticks it suggests against the ones
you bought.
It might be that some property of your RAM makes it impossible for the motherboard
to accept more than two. Normally this is caused by too-large memory chips
or too many memory channels on the each stick.

I also suggest cleaning well all contacts.