Hi @JonnyO2,

Not quite 100% if old and new RAM is being mixed, or if you're just going with the new RAM?

From the Precision T3600 Owner's Manual :  NOTE:  Memory modules should be installed in pairs of matched memory size, speed, and technology. If the memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance.

With just the new RAM, install 1 stick at a time and boot until fault reoccurs.  If error appears right away, just boot with one stick at a time to rule out any bad or non-workable sticks.  And of course, unplug PC  from A.C. anytime working on the inside of it.

New RAM should be returnable if it's not working.

https://www.dell.com/community/Desktops-General-Read-Only/T3600-does-not-recognize-new-RAM/td-p/5080199 

I haven't seen Dells allowing boot when there's memory error.

Also:  https://www.dell.com/support/article/en-us/sln285280/why-my-precision-workstation-t3600-won-t-post-boot-or-recognize-memory-correctly-after-upgrading-memory?lang=en 

Hi, tenía el mismo problema, solo me funcionaba una sola memoria de 4 en el puerto1, esto se debe a un bloqueo de IMC, lo solucione quitando la pila del mother boar y lo deje así, al siguiente día ya me reconoció el 2do puerto de seguro el 3 y 4 ya estarán funcionando, pero aún no lo he probado, espero está solución te sirva y a quien la vea.

Hello, it's been a long time; however, here is my solution.

I had the same error when installing 128GB (8x16GB)—correctable blah blah DIMM4_CPU2.

I tried downgrading the BIOS from A19 to A11 and going from there, same error.

I decided to install just 2 RAM sticks on each CPU (2x2x16GB), it booted fine with 64GB.

My final configuration was CPU1 4x16GB and CPU2 DIMM1_DIMM3_DIMM2 16GB and DIMM4 empty.

Now I'm running my T5600 with 112GB fast and stable.

I hope this helps.

Blessings!