Hello. I don't think that V3 of E5-2400 processors are supported as only E5-2400 V2 are listed as compatible with T420. You may upgrade to V2. See a list of supported V2 processors on the table below: [IMAGE] 
Hello, I am looking to upgrade the Intel Xeon E5-2407 (V1) cpu in a Dell T420. Are the V2 and V3 versions of the E5-2400 series compatible? Or do I have to stick to V1? Thanks!
CPU Upgrade for Dell T420?