Sorry I misunderstood the issues.  Now your issue appears simpler to understand.  390 with H61 chipset only supports Sandy bridge cpu (2nd gen) natively. Some vendor of H61 updated bios to support ivy bridge (3rd gen) including your i5-3470, but Dell by default does not like to do this type of bios update due to marketing preferences. This user unfriendly issue is seen on Dell PC across multiple models.  No wonder you got a microcode error.  If you get the same error with latest Dell bios for this model, that will prove the point.  [URL]
Hi, I have an optiplex that came with a pentium G630. I upgraded it with an i5 3470 but when I turn it on the fans spin up and down forever. I switched the CPU back to stock and when I booted it says there was a CPU microcode update error. I updated the BIOS to A14 and the same thing is still happening. Any ideas why?   Thanks
Optiplex 390 cpu uprade microcode update error