I run a Dell PowerEdge R420 as my Proxmox host, and this weekend I set out to swap its dual E5-2420s for a matched pair of E5-2450 v2s: same socket, more cores, higher clock, mainly to make Home Assistant feel snappier. BIOS was already flashed to a v2-compatible version beforehand. Should have been a straightforward swap. It was not.
First boot, first failure
POST halted immediately on a memory-training failure. Pulled the heatsink back off and found bent pins in the CPU2 socket, a fiddly, nerve-wracking fix I handled with the classic mechanical-pencil-tip trick, straightening them one at a time under a lamp. That worked; both CPUs and all 32GB of RAM trained clean on the next boot.
Progress, but not victory. The very next POST screen: "PCIe Training Error: Integrated RAID," then "System halted."
Down the rabbit hole
I pulled the lifecycle controller logs and started reading. The pattern was ugly and consistent: PG (power-good) voltage out-of-range errors, on every rail, for both CPUs: VCORE, VTT, VSA, PLL, memory VTT, memory VDDQ, plus some system board rails for good measure. That's not "one bad chip" language. That's "the board and the CPUs are disagreeing about power sequencing" language.
I went through the standard troubleshooting ladder: reseated the DIMM, swapped memory sticks between slots to see if the fault followed the stick or the slot, reseated both CPUs, did a second pin-straightening pass on both sockets, reseated the riser, reseated the PSUs, did a full AC drain, tried booting on CPU1 alone. Nothing changed the outcome.
The decisive test
The test that actually told me something: I put the old E5-2420 pair back in. Clean POST, first try.
That single result rules out almost everything I'd been worried about. The board's fine, the RAM's fine, the PERC's fine, the risers are fine. The problem follows the new CPUs specifically, and since two chips from two different sellers are producing the exact same failure pattern, it's very unlikely to be a bad chip. My current leading theory is firmware: the R420's System CPLD handles power sequencing, and it's sitting on an older revision than what some other v2-upgrade R420 owners report running. CPLD updates for this class of board are typically issued specifically to fix power-sequencing bugs like this, and they're not a public download, Dell issues them on request.
Where it stands
The old CPUs are back in and the server's running normally. This is deferred, not abandoned. Next step is testing each v2 chip alone in socket 1 to see whether the fault is shared (points hard at CPLD) or isolated to one chip (means I just got unlucky with one of the two). After that, it's either a support ticket asking Dell for the CPLD update, or sending one chip back.
Not the weekend I planned, but a good reminder that "compatible socket" doesn't mean "compatible everything."