All Intel Core-i CPUs are built to support ECC. It's just Intel disables ECC on i5 and i7, so they don't compete with the Xeon series.Strange, maybe because of custom BIOS. I have X9 and X10 boards and they don't work with i5 and i7 CPUs, but i3 works. Also have tested non-ECC memory and the boards won't boot.
This includes almost all i3 and low-end Pentium/Atom CPUs all supporting ECC. Some i5 models over a couple of Tick generations had ECC as they were basically the i3 of the next Tock generation.