Imac 2017 hung at led 2

Hi,

I was replacing CPU to my Imac 2017 4k, changing the i5 7400 with another Kaby Lake, a Xeon e3 1270v6, I am having problem to cpllete the task because it stop at second LED, 1 and 2 are fully on (in some previous attempts second was one time bliking), now it seems to not talk to GPU, that is onboard, I haveI already tried also the temporary remove of backup battery, but nothing.

Iit seems to do an SMC reset but don't go on with 3rd led.

Any suggestion?