Diagnosing power issue on iMac

[Zataralos]

Hoping someone can provide some direction. I changed the EFI chip on a 2017 iMac 21.5 retina model and now it won’t boot. What I notice is that when I first plug it, in the diagnostic LED1 is on (which is good, getting power) but the second LED (LED2) starts flashing on and off every 5 seconds. This is before I press the power button (which does nothing). Machine won’t boot, no chime, no fans, nothing. Any thoughts? Could I have introduced a short somewhere? What can I check?

Thanks