iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

[worldofskin]

Hi guys. Hope you are having a great day. This imac iMac A1312 2011 27" with this logic board 820 - 2828. Came to the shop becouse it doesnt boot.I started the apple diagnostic and got 4mem error. I disassembled it, cleaned it and replaced the ram. For a little while it worked normally. But after a while it got to the same state. I tried changing all the ram with a good known working ones. Nothing, i tried a different drive to boot from, nothing. I tried to not boot from sata but i disconnected drive and connected only via usb. nothing. I tried to change the graphic card nothing. I dont have a second processor but my friend tryed it in a different pc motherboard and it worked. Please if you have any idea or advice what could i try post it. Thank you in advance have a great day.

[mon2]

Re: iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

Review the RAM power rail against the schematic. Is it stable?

What are the voltage readings of the following?

PM_MEM_PWRGD
PP1V5_S0_CPU_MEM

Test with a single known good module.

Perform a SMC reset and test again.

[worldofskin]

Re: iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

The readings are
PM_MEM_PWRGD 1.491
PP1V5_S0_CPU_MEM 1.493
and after smc reset
PM_MEM_PWRGD 1.490
PP1V5_S0_CPU_MEM 1.492

I took the readings without a screen and for some reason the mac booted and from that time it boots normally. Dont know why. I ran some ram test with one and then 3 sticks. all test passed. do you think it could be just a bad connection on one of those circuits?