Question ASUS Z170-A powers on but no boot. CPU_LED blinks once every 12 seconds

Jan 10, 2022

Wondering if anyone had similar fault. I've had no luck looking for this specific symptom online or in ASUS manuals.

MB: Asus Z170-A
CPU: Corie i5 6600k
Memory: 4x Corsair Vengeance LPX DDR4 (32g total)
GPU: GTX 1070

Background, this was a running system, it ran well for long time. Last night it would power on all fans but not post. CPU_LED would blink once then turn off for 12 seconds, then blink again.
Tried to remove power cord for a while and remove ram sticks to re-seat them. Then the system turned on correctly. Ran stable with no issues, even gamed for a couple hours.

This morning It was the same symptom. Power on fans, but no post, same 12 second blinking on CPU_LED. Tried to reseat all memory sticks but this time no luck.

Tried so far:
Remove, clean and re-seat CPU
Remove, clean and re-seat Memory
Remove all peripherical I/O.
Remove GPU
Tried different PSU
Reset CMOS

At this moment I stripped down the system to just the CPU (removed cooler) and PSU, even removed all memory sticks.
The symptom is the same, CPU_LED blinks once every 12 seconds

Any ideas? I'm starting to suspect cold solders somewhere in the MB but would like to know if someone has any ideas on how to narrow down the fault.