I'm in the same situation as this post, except different motherboard, MS 6156 BX7

Asus P2L97 Mainboard Power Switch not working (but paperclip works)

There was a lot of insightful tips earlier. I checked these on the ATX connector with an oscilloscope.
1. 5V standby is on
2. PS_ON always high
3. PWR_OK always low

But I don't know which chip uses the 5V power and controls PS_ON. I've had this board for >= 2 years and only used it for < 100 hours. I don't recall anything abnormal that caused the power button to stop working. I can power on by shorting PS_ON like in the other case.