Diagnostic led #1 on, but #2, turns on then off.

Hi,

I have a mid 2011 a1312 imac 27” i5 processor.

Recently did a ssd upgrade for hybrid drive. Imac run smoothly and everything is fine. But after awhile my imac does not have any boot chime and is not turning on.

Took out the Lcd and checked the diagnostic led.

"LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.

LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.

LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

The problem comes, my #1 led is on and bold. And when I press the power button, #2 led turns on for awhile, then turns off. And then turns on for a few seconds and off again.

Tried to check my psu, removed the connectors and then plug back in again. Imac turns on and works for a few days, then now the same issue surfaced again.

Would like to ask, if this an issue to the motherboard itself or psu. Because I can't seem to find info on #2 led turning on then off repeatedly. Looping and looping but not turning on.

Do advise.

Thanks!