iMac 27" A1312 doesn't power on

Hi,

My imac 27” a1312 does not power on. Only led # 1 is on when the computer is plugged into the wall outlet. When I pressed the power button, the fans came on and then shut off. LED # 2, 3, and 4 never came on. I measured the voltages at the pins on the psu as shown in the picture. Only pins 3 (12V) and 5 (3.8V) have power. The rest shows 0V. Does it mean that I need to shop for a new psu? Thanks!

here is the picture

Hi,

My imac 27” a1312 emc 2429 fans turn on for 2 sec and then turn off when I press the power button. Everything is silent. Only the first diagnostic LED is on. I replaced the power supply, but the same problem remained. So, the power supply is not at fault. When the power button is pressed, pin 11 on the logic board shows 9.6v, but then goes back down to 0V. The 12th pin on the logic board shows 12V the whole time. This means that the power supply is OK. The 14th pin shows 4V the whole time. I pulled out the ram modules and put one in at time, but still have the same problem. Do I need to replace the logic board?

Thanks!

Rob