imac doesn't boot

My imac is 27" a1312 model. I bought it without a keyboard and mouse. When I pressed the power button, I heard the fans running for a couple of seconds and the fans stopped. The screen is black the whole time. I then removed the screen, hard drive, keyboard, mouse, and all peripheral devices and repeated the booting process but the computer still didn't turn on. The first diagnostic LED turned on and stayed on as long as I plugged in into the outlet. If I unplug the power cable, the first diagnostic LED turns off. I even tried to jump the power button connections to check if the power button was bad. The fans turned on for a couple of seconds and then turned off and nothing happened. I removed all the RAM modules and plugged each RAM module back into the motherboard one by one and reboot the computer each time. but the computer still don't turn on. Any help is greatly appreciated. Thanks!