Why won't my iMac turn on?

3 possibilities:

1.bad power supply, (either try to repair by locating wrong component and replacing it , or but new psu for your model)

2.blown capacitors (look for any odd loking fatten caps on the board , replace if need be)

poor connection somewhere on the logic board.

good luck

I just wanted to concur. I had the same symptoms, led 1 on when plugged in. When power button pressed, I would only here the disc drive make a sound and nothing else. No fans, no HD, no monitor, no chime. The LEDs would light up on 1, 2, and 3. I removed the power supply and noticed one of the capacitors was bulging on the top. One of the chips seemed to be slightly charred on the other side. For my 24 inch a1225 emc 2211 I ordered a refurbished power supply from eBay for $140. The part number was PA-3241-02A1. I reinstalled and it booted right up. I used some duct tape to lift the glass off the computer sinc I didn't have suction cups. Just took a piece and placed it near the top and careful tugged on it till the glass released.

This is exactly what im seeing right now!! right to the T! have we found any answers yet for it?? i thought it was my PSU as well but was getting those 3 LEDS, so i checked the video card thinking that was it, but that looked fine. and if it was that the fans would spin, but im not even getting spining fans, just the optical drive spinning up, would love if anyone knows what i can try??

I had the exact same symptoms (3 LEDs lit as described along with the other symptoms) with my 24" iMac EMC 2211 and replacing the power supply fixed it.