I have the 20" alu iMac which refuses to start. A1224

From cold the iMac led diagnostics show led 1 as on, but as soon as I press the power button LEDs 2, 3 and 4 quickly light accompanied by the optical drive fan. Then nothing.

Even led 1 goes dead..

I have to leave the machine off foe a few minutes before the led 1 will power on again!

I have already swapped the power supply but have the same issue.

Could this be the power cable 922 8188 as documented on some YouTube sites?

Thanks for your help in advance.