All 4 Diagnostic LEDs flash then go off

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 seen the diagnostic readout for the imac as documented below, but that states the LED's remain lit. Mine do not.

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, can anyone help or am i wasting my money buy throwing more at a dead logic board?

Thanks for your help in advance.

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• Verify AC source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that MXM video card is seated properly

• Verify logic board

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

-If external display works then verify/replace LED backlight board

-If external display works then verify/replace LCD panel

-If external display does not work verify/replace logic board