Revision of iMac Mid 2007 strats up screen goes black (LED 4)

Hi All Repairmen,

Can you help me solve this issue ?

After reapplying thé CPU and GPU heatsinks, thé CPU fan runs wild, after a few seconden when after starting the iMac.

LED 4 (LCD) goes OFF after 2 minutes approximately.

I did already reset the SMC and PRAM.

Will reinstalling MacOSX on a new harddisk and original disks help ?

I will be glad hearing from you.

When I ran ASD 3S116 the GPU Proximity Sensor test gives an error TEST FAILED.

Is it possible that the heatsink assembly is broken ?

Another tought , I applied a thermal pad over the two memorychips in one peace.

Maybe this way the GPU Proximity Sensor can't generate a readout ?

When started I have video signal on the screen. After 2 minutes it goes black. The LED 4 goes OFF.

I checked the thermal sensors and they are installed correctly.

Maybe the LCD thermal sensor is disconnected or is covered with dust ?

I blew out a lot of dust under the plastic wrapping on the backside of the LCD panel.

Maybe you can help me further ?

New developments, replaced the GPU Heatsink with an other videocard ATI HD 2600 Pro:

Problem with ASD testing: GPU 0 Die Test 1

Error: Sensor reading above alarm point.

Replace the ATI HD 2600 Pro videochip ?