Help: CPU-fan runs max - ASD test iMac 24" early 2008

[jebe68]

Out of the blue one evening, while the iMac was on, a fan kicked in.
And after restarting, and resetting SMC, it never returned to normal speed.

The Apple Hardware Test had this result:
4SNS/1/40000000:TLOP-130.000

So I opened the iMac to see if the LCD sensor cable was disconnected or damaged
(as a year ago I have replaced the GeForce 8800GS videocard and swapped the HDD with SDD, ever since the iMac didn't have any problem).
But no signs of damage or so; and after restarting the fan keeps blowing at almost max

Who can tell me what part(s) is/are the problem with the following ASD test results???? Motherboard? Cables? Sensor(s)????
The ASD* test (ASD OS & ASD EFI) logs returned 4 errors.

First the ASD EFI test:

Temp (TA0P) - Ambient air temperature sensor test 1 - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR - Sensor is reading above the high limit
- TEST FAILED -

Temp (TG0D) - GPU 0 Die Temp test 1 - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR - Sensor is reading above the high limit
- TEST FAILED -

Temp (TL0P) - LCD proximity temperature test 1 - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR - Sensor is reading above the high limit
- TEST FAILED -

SMC test 1 SMC PID Sensor check
-Checks the SMC PID Sensor warning counter that is monitoring sensors of each PID loop erratic behaviour.
ERROR - The SMC’s PID Sensor check has been tripped
- TEST FAILED -


And the (similar) errors from the ASD OS test:
*
Temp (TA0P) - Ambient air temperature sensor (test#1) - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR --13007 [The SMC could not initialize this sensor] -- (1.00) TEST FAILED

Temp (TG0D) - GPU 0 Die* Temp (test#1) - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR --13007 [The SMC could not initialize this sensor] -- (1.00)*TEST FAILED

Temp (TL0P) - LCD proximity temperature (test#1) - Sensor Reads Within Operating Range
- Check to ensure that sensor reads within operating range.
ERROR --13007 [The SMC could not initialize this sensor] -- (1.00)*TEST FAILED

SMC: SMC PID Sensor check (test#1) - SMC PID Sensor check
-Checks the SMC PID Sensor warning counter that is monitoring sensors of each PID loop erratic behaviour.
ERROR -- 1 [The SMC’s PID Sensor check has been tripped] -- TEST FAILED