Help: CPU-fan runs max - ASD testresults: what is wrong?

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 😕

So, 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