Heatsync, sensor is reading above limit

Hi, I have an iMac that after made a test with ASD, it reports this error:

Temp (TG0H) - GPU 0 Heatsync test 1 - Sensor Reads Operating Range - Check to ensure that sensor reads within operating range.

ERROR - Sensor is reading above the high limit

-TEST FAILED -

Where is that sensor, is it possible to fix that or the logic board is dead?

Thank you

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