As far as i know it's a common issue with those cards,
The ADT7473 is not just a PWM controller, it also monitors the temp sensors and reports the thermals.
So the temperature reporting is also affected (On some sensors it reports 1,000C, on the GPU core it reports crazy low values like -2,147,483,648C)
Examples:
https://www.reddit.com/r/graphicscard/comments/ocqs5i/my_diagnosis_temperature_sensors_in_gpu_are_hosed/
https://forums.tomshardware.com/threads/radeon-hd-6970-fan-is-spinning-at-very-high-rpm.3365747/
https://forums.tomshardware.com/threads/amd-hd-6970-reports-128-c-and-fan-at-100.2638437/
Also i wouldn't throw a card for such minor issue just because it's cheap on the 2nd hand market, It's a waste.