In my own guess it's not related to the temp sensor this error "Can't open core change to next voltage and freq" seems related to the power supply if not, the hashboard might be shorted.
I'm not so familiar with a tester what exact tester do you use is that the test fixture from bitmain or zeusBTC?
I'd like to suggest if you want to check or diagnose the hashboard use the diagram from zeusBTC or check those test points and voltages on the hashboard you can follow the guide from this link below.
- https://www.zeusbtc.com/articles/information/171-antminer-s9-and-s9i-manual-download