2017 M2 cooling fan issues

I was getting engine temp high alerts. Found that the fan was working sporadically at first, but wasn’t at full RPM. Then it would just stutter back and forth a 1/2” or so. Bought a used fan and same exact result.

There are 15 volts at the wiring harness and at the relay behind the radiator reservoir. Have found a lot of conflicting info on what sensor and at what location, I should be looking at, to try next.

So question is, what sensor and where is it located? And are there two Or just one that could affect the fan? The fact that I have full volts at the fan has me thinking it’s not the fan. Help! 😀

Thanks in advance!!