Most of those S9s are not under warranty anymore, in fact it's in Bitmain's best interest that these gears burn.

In the newer versions such as the S9k and S9E , they do not have any power options visible, I haven't had the time to check if they are hidden somewhere like the fan control setting, but it seems like they don't want these setting available for new gears that they are responsible for, as for any other gear , why would they bother if you burn the hell out of your miners? that only means you are very likely going to feed bitmain more money after burning your old S9 ! sounds like a good business plan which I have no double that bitmain would do such a thing ,  I can't confirm it, but they lack ethics enough to do such a thing.