I'm only getting 1 print per hot end temp sensor failure.
Running 24v to power the control board, should I power the control board with only 12v ?

Yes

Ok, new hot end sensor, the control board is back to 12v

I'm still getting the error, but it's not every time, sometimes the printer can run for 25 hours (about 4 prints) before getting the min temp error.
other times only 2 prints.

Would I get this error if the hot bed temp gets more than 2 deg below set point ? or is it only for the hot end?
what else should I look at to fix this problem ?