Replaced hot end on Ender 3 Pro

After a blockage o in the hot end where molten PLA was not going to be easily removed I purchased a brand new Creality replacement with new fans, but on trying to increase the hot end temp I get a ‘whining’ sound (alarm?) and the printer shuts off.
The temp on the screen actually shows a negative figure (-14) whic I have not seen before. It’s usually something around ambient temp. I am assuming it doesn’t matter which way the wires are connected to the main board. It doesn’t indicate this.