Issues following hot end replacement

I have just had to replace the hot and cold end unit on my Ender 3 pro and used the parts from my original Ender 3 v2 which also entailed me having to change the thermocouple (again from my old machine) and having checked and double checked all the wiring was ok I rebooted the pro and during the reboot I get the message heaterempature too high and it will progress no further. Can anyone tell me where I’ve gone wrong please. Thanks in advance. Clive.

Problem analysis:

  1. Thermal abnormality
    2.Motherboard abnormality"

Try to heat the nozzle/thermal bed separately to determine the location of the abnormality;
Printhead alarm: loosen the thermal screws (pull out the thermal), if the fault disappears, then it is a thermal problem, the fault remains the motherboard problems
Heat bed alarm: pull out the thermal wire connector, if the fault disappears, it is a thermal problem. Failure remains, the motherboard problem

Thanks Colin I’ll give all that a go. Fingers crossed. Clive