This error refers to a problem with the print core that is being used on the Ultimaker 3 and indicates that it is reading a too low temperature. One of the reasons for this could be a too low environmental temperature, so always make sure that you're not printing in a too cold environment.
If you're experiencing this error message while the environmental conditions are correct, there probably is a problem with the small circuit board on the back side of the print core. In this case the print core cannot be fixed and has to be replaced by another one. Please contact your local reseller for help with this.