Thermistor problem

Hi there 3D. people .i have aproblem with my 3D printer ,its a German Rep Rap Protos V3 and I had to change the Arduino Mega board .Got the new firmware from the nice people at GRR and installed it . All looks good ,BUT ! I keep geting "def " on the screen when I switch it all on ! The thermistors have been changed for new ones still nothing .Do I have to change anything in the software ? Im running Repieter Arduino ,hope you can help its driving me nuts !!