I have to agree with wdcook something is wrong? How many hours do you have on this tractor? We do not have a 6400 but this series should start down to 10 degrees, I would start with the battery's, cables, and starter, if still starting hard would pull the injectors to check they are holding pressure and the pattern is good. Do you use no. 1 fuel or a blend in the winter? straight no.1 is very hard on the pump. My son on the place now but we still have a 1995 7700 and it will start down to zero without being tied to a power pole, all though we do plug it in so it's easer on it. Good luck 101. l