I hava 2006 F350 6.0 diesel and it won't crank when it's cold ?

How cold is cold? If you are in extreme cold you may need a block heater. If your battery(s) are weak they may not have enough power for a cold start. Cold is hard on diesel engines.

If your in temperature zones that go down to 15 to 0 degrees, Definitely put block heater and a trickle charger in. Up here in Ak, these items are a must!

did you ever find the problem? im having the same issue, same trim and year. new batteries, if its 40+ outside truck cranks and starts just fine, if there is a dip in temp it wont even crank even though battery have full charge. this morning was 20 and it didnt start.