‘04 F250 6.0. I’m stumped here, my truck won’t crank only when it’s really cold, as in -30C (-22F or colder). I know the 1000W block heater is definitely working, I just replaced it a couple months ago and tested it with my multimeter to be sure. Batteries are strong, console lights up like it should when I turn the key (and radio, fan, etc all work) but once glow plug/water sensor lights turn off and I try to crank it nothing happens. As soon as the weather warms up 10 degrees or so it will start up - but I do sometimes notice a very slight hesitation when I turn the key before it turns over. It’s like a split second of dreaded silence before it turns over and fires up normally. I also don’t think it’s related to the block heater because even if it’s not plugged in overnight, it should still attempt to sluggishly turn over (batteries also have lots of cold cranking amps). I replaced the starter a few months ago at the same time I replaced the oil/EGR coolers, glow plugs and glow plug harnesses. It’s a reman NAPA starter. I also replaced the starter relay in the fuse box under the steering column. Finally, the last time this happened (2 days ago) I tried to crank it using the wire on the passenger side under the hood with the barrel connector, touching the lead to the positive terminal to send 12v to the starter and nothing happened...until the weather climbed to -20C yesterday and the key started it (with the split second of silence before it turned over). I don’t get the hesitation or how the extreme cold weather is affecting things. Is this electrical or ignition based? Is my new starter failing but only in very cold temps? Any advice or ideas would be appreciated!