Trucks need to fast idle after temps get near the DEF freeze point to keep the heat coil active as well as the fuel heater.According to everything I've read it freezes at +11F. Most tanks up here have a heater cool in the bottom. Problem we have had is the liquid is splashing a sensor then freezing, tripping the sensor and putting an almost new 2023 truck with 16K on it into limp mode. 5mph and no more. Hopefully I see it back before it warms up next week or I'll be annoyed.
40°F is when the friction chart makes a big increase. I always started using the block heater when temps approached 40°F.When I had my diesels I started pluging them up when it got to 40°. Instant heat in mornings is nice.
Most diesels are delivered with a block heater, but often have to purchase the cord.
Be sure to unplug and let the block heater coil cool a minute or two before starting the vehicle.