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I have a Freighliner M 2 with Cummins ISL9 CM2350 L101 2014 year model.
After driving the truck several hours the DEF tank is hot to the touch. I have not checked the temp yet but I would guess coolant temp. I use the truck in California and its hot here during the summers anyway. I have no engine malfunction light on nor a after treatment malfunction light. Is this normal for this vehicle and engine application. I read a post about a ambient air temp sensor malfunction, can anyone shed some light on this condition.

If it is like the mine, the engine coolant is circulated through a coil in the DEF tank. Therefore it should be coolant temp. I have also heard of a sensor malfunction causing hot fluid, but I think that would throw an engine code.

I just crawled under and looked. I have an electric solenoid type valve that would shut the coolant off or allow it to heat the tank. (Similar to what you would have for coolant flow on a bunk heater). Maybe yours is stuck open? So even though it is warm out and should be closed it can’t?? Just making suggestions.

Thank You for the reply, Just wondering on your truck, does the DEF always run at coolant temp, or just in freezing outside temps. I know some trucks only use the coolant in cold temp situations. My tank is hot to the touch even if it's a 95 degree summer day. Maybe something is stuck .

I’ll try to check tomorrow after we drive a bit and let you know. My guess though, follow the coolant lines from the motor to the tank and see if at the tank there is a electric solenoid in the line near the tank. I bet that is stuck open. But I’ll check and see if mine gets hot tomorrow. Also if you have “Insite” you may be able to plug in and see what the ECM thinks the DEF temp is?