Disclaimers

Managing Your Diesel Exhaust Fluid (DEF) in Cold Climates

Date Published: January 2, 2026 | Last Modified: 1 hour ago | 5 minute read

For owners of heavy-duty Ram trucks, the arrival of winter brings unique maintenance requirements. One of the most common questions our service department receives at Redwater Dodge, home of the Rig Ready Rams, is: "Does my DEF freeze in the winter?"

The short answer is yes. Diesel Exhaust Fluid is a mixture of approximately 32.5% high-purity urea and 67.5% deionized water. Because of this high water content, DEF begins to crystallize and freeze at temperatures below 12°F (-11°C). However, your Ram Truck is engineered to handle these Canadian and Northern winters with ease.


The Built-In Solution: Tank Heaters

Stellantis engineers have integrated a specialized heating element directly into the DEF tank. When you start your vehicle in sub-zero temperatures, the Selective Catalytic Reduction (SCR) system detects the frozen state of the fluid. The internal heater thaws the fluid over time, ensuring it is available when the engine reaches operating temperature to scrub NOx emissions.


The "Half-Tank" Rule: Why It Matters

While the system is automated, there is a professional tip that separates seasoned diesel owners from the rest. We recommend keeping your DEF tank roughly half-full during the peak of winter. There is a scientific reason for this: If your tank is topped off to the brim, the heating element may only be able to thaw a small core of the fluid. If the system attempts to draw fluid but finds the rest of the tank remains a solid block of ice, it can trigger a "Service DEF System" or "Check Engine" light. By leaving "expansion room" and keeping the volume lower, the heater can thaw the entire contents of the tank more efficiently, ensuring your truck stays out of "limp mode" and on the road.



Ready to Find Your Dream RAM Diesel Truck?

Whether you want a stock RAM Diesel truck, a built lifted RAM Diesel, or you are looking to get a custom Rig Ready RAM Diesel started, we are happy to help with every step of the process.


Contact Us Today at Redwater Dodge for all your RAM Diesel Truck needs:

Call: 780-942-3629 

Common Questions About Diesel Exhaust Fluid


Q1: Does DEF freeze in the winter?

A1: Yes, DEF freezes at roughly 12°F (-11°C).


Q2: Can I drive my Ram truck if the DEF is frozen?

A2: Yes. Your truck is designed to start and run while the DEF heater thaws the tank. The PCM (Powertrain Control Module) allows for a warm-up period before it expects DEF flow.


Q3: Why should I keep my DEF tank at half-full in winter?

A3: A half-full tank thaws much faster than a full tank. This prevents sensor errors and ensures the system has liquid DEF available sooner, reducing the risk of a Check Engine Light.


Q4: Will DEF expand when it freezes?

A4: Yes, because of the water content, DEF expands by about 7%. This is why you should never "top off" the tank in extreme cold—the expansion needs room so it doesn't crack the tank or pump housing.


Q5: Does freezing degrade the quality of the DEF?

A5: No. Unlike fuel, DEF can undergo repeated freeze-thaw cycles without losing its chemical effectiveness or breaking down.


Q6: Should I add anti-freeze or additives to my DEF?

A6: Absolutely not. Never add anything to the DEF tank except ISO 22241 certified fluid. Additives will contaminate the SCR catalyst and result in multi-thousand dollar repairs.


Q7: How long does it take for the DEF to thaw?

A7: Depending on the ambient temperature, the internal heaters usually provide flow within 20 to 60 minutes of driving.


Q8: Will a frozen tank cause my truck to go into "Limp Mode"?

A8: Not if the system is functioning correctly. Limp mode usually occurs if the heaters fail or if the tank was too full to thaw properly, leading the computer to believe the system is empty.


Q9: How do I know if my DEF heater is failing?

A9: You will typically see a P20G6 or similar diagnostic code and a "Service DEF" message on your cluster.


Q10: Does the brand of DEF matter in winter?

A10: As long as it meets the ISO 22241 standard, the brand is less important than the storage. Always ensure the jugs you buy haven't been sitting in direct sunlight for months.


Q11: Where can I get my DEF system inspected?

A11: If you're worried about winter performance, call Redwater Dodge at 780-942-3629 to schedule a system diagnostic.