Car care · Elgin Auto Sales & Service Center · Elgin, IL
What Is DEF Fluid? A Diesel Exhaust Fluid Guide
Diesel trucks with a Selective Catalytic Reduction system need diesel exhaust fluid, or DEF, as well as diesel fuel. Here is what DEF does, how to refill it, how to store it and what the warning messages mean, using the 2025 Ram 2500/3500 owner's manual as the example.
NOxconverted to nitrogen and water vapor
ISO 22241API Certified DEF to use
12 degrees FDEF may freeze at or below
5 mphpossible limit if low-DEF warnings are ignored
The short answer
DEF is a fluid that a diesel's Selective Catalytic Reduction (SCR) system injects into the exhaust. The 2025 Ram 2500/3500 owner's manual says that when it vaporizes upstream of a catalyst, it converts smog-forming nitrogen oxides (NOx) into harmless nitrogen and water vapor. It has its own tank and fill point, separate from the diesel fuel, and never goes in the fuel tank. If the DEF tank runs low and you ignore the warnings, the truck can be limited to a maximum speed of 5 mph.
What diesel exhaust fluid does
The Ram 2500/3500 manual says these diesel trucks have a Selective Catalytic Reduction (SCR) system to meet diesel emissions standards required by the Environmental Protection Agency. A small quantity of DEF is injected into the exhaust upstream of a catalyst, where, when vaporized, it converts smog-forming nitrogen oxides (NOx) into harmless nitrogen and water vapor.
DEF is sometimes known by the name of its active component, urea. The manual describes it as a liquid reducing agent that reacts with engine exhaust in the presence of a catalyst.
The DEF injection system includes a DEF tank, pump, injector, electronically heated DEF lines, a DEF control module, NOx and temperature sensors and the SCR catalyst. The manual notes that you may occasionally hear a clicking noise, and that the DEF pump runs for a time after the engine is shut off to purge the system. Both are normal operation.
From the 2025 Ram 2500/3500 Owner's Manual, Diesel Exhaust Fluid (p. 186), Diesel Exhaust Fluid (DEF) (p. 333) and Noise (pp. 133-134), read November 12, 2026.
How to refill DEF
The 2025 Ram 2500/3500 manual gives these steps and limits for refilling the DEF tank:
| Step or situation | What the manual says |
|---|---|
| Which fluid | Mopar Diesel Exhaust Fluid (API Certified) or an equivalent API Certified to the ISO 22241 standard. Fluids not API Certified to ISO 22241 may result in system damage. |
| Where it goes | Remove the cap from the DEF tank, located on the driver's side of the vehicle or in the fuel door. The DEF fill is separate from the diesel fuel fill. |
| Filling at a DEF pump | Insert the DEF nozzle, start refilling and stop at the first shut-off, which indicates the tank is full. Do not keep filling, to prevent spillage. |
| Filling from a container | Check the expiration date and read the label first. After the indication appears on the instrument cluster display, add no more than 4 gallons (15 liters). |
| Do not overfill | Do not top off the DEF tank. Excessive overfilling can result in a check engine light, a fault code and inaccurate level readings. |
| Tank size | The 6.7L diesel's DEF tank holds approximately 7.5 gallons (28.4 liters). |
From the 2025 Ram 2500/3500 Owner's Manual, DEF Fill Procedure (pp. 187-188), Fluid Capacities (p. 383) and Engine Fluids And Lubricants (p. 385), read November 12, 2026.
Cold weather and storage
Freezing. The manual says DEF may freeze at temperatures at or below 12 degrees F (-11 degrees C), and that the system has been designed to operate in that environment. An electric heater inside the DEF tank works automatically when necessary, and if the DEF supply freezes, the truck will operate normally until it thaws.
Do not overfill in winter. If the tank is overfilled and the DEF freezes, the system could be damaged. When filling from portable containers, the manual says you may safely add a maximum of 2 gallons (7.5 liters) when the DEF gauge reads half full.
The gauge can lag. In cold conditions, 12 degrees F (-11 degrees C) and below, the manual says the DEF gauge needle can stay in a fixed position for extended periods, and that this is normal. After a cold refill, the tank heater may warm the fluid so the gauge updates after some run time; under very cold conditions, it is possible the gauge may not reflect the new fill level for several drives.
Storage. The manual calls DEF a very stable product with a long shelf life: kept between 10 and 90 degrees F (-12 and 32 degrees C), it will last a minimum of one year. Containers or parts that touch DEF must be DEF compatible, such as plastic or stainless steel; copper, brass, aluminum, iron and non-stainless steel can corrode.
From the 2025 Ram 2500/3500 Owner's Manual, Diesel Exhaust Fluid Storage, Adding Diesel Exhaust Fluid and DEF Fill Procedure and Filling The DEF Tank In Cold Climates (pp. 187-188), read November 12, 2026.
DEF warning messages and the 5 mph limit
The 2025 Ram 2500/3500 manual describes these low-DEF and DEF fault messages in the instrument cluster display:
| Message | What the manual says |
|---|---|
| DEF Low Refill Soon | Shown when the low level is reached, with a single chime. Approximately 5 gallons (19 liters) of DEF is needed to refill a pickup's tank at that point. |
| Speed Limited to 5 MPH in XXX mi Refill DEF | Shown continuously if DEF Low Refill Soon is ignored, with more frequent chimes unless up to 2 gallons (7.5 liters) of DEF is added. |
| 5 MPH Max Speed on Restart, Long Idle or Refuel Refill DEF | Shown when the counter reaches zero. The truck will be limited to 5 mph if it is shut off and restarted, idled for about an hour or more, or refueled. Add a minimum of 2 gallons (7.5 liters) of DEF to avoid this. |
| Incorrect DEF Detected See Dealer | Drive to your nearest authorized dealer and have the vehicle serviced as soon as possible. |
| Service DEF System See Dealer and other fault messages | If the system detects a fluid other than DEF, component failures or tampering, the vehicle may be limited to 5 mph if it is not serviced within less than 200 miles (322 km). |
From the 2025 Ram 2500/3500 Owner's Manual, Diesel Exhaust Fluid (DEF) Warning Messages and Fault Warning Messages (pp. 113-114), read November 12, 2026.
What not to do
Never put DEF in the fuel tank. The manual says not to start the engine if DEF is accidentally added to the diesel fuel tank, because it can cause severe engine damage, including failure of the fuel pump and injectors.
Never put anything but DEF in the DEF tank. Diesel fuel, fuel additives, gasoline or any other petroleum-based product will contaminate the whole DEF system, even in very small amounts (less than 1 oz. per 78 gallons), and the system will require replacement. Use a container, funnel or nozzle that is new or has only been used for DEF.
Clean up spills. If DEF is spilled, clean the area right away with water and use an absorbent material to soak up spills on the ground. Stop filling immediately if DEF splashes or wells back in the filler neck.
From the 2025 Ram 2500/3500 Owner's Manual, Diesel Exhaust Fluid Storage and DEF Fill Procedure (pp. 187-188), read November 12, 2026.
DEF questions at Elgin Auto Sales & Service Center
The Ram 2500/3500 diesel maintenance plan includes checking the DEF tank and refilling it if necessary at each scheduled interval. For DEF fault messages such as Incorrect DEF Detected or Service DEF System, the manual says to see an authorized dealer. For general maintenance questions, contact our service department or see our service specials.
Our service department at 1200 E. Chicago St. in Elgin is open Monday to Friday 8 a.m. to 5 p.m. and Saturday 8 a.m. to 4 p.m., and is closed Sunday. Get directions.
From the 2025 Ram 2500/3500 Owner's Manual, Maintenance Plan - Diesel Engine (p. 318) and DEF Fault Warning Messages (pp. 113-114), read November 12, 2026.
DEF fluid FAQ
What is DEF fluid made of?
The 2025 Ram 2500/3500 manual says DEF is sometimes known by the name of its active component, urea. Use DEF that is API Certified to the ISO 22241 standard.
What happens if you run out of DEF?
The Ram manual says warnings come first. If they are ignored, the truck can be limited to a maximum speed of 5 mph after a restart, a long idle or refueling. Adding a minimum of 2 gallons (7.5 liters) of DEF may be required to restore normal operation.
Can DEF freeze?
Yes. The manual says DEF may freeze at or below 12 degrees F (-11 degrees C). The DEF tank has an electric heater, and the truck operates normally until frozen DEF thaws. Do not overfill the tank, because an overfilled tank that freezes could be damaged.
Does DEF go bad?
The manual says DEF kept between 10 and 90 degrees F will last a minimum of one year. Check the expiration date on a container before you pour it in.
What if I put DEF in the fuel tank?
Do not start the engine. The manual says DEF in the diesel fuel tank can cause severe engine damage, including fuel pump and injector failure. Have the vehicle serviced.
What are your service hours?
Monday to Friday 8 a.m. to 5 p.m. and Saturday 8 a.m. to 4 p.m. We are closed Sunday.
Questions about DEF?
Visit us at 1200 E. Chicago St. in Elgin.
Sources
DEF information is from the 2025 Ram 2500/3500 owner's manual, read November 12, 2026. Other diesel vehicles differ; follow your own manual. Your own vehicle's labels and owner's manual are the authority for your vehicle. Our address and service hours are from our own website, checked the same day. Photo is of our dealership.
- 2025 Ram 2500/3500 Owner's Manual (Mopar): DEF Warning Messages, Noise, Diesel Exhaust Fluid, DEF Fill Procedure, Maintenance Plan - Diesel Engine, Diesel Exhaust Fluid (DEF), Fluid Capacities, Engine Fluids And Lubricants (PDF pp. 115-116, 135-136, 188-190, 320, 335, 385, 387; printed pp. 113-114, 133-134, 186-188, 318, 333, 383, 385), FCA US LLC via vehicleinfo.mopar.com, read November 12, 2026
- Service page: address and hours, Elgin Auto Sales & Service Center, read November 12, 2026