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What You Should Know About Diesel Exhaust Fluid (DEF)

According to the information provided on a DEF container, Diesel Exhaust Fluid (DEF) is for use in diesel engines equipped with Selective Catalytic Reduction (SCR) systems and is formulated to meet 2010 EPA regulations to reduce NOx emissions from diesel exhaust.  It is non-toxic, non-flammable, non-polluting, and meets ISO 22241 Specifications. But what does that mean to you as RVers?


In 2010, the Environmental Protection Agency (EPA) adopted new emission regulations requiring engine manufacturers to reduce harmful Nitrogen Oxides (NOx) and Particulate Matter (PM) in the exhaust of medium to heavy-duty vehicles.  At the time, most manufacturers turned to the Selective Catalytic Reduction method (SCR) which is a treatment used after the combustion in an exhaust chamber.  Some manufacturers such as Monaco and Navistar in particular elected to use a proprietary method known as Exhaust Gas Recirculation (EGR).  Just like it sounds, this system cools and recirculates the exhaust gas back to the engine to reduce NOx and PM.  After several years of testing and redesigning, the EGR system did not pass the emission standards and has gone by the wayside.  The SCR method allows engine manufacturers to tune the engines for the utmost fuel efficiency and power and treating the emissions downstream.

To be even more specific we consulted the Cummins Filtrations technicians, one of the leading manufacturers of diesel products for the RV industry.  They stated that DEF is 67.5% deionized water and 32.5% high purity urea which is a nitrogen compound that turns to ammonia when heated.  So basically a DEF dosing valve shoots the solution on the exhaust after the engine just before the catalyst where it vaporizes and decompresses to form ammonia and carbon dioxide which eventually turns the NOx to harmless nitrogen and water.

NOx sensors are located before dosing valve and after to monitor the incoming exhaust which will provide the appropriate amount of DEF to be applied and after to monitor the NOx level to determine it complies with EPA regulations.

Here are some common questions we receive on DEF:

How do I fill my DEF tank?

DEF is stored in a separate tank with the volume depending on the size of the vehicle.  Larger motorhomes will have large tanks that can be filled with bulk pumps at fueling stations while smaller vehicles such as diesel trucks used as tow vehicles have small tanks sometimes located under the engine hood.

DEF Storage Tank

If you have a larger motorhome, check to see if you have a filling spot on each side of your vehicle.  DEF fueling stations are usually just on the driver side which makes it hard to fill if your location is on the passenger side only.

DEF Fill Location on Passenger Side

Diesel Prices

DEF Nozzle

What happens if I run out of DEF?

You can not operate the vehicle without DEF!  Some models have an actual level gauge, others just have a warning light.  At 10% a warning light comes on, at 5% it starts to blink, at 2.5% it starts to flash and some motorhomes will have a buzzer.  When it is empty some vehicles go into a “limp” mode that only allows 5mph until the tank is full, others will not even start!

Where can I find it and what should I look for?

DEF has become more readily available today at fueling stations such as Flying J, Pilot, Bosselmans, and others.  It is also available at smaller stations and convenience stores such as Walmart, and most auto parts stores, as well as amazon.com

Does DEF have a shelf life?

DEF does have a shelf life, two years if it’s stored in a cool underground tank, but typically 6 months to one year in portable jugs.  Look for the date code stamped on the jug, if it does not have one, look elsewhere.  If the containers are exposed to the harsh sun most of the day above 86 degrees, it will reduce the shelf life to 6 months!  And always look for the American Petroleum Institute Certification Seal.

How much will I use and should I carry extra?

The amount of DEF applied depends on the amount of fuel you use or your vehicle’s miles per gallon.  Most tow vehicles such as a diesel truck will have a tank capacity of about 5-7 gallons and will get approximately 3500 miles per tank.  Larger units will have up to 10+ gallons capacity.  Most chassis manufacturers we have talked to indicate you will have approximately 2-3% usage.  So if you have a big rig that gets only 6 mpg, you will use 1 gallon every 300 miles.  Typically it’s a 50:1 ratio; for every 50 gallons of fuel, you will use 1 gallon of DEF.

Since it is so readily available, I would not suggest carrying extra containers as the liquid is corrosive to metal components even though it is non-toxic and non-flammable.  If a container were to rupture or freeze and split, the liquid could cause damage to metal components and wiring inside the compartment.  You can switch or mix brands if you find yourself needing some on the road and the nearest fueling station does not carry your brand. Do not reuse containers! Bulk DEF at the pumps is usually cheaper, however, reusing the containers could create problems with contaminants getting into the system from the used container.  When opening a new container, make sure all the protective foil under the cap is removed and does not go into the reservoir.  Also, make sure the paper or plastic seal inside the cap is in good condition and will not get into the fill neck.

Do I need gloves or protective clothing?

DEF is non-toxic, non-flammable, and typically is the least harmful of all the engine fluids.  However, it does have the chemical strength of household ammonia so if you are sensitive to a heavy cleaning agent, it’s a good idea to wear neoprene gloves.  If you spill some on your hands or clothing, simply wash it off with soap and water.

Can DEF freeze?

Yes, since DEF is 67.5% water, it will freeze at temperatures below 12 degrees F, however, most chassis manufacturers have designed a heating system for the tank and lines so it should thaw quickly and not affect performance.  Do not add any type of antifreeze or fuel additive to DEF in extreme temperatures!

What maintenance is required?

There is little maintenance other than using approved DEF and to make sure you do not get contaminants in the jug or funnel.  Some models have a filter or screen that can be removed, inspected, and cleaned yearly.  You can always check with your chassis manufacturer for recommendations.

Thank You To discoverdef.com and Cumminsfiltration.com

About the author: Dave Solberg: Managing Editor, RV Repair Club

For the last 25 years, Dave has conducted RV maintenance and safety seminars, developed dealer and owner training programs, written RV safety and handyman articles, authored an RV handbook reference guide and logged over 100,000 miles on the road in an RV.

RV Repair ClubRV Repair Club is your go-to online resource for enthusiasts who want quality RV maintenance, repair and upgrade information – a community where passionate RVers can come together to gather knowledge and share their experiences.

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