Thursday, June 8, 2023


Ask Dave: All about DEF shelf life and how long it lasts in the tank

There have been many questions regarding diesel exhaust fluid (DEF) that is required in diesel units since 2010. Recently we had questions from several readers about the shelf life of DEF. I believe an article written for suggested DEF had a shelf life of only one year. So the questions started pouring in about what to do if you don’t use all the DEF in your motorhome and tow truck? Do you need to somehow get all the existing DEF out of your tank when the clock strikes one year? And how do you do that since there is no drain valve on the bottom, and you can’t just dump DEF out on the ground?

This has become more of an issue these days as diesel fuel prices climb and more RVers with a diesel engine are taking shorter trips. Also, some are just parking the big rigs for an extended period. So I did some research and contacted several DEF manufacturers, Freightliner Custom Chassis Corporation (FCCC), and Cummins, the major engine manufacturer for the Class A diesel units.

What is DEF and why does it have a shelf life?

In 2010 the federal government implemented a mandate for lower emissions for diesel engines and most engine manufactures went with the Selective Catalytic Reduction (SCR) method. That sprayed the exhaust in a chamber with diesel exhaust fluid (DEF), which breaks down the NOx into nitrogen and water. Some old-time owners of a Monaco product at that time might remember the battle between Monaco and later Navistar, who chose to reroute the exhaust back through a chamber and re-burn it rather than using the SCR and DEF. This was called Exhaust Gas Recirculation (EGR), which Cummins does to a small amount. However, Monaco chose to do a much more extensive amount rather than requiring a DEF tank. They tried this method for several years but were never able to pass emissions certification and it was discontinued.

DEF is a liquid that contains 67.5% distilled water and 32.5% urea. It is sprayed in the final exhaust chamber and is not an additive to the diesel fuel. It has a separate tank that can be filled by 2.5 gallon jugs in the case of smaller tow vehicles, or added at fueling stations with a pump nozzle.


So I called my contacts at the RV manufacturing level, chassis level, and even one of the major manufacturers of DEF, Blue Sky. All the RV manufacturers had no comment, but rather forwarded me to the chassis or engine manufacturer’s recommendation, which I understand.

Here are the responses I got from the others about DEF

According to Blue Sky, one of the leading manufacturers of DEF:

The shelf life of DEF is a function of ambient storage temperature. DEF will degrade over time depending on temperature and exposure to sunlight. Expectations for shelf life as defined by ISO Spec 22241-3 are the minimum expectations for shelf life when stored at constant temperatures. If stored between 10˚F and 90˚F, shelf life will be one year. If the maximum temperature does not exceed approximately 75˚F for an extended period of time, the shelf life will be two years.

According to (engine manufacturer):

Our “official” statement is we go off the manufacturer’s recommendation of “life of DEF”. The DEF system does monitor DEF Quality and will alert the driver if it does not meet Cummins quality levels. Each container will have a shelf life stamped on it.

According to Daimler Worldwide and specifically the Freightliner Custom Chassis Corporation (FCCC):

DEF life will vary based on temp and direct sunlight, so having the tank in a compartment like it is on RVs is much better than on a vehicle where the tank is exposed to direct sunlight. FCCC has not seen a large number of people having an issue with DEF quality, but I would recommend to always leave some room in the tank so that if you do get an indicator that you can still put some fresh DEF in the tank and help revive the DEF quality. In some cases service centers have had to force air into the tank with a hose and then have a second hose to allow it to drain, but that’s certainly not the norm.

 We can’t be the only ones…

The response from my FCCC contact got me thinking that there are other industries that experience the same down time of storage as RV owners. So I called a contact that supplies lubricants and DEF to the agricultural industry. Being in the Midwest, some farm equipment sits for almost 11 months before hitting the dirt, such as combines. So what do they recommend, and do they see any DEF contaminations?

He told me they recommend to leave the DEF tank as close to empty as possible when they are done for the season and getting ready to store the unit. This falls in line with FCCC’s recommendation of “always leave some room in the tank.” Granted, most farmers would put an expensive piece of equipment back into storage in a year or so. But it was interesting that they have not had an issue with the dreaded “Check Engine” light since the DEF era!

How can you check the quality of DEF?

I talked with a Cummins technician as well as my FCCC contact. Both said that the system has a monitoring device to tell the level and quality of the DEF going through the system. However, this made me think of the articles Russ De Maris and others at have written about being stranded when the DEF was low or the sensors were not working well. I would much rather monitor the quality myself before getting into “limp mode” once the quality was not acceptable!

There are a few devices that will allow you to check the quality of DEF, such as the refractometer and concentration tester. The refractometer can check measure the urea concentration to approximately +/- 4%. The “old style” tube-type concentration tester floats discs to measure urea concentration and is temperature compensating. There is also a test strip that you can dip into the DEF and it will either stay the same color (blue) or any color change indicates contamination. The problem with these testing devices is you need to get a sample of the DEF in your tank and sometimes that is underneath the rig and in a long and winding road of tubes!

OK, Dave. Land the plane!

I have a good friend that is a pilot and he and I can talk forever about a subject. His wife will “kindly” chime in, “Can you guys please land the plane!”

From what I get from the DEF manufacturer, they state there is a one-year shelf life to cover any warranty issues, since they cannot control the exposure to sunlight, temperatures, and other issues. However, they state that it can last up to two years if stored properly. I even found one article that said do not use anything more than three years old. This tells me they don’t have much research to prove what actually causes some of the DEF-related issues and are playing it safe.

Personally, I believe if you are going to store your DEF-required vehicle for a long period of time, you need to manage the level. Do what FCCC recommends and keep it as low as possible when storing the vehicle. Blue Sky states that anything over 75 degrees will start to weaken the DEF. So you can imagine what happens when the unit is sitting in Arizona at 100+ degrees!

Bottom line

Keep the tank as low as possible during storage and bring it back up to quality standards when hitting the road. And if you don’t plan to put on enough miles to bring the DEF tank back down by storage time, don’t fill it, but rather keep it at a manageable level.

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

Read more from Dave here


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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


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11 months ago

My wife and I took a trip across country for 5 months with our travel trailer. During our trip we noticed it was getting harder to find DEF. We got home in March and have a unused 2.5 gallon bottle. If unopened does it still only last 1 year or does the year start when you open it.

11 months ago

Dave ~ thanks for your research and article. I was one of this folks asking you DEF questions. Last year I began using the leave room in the tank method. I drive until the add DEF light comes on, which should mean only 2 or 3 gallons remain. I then add a gallon or less and drive again until the add light comes on. Prior I was topping off before major trips. Even so, I never had a problem despite freezing temps in Bend, Oregon and triple digit temps in Arizona.

11 months ago

Living in the cold northeast, where temps spend the winter consistently below freezing for several months, I’m concerned about running the DEF very low before putting the RV into storage. Won’t it freeze, and what additional problems could that present?

Doug Musick
11 months ago

The EPA recently changed the requirements for how engines respond when the DEF sensors indicate problems. This is because most bad DEF codes were due to bad sensors and not bad DEF. Many many people had unnecessary tow and repair bills as a result.

11 months ago

I don’t know why you can’t dump old DEF on the ground, urea is fertilizer.

Thomas D
11 months ago

My 2011 chev is broken right now a heater in the tank is broken. We really need it. It’s 87 degrees right now, genius. Ive had trouble from the beginning in 2010. We clean the air and yet,I watch planes take off all day and spew black s they go. I looked it up. An Airbus 320a burns about 930 gallons an hour. A trip to mesa az takes about 3000 gallons of jet a which is a kissing cousin to diesel. Over 2000 planes were cancelled this weekend. Over 6 million gallons of fuel was not burned. There are no pollution control’s on jet planes. If government is so worried about us, why don’t the same rules apply? Oh of course, money talks and airlines can afford lawyer’s and payola.

Ron L.
11 months ago

But you didn’t answer the questions…. “what to do if you don’t use all the DEF in your motorhome and tow truck? Do you need to somehow get all the existing DEF out of your tank when the clock strikes one year? And how do you do that since there is no drain valve on the bottom, and you can’t just dump DEF out on the ground?”

11 months ago

Most rear diesel motor homes have the DEF tank located within inches of the SCR and exhaust system with limited shielding. Some have posted that such a location near so much heat could prematurely degrade the DEF in the tank. Seems like a legitimate possibility.

That said, I buy and use the lowest cost boxed DEF I find that has the API certification. I test with a refractometer, which I hope doesn’t really have a +/- 4% error or it’s worthless since the DEF concentration has to be +/- 0.7%! I question where that 4% stat came from…seems incorrect for an acceptable accuracy level. I leave my DEF level low going into winter storage. Keep a spare box climate controlled. So far, so good.

11 months ago

Off road diesel and road diesel is the same product.The difference in price is the state and federal highway tax is not paid on dyed off road diesel.

Mike Stanbro
11 months ago

Excellent write-up on this topic. Definitely something I never stopped to consider. I quit buying the 2.5 gallon jugs and now get DEF at a truck-stop fuel island when I realized the savings of buying it in bulk.

Tommy Molnar
11 months ago

I filled my DEF tank when leaving Reno earlier this year. 2,000 miles later in Houston, my Banks iDash shows a 67% level while my Ford dash computer shows “below 50%”. No actual gauge, just a sentence. Almost three months later the iDash and the Ford dash still show the same. no movement or change. We have taken several trips and put some miles on. Seemed strange so I went to the local Ford dealer and inquired. The ‘tech’ said the measuring level ball is probably faulty. I didn’t bother to ask how much to fix it. I said, “How about I just add some DEF?” So I bought a jug of Ford DEF, added the whole two gallons, checked the iDash and it said 100%. Ford dash says “full”. Problem fixed, I guess.

Bob p
11 months ago

It’s been my understanding that off-road vehicles such as farm or construction equipment didn’t fall under the def requirements. Am I wrong? Off-road diesel is also a different color than on road diesel, it has a red dye added and is cheaper, but if you’re caught on the road using off-road diesel it is a very stiff fine. It’s also illegal for stations to sell off-road diesel to anyone other than agricultural or construction equipment.

11 months ago
Reply to  Bob p

no the red dye is/was a product brand thing. taxed fuel and un-taxed fuel each have a part of a dye in it. so if you use un-taxed fuel in your taxed tank it will turn brown. it is ok the other way around as off rd motors and even the reefers on the trailers use non taxed fuel.
used to be there was a tax line to get the tax back, but the irs did away with that.
no def needed on the reefers, not sure about farm equipment.
at least as of this date. 6/21/2022.

11 months ago
Reply to  larry

please see my reply above

11 months ago
Reply to  Bob p

All diesel vehicles, Agriculture or over the road after a certain year require DEF.
You can not fill your over the road diesel vehicle with the fuel you have in your farm tank for your tractors, if they check you it is a huge fine. And the dye is not a “brand” thing, it is there so law enforcement can be sure you have the right fuel in your vehicle.
I am a farm wife, so I know

11 months ago
Reply to  Jeanne

To you all. Jeanne has the correct answer.

11 months ago
Reply to  Bob p

Farmers with new equipment deal with DEF every day and buy in bulk, not just by the gallon. Dye has been in the different diesels for decade upon decade to differentiate non-road and over the road diesel as non-road diesel is not road taxed like gasoline and road diesel.

Jim Beirlein
11 months ago

Thanks for this article, Dave. Though my truck is not in “storage”, I am not driving it much due to the price of diesel. I have a jug of DEF in the garage and enough room in the tank for it but will now hold off putting it in. It’s in a cool location and out of sunlight so it should be good for at least two years. I will now keep that space in the tank in case I get a quality warning message on the truck.

Curt Rissmann
11 months ago

I drive a 2018 2500 Dodge, 6.7 cummins equipped with the DEF system. I have 62,000 miles with no problems. I pretty much use the “Blue DEF”. On occasions I have used cheaper brands and once even used DEF from a pump, again with no problems.

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