If you’ve got a diesel truck or RV, it’s important to understand something about DEF. What exactly is DEF for a diesel engine and where are you most likely to find it? What happens if your rig runs out of it, and does DEF go bad?
In this post, we’re covering all these questions and more, for the benefit of diesel truck/RV owners out there who need to understand this important aspect of their vehicle in order to avoid the potential of some very expensive repairs.
- 1) What Is DEF?
- 2) When and Why Did DEF Come Into Effect?
- 3) Do All Diesel Vehicles Require DEF?
- 4) How Does a DEF System Work?
- 5) Where Can You Get DEF for Diesel Trucks & RVs?
- 6) Do I Add DEF to My Diesel Fuel?
- 7) What Happens If You Run Out of DEF?
- 8) Is There Anything I Need To Know To Properly Handle DEF?
- 9) Can DEF Go Bad?
- 10) Do You Have a Diesel Engine that Uses DEF?
- 11) Free RVing Tips, Tricks, Reviews, Giveaways & More
What Is DEF?
DEF is an acronym for Diesel Exhaust Fluid. Its purpose is to reduce harmful tailpipe emissions that are produced by a running diesel engine. Without DEF, diesel exhaust gases can be very harmful to the environment.
DEF is technically a combination of urea and deionized water.
Diesel exhaust fluid (DEF) is injected into the exhaust stream, where it chemically reacts with the nitrogen oxide (NOx) gases. This reaction converts the nitrogen oxide components into nitrogen gas (inert and harmless), water vapor, and a small amount of carbon dioxide (which is less harmful to the atmosphere the the nitrogen oxide components present before the reaction).
When and Why Did DEF Come Into Effect?
Diesel combustion (without DEF) results in nitrogen oxide-based byproducts that are considered harmful to the environment. They contribute to environmental problems like acid rain, haze, and nutrient pollution. And, when exposed to them, can lead to dangerous respiratory problems in humans.
In 2010, the federal government set new emissions guidelines to address the byproducts of diesel combustion that were determined to be harmful to humans and to the environment.
Back in January 2010, the federal government set emissions guidelines that diesel combustion didn’t meet. So, diesel exhaust fluid (DEF) was developed as a treatment that would allow diesel vehicles to meet the new emission guidelines.
Do All Diesel Vehicles Require DEF?
When emissions standards were tightened in 2010, engine manufacturers had to incorporate DEF for all on-road diesel vehicles and machines to ensure that they complied (off-road vehicles like construction equipment weren’t included). RVs, trucks, and all on-road diesel vehicles manufactured after 2010 require DEF. However, those manufactured before 2010 do not.
For example, our RV was a 2005 diesel pusher that didn’t require DEF. However, we do have some first-hand experience with DEF as it has been a feature of diesel-powered Class B+ rentals we’ve had when RVing overseas… and now our new GMC Sierra with the Duramax diesel engine requires it, too.
How Does a DEF System Work?
DEF is one component of a larger, specialized emissions control system that’s installed in newer diesel-powered RVs and other vehicles. That system is called Selective Catalytic Reduction (SCR), a technology that reduces tailpipe nitrogen oxides (NOx) emissions to near-zero.
What happens is that the harmful diesel exhaust gases leave the engine, and the particulates from the gases get trapped in a diesel particulate filter (DPF). DEF is injected into the exhaust system at the next point in the system, where it meets an SCR catalyst. The emissions then leave the tailpipe with a level of nitrogen oxides that is near zero.
This infographic illustrates how a diesel emissions control system works. (Source: Diesel Technology Forum)
Comparing an SCR system with a catalytic converter on a gasoline vehicle, an SCR system is active (the injection of the DEF fluid), whereas a catalytic converter is a passive system (exhaust gases react with a solid catalyst built into the converter).
It’s important to replenish the DEF in an SCR system periodically. Diesel exhaust fluid is refilled based on the vehicle’s fuel consumption.
SCR is said to be among the most fuel-efficient and cost-effective systems available for the nearly-complete elimination of emissions from diesel engines. In fact, it’s estimated that SCR systems on newer diesel vehicles may improve fuel economy by as much as 3-4%.
Where Can You Get DEF for Diesel Trucks & RVs?
DEF can be purchased at truck stops, auto parts stores, oil change facilities, gas station convenience stores, some grocery stores, and many gas stations. In some locations, DEF is available from its own pump, right alongside the diesel fuel pump nozzles.
You can even buy DEF at Walmart or Home Depot… and online at Amazon:
- ENGINEERED: For use in Selective Catalytic Reduction (SCR) systems on diesel engines
- EFFICIENCY: BlueDEF diesel exhaust fluid is key to helping improve fuel economy by up to five percent
Do I Add DEF to My Diesel Fuel?
No! You absolutely do NOT add DEF to your diesel’s fuel tank. DEF is stored in its OWN tank and is automatically dispensed into the exhaust flow. You should NEVER add it to your fuel tank under any circumstances!
On a diesel vehicle or machine there is a separate DEF tank and diesel fuel tank.
What Happens If You Run Out of DEF?
If your DEF level is getting low, you’ll receive plenty of warning ahead of the tank running empty. Some vehicles even have a dedicated DEF gauge on the dash.
If you’re running low on DEF, it’s possible that your vehicle will have reduced power. This is intentional, to conserve the DEF remaining in the tank.
If you fail to refill your RV’s DEF tank and it runs too low, your vehicle may be immobilized, and you won’t be able to start the engine until the DEF is replenished. Essentially, the rig will go into “limp mode”. In some cases, the engine may even require service.
In our experience, though, there’s no need to run out of DEF. In any DEF-equipped vehicle we’ve driven, a warning light on the dash comes on to alert you that the DEF is running low. At the next fuel stop, buy another container of it and top it back up. Most of the DEF packaging we encounter even has the spout built in, so it’s very easy to add to the tank.
Is There Anything I Need To Know To Properly Handle DEF?
Because most DEF packaging is designed with an integrated spout, you shouldn’t have much to worry about when it comes to coming in contact with it, because you won’t. But, as with all things, there are some precautions you should take.
While not required, it’s a good idea to wear gloves when handling DEF.
- It’s a good idea to wear gloves when handling DEF, to avoid any potential skin irritations. You’re not likely to come in contact with it, but if you do, why risk irritating your skin?
- If you spill DEF on yourself or on the ground, be sure to thoroughly rinse it off with plenty of fresh water.
- Though not corrosive to human skin, DEF is corrosive to aluminum. Avoid leaving DEF fluid in contact with any exposed aluminum and rinse it off as quickly as possible.
- Do not contaminate DEF with any other liquids or substances.
- And, in case it wasn’t made clear above, NEVER add DEF to your diesel fuel tank (or vice versa)!
Can DEF Go Bad?
The shelf life of DEF is said to be one year under optimal conditions, generally agreed to be between around 12° and 90°F.
DEF freezes in storage containers and in vehicle tanks and equipment, though. If temperatures drop below 12°F, your DEF may solidify, in which case it will not perform as intended. In most RV’s, the DEF tank is often warmed by a heater (fed by the coolant system) to ensure it stays a liquid. If, at startup, the tank is frozen, the engine computer will not throw any errors until the tank has had time to thaw.
When you shutdown the RV’s engine, you may hear a whirring/buzzing sound from near the DEF tank. This is likely a pump returning the DEF fluid back to the tank. This helps to prevent it from freezing in the lines.
Also, DEF does degrade over time. But, if you store your DEF at a consistent temperature of about 75°F, you can actually extend the shelf life to as much as two years. This isn’t recommended, however, unless you’re sure about the consistent 75° temperature.
If you notice a change in the color of your DEF, the fluid has lost its integrity and should be drained and replaced.
Do You Have a Diesel Engine that Uses DEF?
If you have a diesel engine of any kind that uses DEF, we’d like to hear about your experiences. Have you had any issues finding DEF when you needed it? Does your RV, vehicle, or machine give you plenty of warning when your DEF begins to run low?
Were you among the unfortunate diesel RV owners who experienced the failure of a part during the pandemic and who couldn’t get it replaced due to supply chain issues? (That issue has been completely resolved, but it was a very big deal when it occurred!)
Drop us a comment and let us know!
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George
Saturday 8th of February 2025
Great article as always! I bought my 2020 F450, used, a couple of years ago. It is the first diesel I have ever owned. The first time the DEF warning light came on, I had no idea what it was until referring to the owner's manual. I had never heard of DEF. So, I made my way to the store and bought my first two jugs of "Blue Def". While opening them, I thought how cool it was that they included the pour spout. About two minutes into the fill, I thought, nice idea poor execution. No matter how many times I tightened it, it just kept leaking. The second jug did the same thing, so I figured it couldn't be operator error. The next time I had to fill again, I thought I'd give the spout a third chance. No luck. Now I just get a funnel and pour straight from the jug. TIP: Pierce a vent hole on the opposite top side of the spout to allow for a smoother pour that won't splash on your shoes...ugh. It's the same principle as pouring from a gallon gas can. Or, as mentioned, fill up at the truck stop. It's much easier and usually cheaper.
Jay
Friday 7th of February 2025
The most convenient, freshest and least expensive place to purchase DEF is at a truck stop, not a gas station, convenience store or auto parts store. It usually costs about $4/gallon. You should avoid paying substantially more than that.
The DEF tank in our MH is 15 gallons. It is a pain to fill from a container. Our diesel tank is 150 gallons. Our Cummins engine DEF:diesel ratio is about 2%. That 15 gallons will last a very long time (theoretically, 750 gallons of diesel or about 5,000 miles) and is a very minor increase in the cost of operating the vehicle.
However, in response to people substituting water in their DEF tanks, the EPA made the manufacturers add a device that tests for the proper concentration of urea in the DEF tank. There were some technical problems with that. For a while, there were a lot of DEF head failures causing the ECM to derate the vehicle to 5 MPH, which is both incredibly inconvenient and dangerous. They all had 10-year warranties by law, but replacements were difficult to obtain especially during the pandemic. The Shaw brand was particularly problematic.
In response, some people reported instructions for building an electronic DEF simulator that plugs into the cable from the DEF tank. It signals to the ECM that the DEF is okay and the tank is 3/4 full. They are inexpensive to build and cost about $75 in parts that are easily available on the Internet. A soldering iron with some very thin solder and some basic tools are all that's required. The instructions are available online. Their use (other than in emergencies) is clearly illegal but they're not illegal to just possess. Compatibility with your type of DEF head may be an issue. It worked fine on our Spartan K2 chassis.
"Big Mike" from Spartan says that they can tell if a simulator has ever been used, but the only way to tell if you built one correctly is to test it on your coach. Since using it leaves no ECM codes, I doubt Spartan can tell, but if your DEF head has failed, you are entitled to a replacement by them anyhow! Hoping that a DEF head failure never gets us stranded on the side of the road, I built one of those simulators and never take a trip without it. Safe travels, everyone.
Ray Davis
Friday 7th of February 2025
I would encourage anyone using DEF to do some research into the product and the suppliers, to make themselves knowlegable with the degradation of the product over time due to differing storage conditions that affect the products shelf life. Date codes are critical to purchasing fresh DEF. Just yesterday, I stopped at two auto parts stores in Texas and both products were over a year old. On the shelves in HD stores, in different states, the manufacturers production dates can vary from a couple of months old to over 18 months. This is true at stores across the spectrum. Fancy names don't make the product better than another. API & ISO approval and standards set the bar and are met by many, but not all. Higher prices do not in this case provide a better product.
Gary B.
Friday 7th of February 2025
Quick addition. Sunlight is bad for DEF. It apparently decomposes the urea. Store in a dark place, or cover the container. Keep up the great work, guys! Cheers!
Ron
Saturday 10th of February 2024
Thank you Peter and John for all of your postings. I read everyone! I always carry a couple of extra boxes of DEF (2.5 gallons each) just in case. These definitely came in handy last summer on our trip to and from Alaska. Not too many DEF pumps in Canada. I also buy our DEF at Costco. It is much cheaper. Around $16. Of course not all Costcos have it and sometimes the stock is not found easily. Cheers! Ron
Gary B.
Friday 7th of February 2025
@Ron, There are starting to be more DEF pumps in Canada, but mostly at commercial pumps that require a passcard for that brand. It is however, usually available by the gallon jug, at gas stations. I buy mine by the 2 gallon jug at stores that sell big truck parts & accessories. Cheers!
TheRVgeeks
Sunday 11th of February 2024
Our pleasure, Ron. Glad you find this stuff interesting! And thanks for the tip about Costco. Our old 2005 Mountain Aire was pre-DEF, so we never had to worry about it. But our new truck is going to need it... and we shop at Costco all the time. It's a perfect fit! 😉