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RV DEF – What You DEF-initely Need to Know About It!

RV DEF – What You DEF-initely Need to Know About It!

If you’ve got a diesel RV, it’s important to understand something about RV DEF. What exactly is DEF for a diesel RV and where are you most likely to find it? What happens if your rig runs out of it, and does RV DEF go bad?

In this post, we’re covering all these questions and more, for the benefit of diesel RV owners out there who need to understand this important aspect of their RVs in order to avoid the potential of some very expensive repairs.

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.

A diesel exhaust pipe with an exhaust stream

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 of 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 on all diesel vehicles and machines to ensure that they complied. RVs, trucks, and all diesel vehicles manufactured after 2010 require DEF. However, those manufactured before 2010 do not.

For example, our RV is a 2005 diesel pusher that doesn’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.

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.

An infographic showing how a diesel emissions control system works

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 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, Home Depot, and on Amazon.

BlueDEF Diesel Exhaust Fluid Synthetic Urea & Deionized Water 2.5 Gallon Jug
  • 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 RV Fuel?

No! You absolutely do NOT add DEF to your RV’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!

Separate DEF and diesel fuel tanks shown

On a diesel vehicle or machine there is a separate DEF tank and diesel fuel tank.

What Happens If You Run Out of RV 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 RV 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 RV 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 rig may require service.

In our experience, though, there’s no need to run out of DEF. In the rigs we drove, a warning light on the dash would come on alerting us that the DEF was running low. At the next fuel stop, we’d buy another container of it and top it back up. Most of the DEF packaging we encountered even had the spout built in, so it was 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.

  1. 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?
  2. If you spill DEF on yourself or on the ground, be sure to thoroughly rinse it off with plenty of fresh water.
  3. 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.
  4. Do not contaminate DEF with any other liquids or substances.
  5. 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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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


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! 😉

Dr. Mike

Saturday 10th of February 2024

".....engine manufacturers had to incorporate DEF on all diesel vehicles and machines to ensure that they complied. RVs, trucks, and all diesel vehicles manufactured after 2010 require DEF."

Not entirely true:

The U.S. Military also has a large vehicle fleet that consumes diesel fuel. However, DEF is not commonly used in bases across the world. Except for vehicles used for local travel, heavy-duty vehicles such as tanks and work trucks are exempt from NOx reduction regulations.

Great article (as usual), but you forgot one very important item: A handheld DEF refractometer. It is used to measure the concentration specifications of the fluid to ensure 32.5% of the fluid is urea.

Phil Fonteyn

Friday 3rd of March 2023

Do you have any tips or procedures for draining and replacing DEF? I have a 2011 Winnebago diesel pusher with a Cummins engine. I know the DEF that is in it is a minimum of 4 to 5 years old. It has absolutely been subjected to temps well below 12 degrees. I would like to drain the tank, purge it, and refill it.

Dr. Mike

Saturday 10th of February 2024

@Phil Fonteyn, You can use a battery (2-D cell) transfer pump ($10 at Harbor Freight) to remove the old DEF. Add some fresh DEF and swish it around the tank whith the pump, then pump it out. Fill the tank with fresh DEF and enjoy!

Bob Garbe

Thursday 16th of February 2023

Ford Transit Class B and C Motorhomes with the 3.2 L 5 cylinder diesel have had persistent issues with the DEF system involving both the tank heater and the level sensors that has necessitated a enhanced warranty for malfunctions in either of these systems. This involves the replacement of the entire tank module with pump, sensors, and heater which are one unit.

Problem, of course, is the fact that you have to wait until the system fails until you can get the problem addressed which invariably happens while out on a trip. The number of Ford dealers willing to work on RV's is steadily deceasing and so this can result in ruined or delayed trips.

Richard Kilgore

Saturday 11th of February 2023

My only problem with DEF is the difficulty I have with actually pouring it into the filler spout on my 2013 Winnebago VIA. Part of the engine compartment frame interferes with the DEF filler tube and makes it almost impossible to avoid spilling some DEF. And, it is impossible to see when the DEF tank is full.


Monday 13th of February 2023

Uggh... we hate it when it's clear the manufacturer never considered what the user-experience would be like!

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