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.

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.

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.

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!

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.

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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