Diesel exhaust fluid (DEF) plays an important role in reducing the environmental impact of diesel engines. DEF helps lower the emissions of nitrogen oxides (NOx) that come out of the tailpipe. But like any consumable, DEF has a shelf life. So how long does DEF last before it goes bad?
What is Diesel Exhaust Fluid?
Diesel exhaust fluid (DEF) is a liquid that is sprayed into the exhaust stream of diesel vehicles. It is used to lower emissions from diesel engines. DEF is composed of 67.5% deionized water and 32.5% urea.
DEF works with selective catalytic reduction (SCR) systems in the exhaust to convert nitrogen oxides (NOx) into nitrogen and water vapor. SCR systems use a catalyst to facilitate this chemical reaction. DEF is required for SCR systems to function properly.
Most diesel vehicles manufactured after 2010 use SCR systems and require DEF. This includes heavy-duty trucks, buses, off-road equipment, diesel pick-up trucks, and more. DEF tanks are refilled as needed, typically every few thousand miles.
How Does Diesel Exhaust Fluid Go Bad?
There are a few ways that diesel exhaust fluid can go bad or degrade in quality and effectiveness:
- Crystallization – DEF is a solution of urea and water. Over time, the urea can crystallize and separate from the water, reducing effectiveness.
- Contamination – Impurities introduced during handling, storage, or refilling can contaminate DEF.
- Degradation – The urea in DEF decomposes slowly over time, especially at higher temperatures.
Crystallization and degradation speed up as DEF ages and is subjected to higher temperatures. Contaminants also make the fluid break down faster. All of these processes reduce the performance of DEF over time.
How Long Does Diesel Exhaust Fluid Last?
The shelf life of diesel exhaust fluid varies based on storage conditions:
- Unopened – 12 months from manufacture date when stored properly between 12°F to 95°F (-11°C to 35°C).
- Opened – 6 months from first opening when stored properly between 12°F to 95°F (-11°C to 35°C).
DEF has the best longevity when stored in cool, consistent temperatures. Heat exposure, frequent temperature fluctuations, exposure to sunlight, and improper storage can all decrease the usable life of DEF.
| Storage Temperature | Expected Shelf Life |
|---|---|
| Below 12°F (-11°C) | May freeze – store above this temp |
| 12° to 86°F (-11° to 30°C) | 12 months or longer |
| 86° to 95°F (30° to 35°C) | 6-12 months |
| Above 95°F (35°C) | Less than 6 months |
As you can see from the table, cool stable temperatures promote the longest DEF shelf life, while warm temperatures degrade it faster.
Signs of Bad Diesel Exhaust Fluid
Here are some signs that your diesel exhaust fluid may be bad or expiring:
- Crystallization – Urea crystals or slushy texture indicates degraded DEF.
- Cloudiness – Clear DEF can turn cloudy white or yellow as it ages.
- Contaminants – Particles, dirt, debris or other solids in DEF are a sign of contamination.
- Color Change – DEF should be clear and colorless. A colored or brownish tint indicates contamination or age.
- Odor – DEF has an ammonia-like odor. A stronger smell can mean aging fluid.
Testing strips are also available to measure the urea concentration and check DEF quality before pouring it into your vehicle.
Impacts of Using Degraded Diesel Exhaust Fluid
Using degraded, contaminated, or expired DEF in your diesel vehicle can lead to:
- Clogged SCR components
- SCR system failures and faults
- Increased NOx emissions
- “Check Engine” warning lights
- Reduced engine power (mph limited to 5-20 mph)
- Higher fuel consumption
To avoid issues, use high-quality DEF within the shelf life and replace older supply. Do not top off or mix old DEF with fresh fluid.
How to Dispose of Bad Diesel Exhaust Fluid
Like other automotive chemicals, bad or expired DEF should be disposed of properly:
- Do not pour down a drain – urea can promote algae growth in waterways.
- Check local hazardous waste regulations – some areas may allow pouring expired automotive liquids down the drain at certain facilities.
- Absorb and contain any spilled DEF with kitty litter or sand and dispose of solid material.
- Take to a local automotive store – many accept expired chemicals like DEF for proper hazardous materials disposal.
Be sure to dispose of DEF in accordance with any local regulations. Wear gloves when handling spoiled DEF to prevent skin irritation.
How to Store Diesel Exhaust Fluid Correctly
Proper storage is key to maintaining the quality and longevity of diesel exhaust fluid. Here are some tips for storing DEF correctly:
- Keep DEF between 12°F to 95°F (-11°C to 35°C) for best shelf life.
- Avoid direct sunlight and extreme heat exposure – park indoors when possible.
- Store in sealed containers to prevent contamination.
- Avoid plastic containers – DEF can degrade plastic over time. Use polyethylene, stainless steel, or aluminum containers.
- Keep DEF containers closed when not in use to limit air exposure.
- Maintain consistent cool temperatures. Frequent temperature cycling shortens shelf life.
Storing DEF correctly keeps it from crystallizing, degrading, and becoming contaminated. This maintains its emissions-reducing effectiveness over the full 12-month shelf life.
Conclusion
Diesel exhaust fluid provides important emissions benefits in modern diesel engines. But like any consumable, DEF has a limited shelf life. Unopened DEF lasts for 12 months, while opened containers expire after 6 months. Higher temperatures degrade DEF faster. Any crystallization, contamination, color change or odor indicates aging fluid.
Using degraded DEF can damage your vehicle’s emissions system. Always replace older supply with fresh fluid. And store DEF in ideal conditions – cool, consistent temps between 12°F-95°F (-11°C to 35°C), sealed from contamination, and away from sunlight. Proper storage gives you the full 12-month shelf life from each bottle of diesel exhaust fluid.