How long can DEF fluid sit in tank?

What is DEF fluid?

DEF (Diesel Exhaust Fluid) is a liquid that is sprayed into the exhaust stream of diesel vehicles to lower emissions. It helps convert harmful NOx gases into nitrogen and water. DEF is composed of urea and deionized water. All diesel vehicles manufactured after 2010 use DEF as part of their emissions control systems.

DEF has a limited shelf life and will degrade over time, especially if it is exposed to high temperatures. Properly stored DEF has a shelf life of approximately 18 months. After this point, the urea begins to break down into ammonia, which can damage SCR systems and cause your check engine light to turn on. Let’s take a closer look at how long DEF can remain in the tank before degrading.

How does DEF degrade?

There are two main factors that cause DEF degradation: heat and contaminants.

Heat speeds up the breakdown of urea into ammonia and carbon dioxide. DEF is designed to be stable up to approximately 86°F. Temperatures above this accelerate decomposition. The hot exhaust and high underhood temps of a diesel engine provide ideal conditions for DEF to start degrading.

Contaminants like dirt, other fluids, and metals can also make the DEF solution unstable. Impurities essentially “poison” the DEF and make it break down faster. Even small amounts of contamination can make the urea decompose more rapidly.

As DEF degrades, it forms crystals that can clog SCR components and cause corrosion. Degraded DEF will not properly convert NOx emissions either, leading to excess pollution. For optimal performance, DEF must maintain its purity.

Shelf life of unused DEF

Unused DEF sitting in sealed containers has a very long shelf life. DEF is commercially packaged in sealed jugs, drums, or totes. As long as these containers remain unopened and stored properly, the DEF can last for years.

The DEF manufacturers Arcomm and Peak guarantee their DEF for up to 2 years if stored between 23°F and 77°F. Other major brands also cite shelf lives up to 2 years for unopened packages stored at moderate temperatures.

Once opened, DEF has a much shorter lifespan. Any air or contaminants that get introduced will start to break down the urea. Opened DEF should be used within 6 months. Make sure to seal the container to limit air exposure.

Shelf life of DEF in vehicle tanks

The DEF sitting in your vehicle’s tank has a shorter shelf life than unopened containers. The ambient heat and potential contaminants in the tank accelerate degradation.

Most experts recommend using up DEF within 6 months of it being put into the vehicle’s tank. DEF can often last 9-12 months before any performance issues arise, but degradation will steadily occur.

To maximize the lifespan, avoid topping off the DEF tank unnecessarily. Only add enough DEF to reach the next service interval or refill. The more DEF sits in the tank, the more exposure it has to heat, moisture, and impurities which shorten its shelf life.

Signs of degraded DEF

Here are some signs that the DEF sitting in your tank has degraded and needs to be replaced:

  • Check engine light turns on
  • Excessive diesel emissions
  • DEF quality malfunction message appears
  • DEF pump or injector failure codes
  • Tanks clogs from DEF crystallization
  • Strong ammonia smell from exhaust

If you suspect the DEF is expired, the best option is to drain and flush the tank before refilling it with fresh DEF. Most trucks have a tank drain valve that can be used to empty it out.

Does DEF go bad if not used?

Yes, DEF has a limited shelf life and will eventually go bad even if the vehicle is not being driven. Two factors lead to DEF degradation over time:

1. Exposure to heat

DEF is designed to be stable up to about 86°F. Even if the vehicle sits unused, the temperature in the tank will typically exceed this threshold which starts breaking down the urea. Hot underhood temperatures, ambient heat, and the proximity to the hot exhaust system can raise tank temperatures. This starts the decomposition process.

2. Contaminants and moisture

When DEF sits for extended periods, it can start to crystallize and precipitate out impurities. Condensation inside the tank also introduces moisture. These contaminants destabilize the solution and cause it to expire faster. Even if unused, impurities find their way into the tank.

While degraded DEF may not immediately cause issues if the vehicle sits, it will eventually fall out of specifications. For maximum performance, the DEF should be drained and replaced about every 12 months even if the vehicle is not driven regularly. Heat and contaminants affect stored DEF whether it burns through the system or not.

Extending DEF shelf life

Here are some tips to maximize the lifespan of the DEF in your truck:

  • Avoid topping off tank. Only add what’s needed.
  • Use DEF within 6 months of it being added to the tank.
  • Park vehicle in a shaded area to reduce tank heat if possible.
  • Limit contaminants by keeping the tank cap sealed.
  • Install an aftermarket tank cooling system or insulation wrap.
  • Drain and flush tank yearly as part of maintenance.

Keeping DEF out of high heat conditions gives the best chance for it to remain stable as long as possible. Proper storage and timely draining of old fluid help optimize its shelf life.

Conclusion

DEF fluid has a limited lifespan, especially when stored in hot vehicle tanks. While sealed containers allow DEF to last up to 2 years, the fluid in the tank should be used within 6-12 months if possible. Heat, moisture, and impurities in the tank environment cause DEF to degrade over time. Signs like emissions problems, crystallization, and failure codes indicate the DEF has expired. Draining and flushing the tank annually provides the best way to keep fresh DEF in your system. With proper maintenance, DEF can work effectively at reducing emissions for the long haul.

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