Procedure Perfected: How to Test DEF Fluid and Ensure Its Optimal Temperature

Why don’t we see buses and trucks belching black clouds of smoke anymore? It’s an ingenious combination of diesel exhaust fluid (DEF) and part of the exhaust system called selective catalytic reduction (SCR). 

But DEF can become contaminated, rendering it ineffective. This article will show you what it is, how to test DEF fluid to ensure it is unadulterated, and its optimal temperature for best performance.

Understanding DEF Fluid

DEF fluid plays a pivotal role in maintaining the eco-friendly operations of diesel vehicles. Mandated by the EPA in 2010, it breaks down harmful nitrogen oxide emissions. DEF is composed of 67.5% deionized water and 32.5% refined urea. When injected into the exhaust before the catalyst, it reacts with gases to form nitrogen, CO2, and water, making the air cleaner. Temperature and purity are vital for DEF’s role in the Selective Catalytic Reduction (SCR) process.

Importance of Maintaining DEF Fluid Temperature

For your reference, DEF is also called AUS 32 (aqueous urea solution 32%) and is sold under the brand name AdBlue. Regardless of the name, DEF freezes at 12 °F (−11 °C), and the tank must have a heater to prevent freezing. The EPA requires that DEF flow within 70 minutes of starting an engine. European regulations require it to flow within 20 minutes. 

Most modern diesel engines will not allow starting if the DEF is not flowing. Therefore, you must install DEF storage heaters if you run in freezing conditions.

How Temperature Affects DEF Fluid Quality and Efficacy

As we’ve mentioned, DEF can freeze. The ice crystals can block the DEF system, reducing the flow rate and even causing engine shutdown. Additionally, the expansion of frozen DEF can damage the system’s storage tanks, pumps, and other components. 

On the other hand, exposing DEF to high temperatures can result in the evaporation of water, leading to an increased urea concentration. This condition can negatively impact the SCR system’s efficiency and effectiveness in reducing emissions.

Why Testing DEF Fluid is Crucial

Using contaminated or sub-standard fluid can have serious consequences, severely affecting engine performance and emissions control systems. 

The Risks Associated with Contaminated or Sub-Standard DEF Fluid

Without DEF testing, here is what happens when using poor-quality DEF:

  1. Clogged or Damaged SCR Systems
  2. Increased Emissions
  3. Invalidation of The Warranty
  4. Total System Removal and Replacement

It is possible to flush plastic fuel tanks several times and reuse them. However, expect to replace the rest of the system.

Impact on Engine Performance and Emissions

If the DEF tank becomes contaminated, most likely, you’ll see the check engine light come on. Don’t ignore it! The consequences of running with contaminated or sub-standard DEF include:

  • Reduced Efficiency
  • Expensive Repair and Replacement Costs
  • Can Damage the SCR System
  • Environmental Impact 
  • Legal Consequences

Some engines will shut down automatically, a.k.a. derate mode. Prevent these issues by knowing what to look for in the DEF tank.

How Do You Know If Your DEF Fluid Is Bad?

There are two types of DEF contamination: hard particles and chemicals. A DEF filter can catch most particles but not chemicals.

The first way to check DEF is by visually inspecting the fluid. It should be clear and colorless. If you see particles, cloudiness, or any color, it’s contaminated. 

Secondly, use a refractometer to check the DEF. The refractometer should show you the Urea level of 32.5%. 

Third, you can smell it. DEF should have a slight ammonia scent. If you smell petroleum or other chemicals, assume it is contaminated.

If you suspect your DEF fluid is bad, ask your certified fuel distributor to check. Don’t run the engine if it is contaminated. Replace the fluid immediately. 

The DEF shelf life is about one year. Direct sunlight and storage temperatures above 86°F (30°C) will degrade it much faster.

Steps on How to Test DEF Fluid

Follow these steps below to effectively test DEF fluid and address any issues before they cause significant damage:

  1. Visual Inspection – Start by inspecting the DEF fluid for discoloration or cloudiness.
  2. Smell Test – It should have a slight ammonia smell. Unusual smells can indicate contamination or degradation.
  3. Check for Crystallization – Inspect the exhaust piping and DEF injectors for signs of crystallization or deposits.
  4. Visual Clarity – Ensure the DEF fluid is transparent, without any color or solid debris.
  5. Collect a Sample – If you suspect contamination, collect a sample of the DEF fluid from the top layer of the tank and check for engine oil or hydraulic oil contamination.
  6. Use a Handheld DEF Refractometer – A refractometer DEF tester can provide a more accurate reading of the urea concentration.
  7. Monitor Warning Lights – Don’t ignore any warning lights on your vehicle’s dashboard related to the DEF system. These lights can indicate issues with the fluid.
  8. Engine Performance – Keep an eye on your vehicle’s engine performance. If you notice a sudden power reduction, it could indicate a derate mode condition due to a DEF fluid issue.

Remember, maintaining the quality of DEF fluid is crucial for properly functioning your vehicle’s emission control system. If you have any concerns about the fluid, it is always best to consult a professional mechanic, dealership, or fuel service for further guidance. For more information, please Download the DEF guide.

Interpreting the Results 

Compare the test results with the acceptable range specified by the manufacturer. Maintaining the standard 32.5% urea concentration is paramount for optimal engine performance, emissions control, and regulation compliance. If the test results show that the urea concentration falls outside the recommended percent, you’ll need to consult a mechanic to flush and replace system parts. 

Maintaining Optimal DEF Temperature

Here are some key points to consider for maintaining the optimal temperature of DEF:

  1. Storage temperature – Store DEF in a cool, dry place between 12°F (-11°C) and 65°F (18.3°C). Temperature spikes above 86°F (30°C)  can reduce the shelf life of DEF. It is important to store DEF where temperatures are adequately regulated to prevent degradation.
  2. Freezing and thawing – DEF has a freezing point of 12°F, but freezing does not affect its quality or performance. When DEF thaws, it returns to its original state without any impact on its effectiveness. However, it is essential to prevent the storage containers from being damaged due to freezing.
  3. Avoid extreme temperatures – While high temperatures do not affect the quality or efficiency of DEF, they can accelerate its degradation. So, store DEF within the temperature range of 12°F to 86°F (30°C) to ensure optimal performance.

Follow these guidelines to maintain the optimal temperature of DEF to maximize its shelf life.

Powerblanket Solutions for DEF Fluid

Powerblanket offers DEF storage heaters that prevent freezing and maintain ideal Diesel Exhaust Fluid (DEF) temperatures. These heaters keep DEF at the optimal temperature range to prevent it from freezing. Powerblanket’s DEF storage heaters ensure the DEF does not solidify or crystallize. By maintaining the right temperature, these heaters help preserve the quality and effectiveness of DEF.

The Powerblanket DEF tank heater is the most efficient way to heat and store diesel exhaust fluid. Explore DEF Storage Heaters.

The Powerblanket DEF tank heater is the most efficient and effective way to heat and store diesel exhaust fluid.


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Shelby Thompson

Shelby Thompson is the head of standard product sales for Powerblanket. He has a distinguished military career, having served in both Afghanistan and Iraq. In his time in the Marines, Shelby acquired an impressive skillset that he now uses in his current role. When he's not working, Shelby loves spending time outdoors with his wife, son, and daughter. He is also a semi-keen hunter, fair weather fisherman, and shooter. Unfortunately, Shelby also has something of an unlucky streak when it comes to Fantasy Football at the company.



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