What’s That: DEF Crystallization

Over the past last decade, diesel exhaust fluid (DEF) has become a standard operating requirement  for fleets and off‑road equipment running selective catalytic reduction (SCR) systems. While DEF is widely used across industries, what’s often overlooked is how sensitive it is to storage conditions, handling practices, and environmental stress. These factors alone can directly impact SCR performance and equipment uptime.

 A storage and handling issue in the DEF space that’s not often discussed is DEF crystallization, a problem that can arise in any season. When DEF crystallizes, it can restrict flow to the SCR system, triggering fault codes, reducing engine performance, or even causing forced derates and equipment downtime. Left unresolved, crystallization can lead to costly repairs and unexpected emissions compliance issues, making proper DEF storage, handling, and system maintenance essential for reliable, year‑round operation.

What is DEF Crystallization?

DEF crystallization occurs when diesel exhaust fluid shifts from a liquid into solid urea deposits (crystals). Because DEF is composed of 32.5% urea and 67.5% deionized water, changes in temperature, evaporation, contamination, or improper storage can cause water to separate, leaving behind hardened urea crystals.

Once formed, these crystals can accumulate inside DEF tanks, lines, injectors, filters, and pumps (components designed strictly for liquid flow) causing restricted delivery to the SCR system. When the system doesn’t receive enough DEF to properly reduce emissions, it will trigger fault codes, derates, or even shutdowns. While freezing and thawing cycles can promote crystallization in cold weather, crystallization can happen year‑round when DEF quality and handling practices aren’t maintained.

How Does DEF Crystallization Happen?

Common causes of DEF crystallization include:

• Freezing temperatures
  DEF freezes at 12°F (-11°C). When it freezes and thaws repeatedly, urea  crystallizes and settles in lines, filters, and injectors.
• Evaporation from heat exposure
  High temperatures can cause the water portion of DEF to evaporate, increasing urea concentration and triggering crystallization in warm climates.
• Poor storage and contamination
  Exposure to air, dirt, fuel, or materials that don’t belong will accelerate crystal formation and degrade DEF quality.
• Residual DEF left in components
  Small amounts of DEF left in injectors, pumps, or lines after shutdown can dry out, leaving crystals behind that build over time.

How to Protect Your SCR System from DEF Crystallization

Protecting your SCR system from DEF crystallization requires a combination of proper storage, smart handling, and preventive maintenance. Because crystallization can happen in both hot and cold temperatures, paying mind to the changes in your DEF year-round is important.

1. Store DEF Correctly

• Keep DEF in sealed, dedicated containers made of compatible materials.
• Store between 12°F and 86°F when possible, to minimize freezing and evaporation risks.
• Avoid direct sunlight and excessive heat, which causes water loss and increases urea concentration.

2. Prevent Contamination

• Use clean, DEF-only transfer equipment—never fuel hoses or mixed-use containers.
• Always reseal containers and tank lids immediately after use.

3. Manage Cold-Weather Exposure

• Allow enough warm-up time so frozen DEF can fully thaw before operation.
• Avoid turning equipment on and off rapidly in freezing conditions, which increases crystal buildup in the lines and injectors.

4. Reduce Residual DEF Buildup

• Follow OEM shutdown procedures to help purge DEF from injectors and lines.
• Regularly inspect injectors, filters, and dosing valves for early crystal formation.
• Address minor restrictions early before they become system-wide failures.

5. Maintain Overall Engine Health

• Poor combustion, contaminated fuel, or improper lubrication can increase SCR demand and stress DEF components.
• Implement a preventive maintenance program to help reduce excess DEF dosing and lower the risk of crystallization.

Where Mansfield Helps You

At Mansfield, we help operators stop DEF crystallization before it happens and address the root causes (quality, storage, handling, and system stress) instead of relying on reactive fixes. By supplying high-quality, ISO-certified DEF, Mansfield ensures fleets are using fluid that meets strict purity standards, reducing the risk of contamination-driven crystal formation. Mansfield also works with customers to right-size DEF storage and dispensing equipment, helping minimize air exposure, temperature extremes, and long dwell times that allow water to evaporate and urea to crystallize. Through reliable logistics, inventory planning, and delivery scheduling, Mansfield helps prevent scenarios that increase crystallization risk. Reach out to a team member today!

Ready to protect your SCR system from DEF crystallization? Now You Can.