DEF Quality Reference

What it is

Diesel exhaust fluid (DEF) is a solution of high-purity urea in deionized water, used by selective catalytic reduction (SCR) systems to reduce NOx. ISO 22241-1 sets its quality: 32.5% urea by mass (allowed range 31.8% to 33.2%) and tight limits on metals, phosphate and other contaminants that damage the catalyst. Product meeting the standard is often marked as API certified or ISO 22241 compliant.

How to use it

Check DEF that is suspect, old or from an open container. A DEF refractometer reads urea concentration directly. A reading outside the allowed range, a cloudy or colored fluid, or a smell other than a faint ammonia odor means the fluid should not be used.

Quick reference

Key points

• Use only clean, dedicated DEF funnels, pumps and containers. Never reuse a container that held anything else.
• Contamination with fuel, coolant, tap water or dirt can damage the SCR catalyst and dosing system. Small amounts matter.
• Frozen DEF thaws and is usable. Vehicle systems have tank heaters for this.
• Expansion on freezing is about 7%. Do not overfill containers stored in the cold.
• DEF crystallizes into white deposits when it dries. Rinse spills with water.

Common mistakes

• Pouring DEF into the fuel tank, or diesel into the DEF tank. Both need immediate attention per the engine maker's procedure.
• Storing DEF in a hot trailer or in direct sun for long periods.
• Using a brass or galvanized fitting on a DEF dispenser.
• Topping off from an old, opened jug without checking it.

Governing standards: ISO 22241-1 (NOx reduction agent AUS 32, quality requirements); ISO 22241-2 (test methods); ISO 22241-3 (handling, transportation and storage). Engine maker's DEF requirements.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.