Diesel exhaust fluid (DEF) indicates how well a diesel’s emissions systems have been maintained. In fact, the DEF’s specific color clearly shows whether or not the system is healthy or filled with contamination. Each color holds the key to what might be wrong with the DEF and if that problem requires separate care before refilling the system with brand-new DEF.
The guide below identifies the various colors associated with DEF to help diesel owners and mechanics determine fluid health. Clean DEF is crucial to continued optimal engine performance and emissions control. As you diagnose the health of your diesel’s DEF, use diagnostics and repair tools from Identifix—like Direct-Hit® Mobile—to access everything you need from under the hood.
Not all diesels require diesel exhaust fluid; however, most modern diesels with a selective catalytic reduction (SCR) system do. DEF is comprised of 32.5% urea and 67.5% deionized water. Keep in mind that deionized water is not the same as distilled based on the variances in purification processes.
DEF is heavier than diesel fuel and engine oil. Its active component, urea, is nontoxic, nonflammable, biodegradable, and pH-neutral. Urea’s primary directive is to bind with the nitrogen oxide (NOx) produced by diesel engines, chemically separating it into nitrogen and water.
Diesels require DEF for smooth and efficient operations. DEF must be routinely inspected and either replaced or replenished based on its condition. Some vehicle makes and models will only start with the proper amount of DEF at their disposal.
DEF is typically clear, slightly yellow, or light blue. Its exact coloring can depend on its environment, as it looks bluer next to copper or brass and rust-colored against steel. If DEF appears cloudy or dark and has a strong ammonia smell, it’s contaminated and should be replaced.
If your DEF fluid is no longer viable, dispose of it properly by taking it to your local hazardous waste recycling center. Never pour the DEF down the drain or dump it on the ground. Mixing it with other substances invites an unwanted potential chemical reaction.
Although tainted diesel exhaust fluid should be removed from the system, it’s vital to determine the source of contamination before replacing it. Without this critical step, you could waste valuable resources on a malfunctioning system and risk leaving the underlying cause unaddressed. Troubleshooting DEF system issues requires evaluating any noticeable symptoms, including the following:
Most DEF system issues arise from crystallization or contamination. When the urea present in DEF contacts the air due to overfilling or a leak, it crystallizes into hard deposits. Wash away urea crystals with deionized water.
Crystallized urea can form due to unsealed DEF pumps or injector nozzles. Formed deposits tend to clog the dosing unit, lines, and catalyst. These blockages reduce the system’s ability to convert NOx, resulting in many of the symptoms listed above.
Dirt, metal, water, and other fluids can also contaminate the DEF system. Improper storage and handling can introduce organic or metal debris into the system, prematurely stressing the DEF pump. Diesel fuel, engine oil, and coolant can also contaminate DEF, reducing its performance.
It’s a common misconception that you can top off your DEF with water. Even if you use deionized water, you must maintain the proper urea-to-water ratio to ensure optimal performance. DEF can also gel or freeze below 12 degrees Fahrenheit and should never be thawed with additives that can dilute the proper urea-water ratio.
Although frozen DEF expands, it maintains its golden ratio once thawed. However, it must be appropriately stored and handled to ensure reliable performance once it is introduced into the diesel’s DEF system. The guidelines below explain how to store DEF properly:
DEF’s shelf life varies depending on how you store it. With proper storage, DEF can last up to two years, so having a bottle or two on hand to top off your diesel’s exhaust fluid system isn’t a bad idea. Routinely inspect the DEF level and quality within the tank, topping it off every few thousand miles or per the manufacturer’s recommendations.
DEF systems are vital to diesel emissions and performance, requiring routine inspection and maintenance like any other system in modern vehicles. Reading the color of diesel exhaust fluid can help you identify potential concerns and—if necessary—make the appropriate repairs. With the proper knowledge and resources, DEF system maintenance becomes yet another routine service interval.
Rely on Identifix Direct-Hit® Professional and Pro Academy for instant access to the resources you need to diagnose and repair issues at the source—all at your fingertips. Talk through repairs via our virtual technician hotline to find the proper fix fast. Contact us to learn more about the full suite of Identifix products.
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