Diesel Exhaust Fluid (DEF) is required on every diesel engine built in North America after 2010. It’s an aqueous urea solution — 32.5% high-purity urea, 67.5% deionized water, conforming to ISO 22241-1 — injected into the exhaust to neutralize NOx emissions. This FAQ answers the 25 most-searched questions about DEF, from “what is it” through “why is my truck in limp mode.” For comprehensive coverage, see our Diesel DEF Complete Guide. NüDef solves DEF’s biggest field problem: crystallization in storage and in your SCR system.
DEF stands for Diesel Exhaust Fluid. It’s a clear, non-hazardous fluid (32.5% urea, 67.5% deionized water) that diesel trucks built after 2010 spray into their exhaust to meet EPA emissions limits. DEF doesn’t go in the diesel tank — it goes in a separate, smaller tank, usually filled at the same time you fill diesel. Without DEF, the truck enters limp mode. For everything DEF in one place, see our Diesel DEF Complete Guide.
Check the production date or "best by" date on the container. Older DEF that's been sitting in retailer warehouses is more likely to be off-spec when you finally use it. A 2-month-old jug is fine; a 14-month-old jug deserves a second look.
Between 50–75°F is ideal. Don't leave open containers exposed to air, sunlight, or temperature swings. UV light degrades urea over time. Hot warehouses (above 95°F) cut DEF shelf life in half.
NüDef is a DEF treatment, not a fuel additive. Pour it into the DEF tank or DEF storage container, never into the diesel fuel tank. Mixing DEF additive into diesel fuel doesn't prevent crystallization and could cause fuel system issues.
P207F, P20EE, P20EF should be addressed quickly. Continued operation with these codes can damage the SCR catalyst — a $1,500–$12,000 repair depending on truck. Don't clear the code and keep driving.
Adding NüDef to fleet DEF inventory at $50/month per truck is dramatically cheaper than the $4,000–$12,000 SCR repair bills that follow crystallization. Prevention math always beats reactive repair math in fleet operations.
| Question Category | Common Search Term | NüDef Resource | Quick Answer | Notes |
|---|---|---|---|---|
| What is DEF | "what is def diesel" | <a href="/diesel-def-complete-guide/" style="color:#1a8f3f;">Diesel DEF Complete Guide</a> | 32.5% urea + 67.5% water solution | EPA-required since 2010 |
| DEF Specs | "iso 22241" | <a href="/diesel-def-complete-guide/" style="color:#1a8f3f;">Specs section</a> | ISO 22241-1 standard | Required for SCR compatibility |
| DEF Consumption | "how much def per gallon" | <a href="/how-much-def-additive-per-gallon-nudef-dosing-guide/" style="color:#1a8f3f;">DEF Dosing Guide</a> | 2–3% of diesel consumption | Variable by engine |
| DEF Cost | "where to buy def" | <a href="/where-to-buy-def-additive-online-retail-wholesale-guide/" style="color:#1a8f3f;">DEF Buying Guide</a> | $3.99–$5.99/gallon retail | Bulk $2.85–$3.85/gal |
| DEF Storage | "def freeze point" | <a href="/how-to-keep-def-from-freezing-top-tips-for-winter-protection-for-diesel-engines/" style="color:#1a8f3f;">Winter DEF Storage</a> | Freezes at 12°F (-11°C) | Cold-cycle OK chemically |
| DEF Codes | "p207f p20ee p20ef" | <a href="/def-trouble-codes-explained-p207f-p20ee-p20ef/" style="color:#1a8f3f;">DEF Trouble Codes</a> | Various SCR fault codes | Diagnosable + fixable |
| DEF Crystallization | "def crystallization" | <a href="/what-causes-def-crystallization/" style="color:#1a8f3f;">Crystallization Guide</a> | Cold AND hot crystallize | NüDef prevents both |
NüDef is the proven DEF additive that prevents crystallization in your SCR system. One bottle treats 25 gallons of DEF, works with every DEF brand, and is sold direct from NüDef or through our fleet wholesale program. Stop reactive SCR repairs — prevent the problem.
