📖 What It Does
Reduces NOx emissions from diesel exhaust by up to 90%. Every diesel pickup, SUV, and on-road truck sold in the US since 2010 uses DEF and SCR to meet emissions limits.
⚙ How It Works
A small DEF tank (typically 3–8 gallons) stores the fluid. A heated pump and dosing injector spray DEF into the exhaust stream when the SCR catalyst is hot enough. Inside the SCR, ammonia from the urea reacts with NOx on a precious-metal-coated substrate, releasing only nitrogen and water vapor.
⚠ Symptoms When It Fails
Running out of DEF will limit power, then prevent the engine from starting after a countdown of restarts. Other issues include crystallized DEF in the injector, a failed dosing pump, or contamination from non-DEF fluid (which is catastrophic). See full symptom guide.
💰 Replacement Cost
DEF refill: $3–$10 / gal. A gallon of DEF runs $3–$5 at truck stops or $7–$10 at the dealer. Average truck uses ~2 gallons per 800 miles.