Diesel exhaust fluid is essential for the maintenance of a diesel vehicle, as it reduces engine emissions and makes it safer to drive and less harmful to the environment. Diesel exhaust fluid, which meets stricter EPA standards to reduce nitrogen oxide emissions and particulate matter, is the key to making it more emission compliant. The best diesel exhaust fluid meets the EPA's standards and is free from impurities, which can damage the vehicle’s selective catalytic reduction system. The cost of diesel exhaust fluids can range from 50 cents per gallon to up to $12 per gallon, depending on the size of the quantity purchased. The shelf-life of the liquid is dependent on temperature, with storage temperature increasing from 75 degrees Fahrenheit to 86 degrees Fahrenheit, and if stored in a location where the fluid can last anywhere from 18 to 24 months, it may last up to 18 months. The manufacturer's price-per-gallon is often dependent on brand recognition and retailer preference, but the larger quantity purchased tends to be lower.