Couch Oil is your trusted supplier of quality diesel exhaust fluid (DEF). 

DEF systems are required for emissions control on most modern diesel engines. DEF works like a chemical catalytic converter in diesel engines, keeping trucks and machinery operating within federal emissions guidelines. The vehicle’s selective catalytic reduction (SCR) system injects DEF into the exhaust at a specific point to reduce harmful pollutants, especially nitrogen oxides (NOx). With modern emissions controls, most diesel engines will not start without DEF. If the DEF supply runs out when on the road, vehicle speed will be limited until the DEF tank is refilled.