Diesel Exhaust Fluid Level Warning Lamps Diesel Exhaust Fluid Level Warning Lamps Diesel Exhaust Fluid Level Warning Lamps A four light bar segment indicates the Diesel Exhaust Fluid (DEF) level in 25% increments. Low DEF levels will trigger a decrease in the engine's performance. The use of improper DEF fluid will trigger a decrease in the engine's performance. In an empty or an ignored state and the diesel fuel tank is filled without filling the DEF tank, the vehicle's speed will be limited to 5 mph until DEF is detected in the DEF tank. Figure 8. Driver Card 52 All information subject to change without notice. Copyright © 2017 DETROIT DIESEL CORPORATION DDC-SVC-MAN-0192