Derate

📖 YOUCANIC Automotive Glossary

Derate is a progressive engine power reduction strategy primarily used on diesel vehicles where the ECU reduces maximum available power in stages based on the severity and duration of emissions system faults. Unlike Limp Mode which is typically an all-or-nothing power restriction, derate can be graduated — for example, a first-level derate might reduce power by 25%, a second level by 50%, and a final level may limit the vehicle to 5 MPH or prevent restart entirely. Derate is most commonly associated with diesel aftertreatment faults: SCR efficiency below threshold, DEF quality or level issues, DPF excessive soot loading, and NOx sensor failures.

EPA and CARB regulations require diesel vehicle manufacturers to implement inducement strategies that progressively restrict vehicle performance when emissions systems are not functioning — this is what derate enforces. The countdown typically begins with a dashboard warning, then progresses through power reduction stages over a set mileage or time period if the fault is not repaired. The YOUCANIC UCAN-II can read the specific aftertreatment DTCs causing derate, monitor SCR efficiency and DPF soot loading, and after repairs, perform the necessary resets to clear the derate condition and restore full power.