OBD is a government-mandated standard that requires engines to actively monitor and test emissions-related components and systems to detect malfunctions that adversely affect emissions. An engine's OBD system monitors nearly every component that can affect the emission control system. If the OBD system detects a malfunction that could cause an increase in exhaust emission levels, the OBD system illuminates the Malfunction Indicator Lamp (MIL) on the vehicle instrument panel to alert the operator that the engine is in need of repair. The level of OBD monitoring required can vary, depending on factors such as Gross Vehicle Weight Rating (GVWR), model year, certification level, and applicable government regulations.