Malfunction Indicator Lamp: Description and Operation

The Malfunction Indicator Lamp (MIL) looks the same as the MIL you may already be familiar with, such as the Service Engine Soon or Check Engine lamp. The OBD II regulations require that the MIL illuminate according to a strict set of guidelines. Under OBD II the MIL is illuminated when the PCM detects a malfunction that will impact the vehicle emissions.

The MIL is controlled by the PCM. The MIL will be illuminated if an emissions-related diagnostic test indicates a malfunction has occurred. The MIL will remain illuminated until the system or the component passes the same test for 3 consecutive trips.

A vehicle that is experiencing a misfire malfunction that may cause damage to the 3-way Catalytic Converter (TWC) will flash the MIL once per second. The MIL will continue to flash once per second until the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst. The MIL will stop flashing and remain ON steady once the vehicle is outside the speed and the load conditions that may cause damage to the TWC catalyst.