The on-board diagnostic (OBD) generation two (II) system, equipped on all vehicles manufactured from model year 1996 to the present, performs monitoring of emission control systems continuously and non-continuously. Fuel control, engine misfires, and the comprehensive component monitor (which tests all engine and transmission sensors [inputs] and actuators [outputs] for electrical faults) are monitored continuously. The computer will set a hard code and command the MIL to illuminate upon the first fault detection of a continuous monitor. In the event of an engine misfire severe enough to damage the catalytic converter, the computer will command the MIL to flash. The catalyst, exhaust gas re-circulation, fuel evaporative control, oxygen sensor, heated oxygen sensor, secondary air injection, and diesel exhaust after-treatment systems are monitored non-continuously. These non-continuous monitors are tested once per drive-cycle when prerequisite operating conditions are met. Depending on the manufacturer, some systems like the fuel evaporative system may be tested even when the engine is turned off. If a fault is detected during a drive-cycle of a non-continuous monitor, the computer will store a pending code. If the same fault is detected during the second consecutive drive-cycle, the computer will set a hard code, record a freeze frame data of various inputs at the time the fault was detected, and will command MIL to illuminate.
Once lit, the MIL will remain on until the vehicle has completed three consecutive drive-cycles in which the fault is not detected. The MIL will also turn OFF when stored diagnostic trouble codes are cleared by a scan tool or when power to the computer is lost. However, the MIL will only continue to remain OFF if the fault is successfully repaired.