On board diagnostics include on-board monitoring strategies capable of detecting emission-related malfunctions while driving the vehicles on road.
The Malfunction indicator light (MIL) which is present on the panel will illuminate when a malfunction is detected. But certain conditions should be satisfied before MIL illuminates, i.e. when OBD determines the malfunction it generates a Diagnostic trouble code (DTC) or fault code and stores it in the ECU. If the malfunction is detected again before the next driving cycle in which the suspected system is monitored, the MIL illuminates continuously, and a confirmed fault code is generated from engine data. If in case the malfunction is not detected till the end of the driving cycle, the stored fault code will be erased. If the malfunction is not detected in the next driving cycle, the MIL can be turned off but the trouble code is still stored at least for 40 engine warm-up cycles.
To measure the frequency of monitoring, an in-use monitor performance ratio is used. It defines the operational frequency of several monitoring strategies relative to specific drive cycle conditions.
It can be defined as the “Number of monitoring events/Number of driving events”, It means the ratio of the number of times the conditions have existed under which a monitor should detect a malfunction to the number of driving cycles applicable to that monitor. Each component such as fuel system, cooling system, exhaust gas sensors, other emission control system monitoring, etc. requires its own ratio. This IUMPR is measured by incrementing a counter when the monitoring conditions operate on-road and also tracking when the vehicle driving cycle meets specified milestones.
To view IUMPR value you can use diagnostics tools like Silver Scan tool or DiagRA D. Each software comes with all the functions you need to carry out complete OBD diagnostics and archive them in a legally secure manner. Measured values are displayed numerically and graphically. The below figure shows the IUMPR measured in the silver scan tool.