OBD II Software and Diagnostics
Now that we have looked at the components and some component functions of the systems controlled by the OBD II PCM, let’s turn our attention to how the OBD II software and diagnostics ensure that the systems are functioning correctly during all driving conditions.
One of the PCM's most important jobs is to make sure all the engine's sensors are working properly and that the engine isn't polluting. This is accomplished through self diagnostic software internal to the PCM. Even with pre-OBD II systems a certain amount of self-diagnostic capability was required to detect problems that might upset the smooth operation of the engine. On these systems diagnostics were rudimentary; if a sensor’s electric circuit went open or shorted, the gross failure would set a DTC fault code and illuminate the check engine light. So, a sensor that was not working correctly but had not failed completely would not set a code or illuminate the check engine light even if it was creating performance issues or excessive tail pipe emissions.
OBD II builds on this foundation and adds several new layers of diagnostics driven by controlling any chance of increased emissions. OBD II checks most engine functions while the vehicle is being driven. It is designed to detect almost any problem that can cause emissions to exceed the federal limit by 1.5 times. Now the PCM’s software can detect engine misfires, how well the catalytic converter is performing and whether the vehicle is leaking fuel vapors into the atmosphere. Almost any vehicle system problem that causes emissions to exceed the federal limit will cause the Check Engine light to be illuminated, even if there is no noticeable drivability problem.
OBD II diagnostics continue to use the Check Engine lamp to alert the driver when a fault occurs, and it still stores DTC fault codes that correspond to specific kinds of problems. What’s new is a feature to track problems as they happen in real time, capturing a snapshot of system parameters and readings when the problem occurred. This feature is called freeze frame data and it’s a great tool for helping repair professionals isolate the root cause of the problem that illuminated the check engine light.
Part of OBD II enhanced software programming is a series of complex diagnostic routines that run during specific operating conditions when the vehicle is being driven. These routines, called readiness monitors, exercise the various emission systems checking for expected results. After the OBD II system has run a monitor the results are compared with a set of expected results. If the results are the same the monitor passes and if the results are not the same the monitor fails which sets the stage for triggering a DTC fault code and illuminating the check engine light. The OBD II system monitors some functions every time the engine is running. These routines are called continuous monitors and they constantly execute while the vehicle is operating. The continuous monitors check for real time failures that could result in elevated emissions. There are three continuous monitor routines:
- The comprehensive component monitor continually checks readings from all the sensors and controlled
components to the PCM. - The misfire monitor detects ignition and fuel related misfires that may cause emissions to increase.
- The fuel system monitor detects changes in fuel mixture control that may cause emissions to increase.
Other OBD II monitors are only active during certain times. These are called non-continuous monitors and they execute only under specific conditions and driving behavior. Non-continuous monitors include the catalytic converter efficiency monitor, the EVAP system monitor that detects fuel vapor leaks in the fuel system, the EGR system monitor and the oxygen sensor monitors. The vehicle operating conditions required for the non-continuous monitors to execute are referred to as the enabling criteria. Some monitors require the vehicle to follow a prescribed drive cycle routine as part of the enabling criteria. Drive cycles vary from vehicle to vehicle and for each vehicle system being tested. Some non-continuous monitors require two drive cycles to complete testing due to the need for cool down and warm up periods in between. Depending on the non-continuous monitor, variables such as speed, engine and ambient temperatures, load, fuel level, as well as other conditions must be met for the monitor to perform its test.
The OBD II system communicates the results of its diagnostic monitoring efforts through two reporting mechanisms: diagnostic trouble codes (DTC) and readiness monitor status. If a particular monitor fails, the PCM stores a DTC fault code, captures relevant freeze frame data and illuminates the check engine light. Through readiness monitor status the OBD II system indicates which non-continuous monitors have met the right enabling criteria and run since the last time the DTC fault codes were cleared from the PCM’s memory. If a monitor has run, its readiness status will be Ready. If conditions to run a particular monitor have not been met yet, the readiness status will be Not-Ready.
A vehicle will fail the (NYVIP2) OBD II inspection if enough monitors are reported as Not-Ready. Generally, OBD II vehicles from model year 1996 to model year 2000, inclusive, will fail if more than 2 monitors are set as Not-Ready; while model year 2001 and newer vehicles will fail when more than 1 monitor is reported as Not-Ready.
For more comprehensive information on drive cycles, monitors, DTC codes and the check engine light, click on the appropriately labeled tab.