Global OBDII Monitors: How They Work
Let?s start first with understanding the role of the ECM (Engine Control Module). The ECM has one primary function. It is responsible for managing the various vehicle systems related to emissions performance. In keeping with this responsibility, the ECM performs tests on these systems to insure their proper performance. If a problem is found, the ECM can then warn the driver by illuminating the MIL (Malfunction Indicator Lamp). The result is a powertrain diagnostic trouble code, or ?P? code.
These tests fall into three basic categories: electrical tests, rationality tests, and functional tests. Electrical tests are tests for open circuits, and short circuits to power or ground. Rationality tests are tests that compare ECM inputs and to see if the inputs make sense. An example would be a comparison of MAF (Mass Airflow sensor) to the TPS (Throttle Position sensor). If the MAF is reporting an airflow rate that the ECM knows is contrary to how much air should be flowing at the reported throttle opening, it will assume that one of the two is reporting incorrectly. Functional tests are performance tests of entire systems or individual components. These tests can be done passively or intrusively by the ECM. A passive test is one where the ECM simply watches while the system or component functions normally, looking for verification of that function from another input. An intrusive test is one where the ECM actually changes the normal state of a system or component and verifies that change with an independent input.
Now, with all that being said, what is a monitor?
A monitor is a grouping of these tests into an individual system function. Any particular monitor can be comprised of just one test, or of several tests. The tests themselves often require that certain conditions be present before the tests will be performed, or ?run?. These conditions are called ?enabling criteria?. Enabling criteria involves engine/vehicle operation as well as the health of other systems the ECM may need to perform the test.
Vehicle/engine operating conditions can include engine temperature, time since start-up, vehicle speed, and whether or not the vehicle is accelerating or decelerating, just to name a few. These operating conditions are called ?drive cycles?, or ?trips? and are unique to each monitor. Often, a general ?drive cycle? is described, designed to meet the needs of all the vehicle?s individual monitors when followed. This allows you to perform a specific test drive that will allow the ECM to recheck any repairs that you?ve made.
Because some tests rely on information from a component that may be related to a separate monitor, the ECM will suspend testing if a fault is first found in that component. When reviewing the individual code enabling criteria in your service information system, you will often find a listing of DTCs (Diagnostic Trouble Codes) that will cause this suspension. One example is the Catalytic Converter Monitor. Most of these monitors rely on input from the oxygen sensors, and if a DTC is already recorded for a problem with one of the sensors, the catalytic converter monitor will be suspended until this fault is repaired. Being suspicious of what the affected oxygen sensor is telling it, the ECM can?t correctly judge the health of the converter.
Once the enabling criteria of an individual monitor are met, the ECM will perform the test(s) associated with that monitor. If a fault is found, and it is one that will have an immediate impact on emissions, the ECM will set the DTC, turn on the MIL and record the operating conditions present at the time of failure in Freeze Frame. These are the ?1-trip? codes. If the fault is not deemed as an immediate threat, the ECM will record the DTC as ?pending? and wait to see if the same fault occurs on the next consecutive test before maturing the code and illuminating the MIL. These are the ?2-trip? codes. Codes related to the Misfire monitor and Fuel monitor have to repeat under the same load/rpm conditions as the first failure in order to mature, and this information is stored in what is often referred to as a ?Similar Conditions Window?.
If the second test passes on a ?2-trip? code, the code will be erased from ?pending?. If a matured code (one that has turned on the MIL) is retested, and passes, three times in succession, the MIL will turn off. However, the code will remain in memory until a specific number of ?warm up? cycles are completed without another failure.
Monitors are further identified as being either ?continuous? or ?non-continuous?. Continuous monitors are the Misfire monitor, the Fuel monitor and the Comprehensive Component monitor. Continuous monitors run as the name implies?continuously. Every time the enabling criteria are met, from key on to key off, these tests will be performed. Non-continuous monitors run only once after the enabling criteria are met, regardless of how long the vehicle is operated or how often the enabling criteria are met during any one key on/key off cycle. All the other monitors listed on your scan tool fall into this category.
Ready Or Not: More On Global OBDII Monitors
Mode $01 lists something called ?Readiness Monitors?. A more accurate description would be ?Monitor Status?, and is a listing of whether or not the individual monitors have completed their testing. You should see all monitors listed as ?ready? or ?complete? when you check this screen. Monitors listed as ?not available? or ?not supported? simply means that these monitors are not applicable to the vehicle you are testing.
Monitors listed as ?not ready? or ?not complete? means that the monitor has not finished testing since the last time it was cleared. Monitor status is reset to ?not ready? (and all related testing information that may have been stored in Mode $06 is lost) whenever the codes or Keep Alive memory is cleared. Status is also reset whenever battery power is lost, whether due to a normal battery changeover or a problem in the wiring to the ECM. If cleared for whatever reason, a fault detected in a completed monitor may prevent the ?not ready? monitors from running and changing status.
Without being reset, the monitor status will changeover to ?ready? after the monitor?s first completion. Monitors may list as ?ready? even if the ECM detected a failure during the testing?so it doesn?t mean that no problem was found. It also means that we don?t know the last time the monitor completed successfully. Back to our oxygen sensor failure and it?s affect on the catalytic converter monitor. The monitors may all read ?ready? even after the sensor failure was recorded, but since the converter monitor uses the sensors for its tests, this monitor will be temporarily suspended. This monitor is a non-continuous monitor, so any test results listed in Mode $06 are also suspect.
Monitor status only tells us whether or not the tests have EVER been run since the vehicle first left the dealer?s lot. For this reason, it is good practice when repairing any powertrain DTC to operate the vehicle until all the monitors have rerun and switched to ?ready?, and then recheck for any other codes that may have been recorded. Continuous monitors will list the first recorded failure of any ?2-trip? codes in Mode $07 (Test Results For Continuous Monitors), making this an easy check. To double check the test results for non-continuous monitors, though, you?ll need to check Mode $06 (Test Results For Non-continuous Monitors). Personally, and because the Evaporative Emissions Monitor often has the most involved drive cycle requirement, I don?t worry if that is the only monitor that hasn?t completed when verifying my repair. That is, unless I?m dealing with a code related to that system.
Different than Mode $07, Mode $06 is a listing by monitor of the actual, individual test results. Often, these test results are unidentified in your scan tool and list results that, at first glance, mean nothing to you as a tech. Today, most service information systems provide more detail on what these tests are and how to translate them. Some scan tools now do this for you, and save you the trouble. But when verifying a repair, your primary interest is in any of these individual tests that show as ?failed?, or have test results that are close to exceeding the maximum or minimum limits as shown on the scan tool screen. If your first scan of the results shows all ?passed? with results well within limits, you can be pretty sure there are no immediate problems you need to address. If not, then it?s time to dive into the service information to determine what system monitor is being tested and how.
That?s an overview of Global OBDII monitors. I hope you found it helpful.