ECMs are good at telling you what to fix.
An OBDII Engine Control Module (ECM) has one primary function: keeping vehicle emissions in line. It's gotten so good at it that many states now rely on what the ECM has to say when certifying vehicles in meeting state emissions requirements. These inspections check, in part, the status of the onboard readiness monitors.
But what exactly is a monitor?OBDII Requirements
OBDII standards require that the ECM be able to verify the functional integrity of its systems in order to ensure that vehicle emissions remain within guidelines. The ECM does so by performing internal tests of these systems and/or their components. Every powertrain Diagnostic Trouble Code (DTC) is related to one of these tests.
There are three basic types of tests the ECM can perform. The first is a test of circuit integrity — electrical tests for open circuits, shorts to power or to ground. The second type of test is the rationality test. In a rationality test, the ECM compares the information from one sensor to another to see if the two are compatible with each other. An example would be the comparison of the Throttle Position Sensor (TPS) signal to the Mass Airflow sensor (MAF) signal. The ECM has an idea of how much air should be flowing with any given throttle opening, so if the MAF signal tries to tell the ECM that the airflow is more or less than expected, the ECM will record the discrepancy.The third type of test is the functional test, further categorized as passive or intrusive. A functional test is one where the ECM is testing the actual performance of a given component or system. If the test is conducted under normal operating conditions, with no direct involvement of the ECM, it is a passive test. If the ECM takes an active role in altering the normal operating conditions, it is called an intrusive test.
Evaporative emissions (EVAP) systems offer good examples of both types of functional tests. A natural vacuum leak detection system is a passive system, observing and reporting on the normal operation of the components. However, when the ECM commands the vent closed and applies engine vacuum to the system via the purge solenoid, then monitors the resulting vacuum decay, it is conducting an intrusive test.
Monitors are groupings of one or more of these tests into subcategories. While the subcategories are standard, the individual manufacturer establishes the tests included in each monitor. You'll find a listing of these monitors in Mode $01 on your scan tool when using it in Global OBDII mode. Checking the current status of the monitors should be one of your first steps when diagnosing powertrain-related codes.
Ready or Not?
When you view monitor status in Mode $01, you'll see one of three conditions: Ready (or Complete), Not Ready (or Incomplete) or Not Supported (or Not Applicable).
Not Supported means just that. A monitor with this term listed beside it is one not used by the vehicle you are testing.
Ready means that the ECM has completed all the tests associated with that monitor since the status was last reset. This only happens if the codes are cleared or battery power is lost to the ECM. It does not mean that the monitor tests all passed and no fault was found. In addition, you can't tell from the status screen when the last time the monitor performed all of its tests.
If, from the very first time the monitors completed their testing when the vehicle was driven new off of the lot, the battery was never changed or the codes never cleared, the monitors will list as Ready. Even if a subsequent run uncovered a problem and set the MIL, the status will remain unchanged. You can't tell from this screen when the monitor last ran successfully simply from this status.Not Ready means that the ECM has not completed all of the monitor's tests. This can be due to a few different reasons. Each monitor has its own unique enabling requirements before the ECM can run its tests. These enabling criteria can include how the vehicle is driven, how much fuel is in the tank or what the engine coolant temperature is, just to name a few. If all of the conditions needed to run all of the monitor's tests have not been met before the driver shut the key off, the monitor will remain in a Not Ready status.
Another enabling criterion that can keep a monitor from completing its testing is the pass or fail of a test used by another monitor. Here's an example of how one monitor can affect another. Most catalytic converter monitors rely on the input of the engine's oxygen sensors. These sensors are tested in a separate monitor. If the ECM detects a fault in one of the sensors, it will suspend the completion of the converter testing until the sensor problem is fixed, even if the sensor monitor is listed as Ready. Be sure to look for pending codes that might be preventing another monitor from running.
Drive Cycles and Trips
This is a good place to throw in a few more terms: drive cycle and trip. A drive cycle is completed when all of the enabling criteria have been met and the ECM has completed all of the monitor's tests. Trip means the same thing. Each monitor has its own defined drive cycle, but your service information may list only a generic drive cycle designed to meet the conditions needed by all of the monitors.Some drive cycles are completed as soon as the engine is started and shut off while others are more involved, with the Evaporative Emissions Monitor (EVAP Monitor) usually requiring the most involved criteria. Regardless of the criteria, once the key is shut off, the drive cycle stops and has to restart. If the testing is incomplete, the ECM may store the results it has so far and pick up again on the next key start. The drive cycle itself, however, is not complete until the monitor associated with it is complete.
Why know this? Have you ever heard of the terms one-trip or two-trip codes? If the ECM detects a problem while conducting a monitor's tests, it will fall back on its programming to see if the fault is one that will cause an immediate, detrimental impact on emissions. These faults need attention soon, and the ECM will turn on the MIL the first time the fault occurs, the first trip.
If not, the ECM will record the fault and look to see if the same problem occurs on the next consecutive test, the second trip. If it does, then the ECM will turn on the MIL. If not, the ECM will erase the first record and forget it ever happened.Continuous or Non Continuous
The ECM will repeatedly perform some monitors' tests during any given drive cycle. These monitors are the continuous monitors, and include the Misfire Monitor, the Fuel Monitor and the Comprehensive Component Monitor. Because the Misfire Monitor and the Fuel Monitor are continuous and being tested over every conceivable load and rpm range, this makes Freeze Frame information invaluable in duplicating the driving conditions that were in place when the fault was detected.Keep in mind that Freeze Frame information is stored when the code matures and turns on the MIL light, and might reflect operating conditions just after the problem was discovered. The driver may have been accelerating or decelerating at the time, so test drive for these problems on both sides of the Freeze Frame record.
On all the other monitors, the ECM will perform their tests only once during any given key on/key off cycle, providing all the conditions needed by the monitor were met. This means you could take a non-stop drive from Tampa to New York, have these monitors done before you crossed the Florida state line and never have the same tests repeated until you reached your destination.
If the ECM finds a two-trip fault, you won't know about it until you've headed back, and only if the same problem occurred on the next consecutive run. What does this mean to you? Let's say you think you've fixed an EVAP problem. EVAP monitors require the most involved drive cycles, and most EVAP codes are two-trip codes. If your customer mainly drives back and forth to his office two miles from home, it may take the ECM weeks to complete two consecutive runs of the EVAP monitor to verify the fix or turn the MIL back on if you missed.
No doubt, OBDII has made diagnosing powertrain faults a lot easier. By understanding how the ECM tests and what criteria is required, you can often mimic these tests in the bay. This allows you not only to identify the fault, but verify your repair, without driving across country!
