Predicting Failures

Predicting Failures

spect that at one time or another, we’ve all had a customer return with a check engine light on with a new fault code that wasn’t there last time they were in. The events probably went something like this:

Customer: “You couldn’t see that when you looked at it last time?”

Shop: “No ma’am, there’s no way to predict that…It isn’t broken until it’s broken… It’s like a light bulb… We don’t have a crystal ball…”

That’s about how it goes, right? The customer thinks we should have some kind of crystal ball to look into the future with and predict failures before they occur. Well, what if I told you that we sort of do? Well, to a certain extent anyway. What we can look at are the test results from mode $06.

What Are “Modes”
The PCM has several diagnostic modes. Depending on your scan tool and your use of it, you may get to see some or all of these modes. Also, depending on your scan tool, you may be accessing the various modes in the PCM without referring to them as “modes.”

  • Mode $01 is powertrain scan data PIDs

  • Mode $02 is freeze-frame data

  • Mode $03 is stored DTCs

  • Mode $04 is the request for clearing diagnostic info

  • Mode $05 is the O2 sensor monitor test results

  • Mode $06 is on-board test results for non-continuous monitors

  • Mode $07 is on-board test results for continuous monitors and pending codes

  • Mode $08 is request control of on-board system tests such as evap leak testing

  • Mode $09 is vehicle information and in-performance vehicle tracking

As you can see by this list, you’ve been accessing some or all of these various modes for quite some time already. Modes $01 through $09 are part of the generic protocol as dictated by the EPA. In order for a scan tool to be fully OBD II compliant, it has to access all of these modes, though some do so poorly and some others manage to squeak by without doing it at all.

For this article, we are going to be looking at test results from Ford’s mode $06. In mode $06, you’ll see test results from the PCM’s monitors. A “monitor” in the PCM is simply a piece of the PCM’s software that is designed to test certain components. These components may be tested at all times during a drive cycle (continuous), or they may be tested only once per drive cycle after certain criteria are met (non-continuous). Non-continuous monitors include, but are not limited to:

  • O2 Amplitude/Reaction Time

  • O2 Heater Circuit

  • EGR Flow

  • Evap Leak Detection

  • Cat Efficiency

  • Secondary Air Monitor

  • Thermostat Monitor

    Continuous monitors include, but are not limited to:

  • Misfire Monitor (some makes display as PID in mode $01, Ford displays as TID in mode $06)

  • Fuel System Monitor (continuous O2 monitoring of air/fuel ratio)

  • Comprehensive Component Monitoring (circuit integrity)

By the end of this article I hope to accomplish four things for you:

  1. Increase the understanding of the “thought” process that the PCM uses to set some codes.

  2. Show how tests results from one test may reveal why another test failed, and possibly have set a misleading fault code.

  3. Show how these test results can aid in no-code driveability diagnostics.

  4. Show how some fault codes can be caught and fixed before they actually set a code and MIL light.

All of the exampled tests here will be from Ford vehicles. The basic ideas and principles can be applied to other makes, but the specific technical data will differ when compared to other makes. Some makes completely separate these test results, and some may toss them all together in one section, like Ford. Some vehicles, like in the Ford screen shots on the following pages, provide an abundance of information. However, mode $6 on some other makes may provide next to nothing.

Also, the data shown here may be displayed differently from your scan tool unless you happen to be using the exact same tool. Some scan tools show these test results in a raw, non-translated form that requires mathematics applied to it to be useful. Some other tools only show a pass/fail status without showing actual values. Some tools group tests together on one screen and some make you access one test per screen.

The two tools used here translate the data into ready-to-apply values. One is a Ford OE tool and the other is an aftermarket tool. If you happen to own a scan tool that doesn’t translate this test data for you, information needed to do so can be found on the web at the manufacturers’ websites. Some good information also can be found at TID, CID and MID

TID, CID and MID are Huey, Dewey and Louie’s cousins. Well not really, but TID, CID and MID are related to each other. These terms are how the different data is labeled and identified in mode $06. TID stands for Test ID, CID stands for Component ID and MID stands for Monitor ID.

TID is the type of test that is being performed by a monitor. For example, under the O2 monitor you will see several TIDs with ID numbers like 1, 2 and 3 (or $01, $02 and $03). Although these tests are performed by the O2 monitor and are testing the O2 sensors, these are each different tests for the O2s. One TID may be for the sensor’s reaction time (displayed as amplitude after a given time frame), where as another may be for the O2 sensor’s switch point. Sometimes the TID is not really a test at all. Sometimes it is simply to show a fixed reference value that the EPA requires the manufacturer to display, such is the case with the O2 sensor switch point voltage.

CID is identification for the part that is being tested. For example, the O2 monitor is running multiple tests on multiple O2 sensors. So, the test results are broken down to display each O2 sensor (or component) and labeled with its own component ID number.

MID takes over the duty of CID on CAN vehicles.

Layered Faults, Misleading Faults and Codeless Faults
As touched on earlier, continuous monitors run all of the time and non-continuous monitors only run once per drive cycle after certain conditions are met. Monitors (especially non-continuous monitors) often require passing results from another monitor before they can run. It is important for a technician to understand this because certain fault codes can block certain monitors from running, which in turn can mean that a problem in one system can actually “hide” under a fault code from another system.

The catalyst monitor, for example, requires a passing status from the misfire monitor and the O2 monitor. That’s because the catalyst monitor uses the O2s to measure the catalyst’s performance, and also a misfire will produce excessive HCs and oxygen in the exhaust. You can’t expect a catalyst to perform within limits if it’s not being “fed” its proper “diet” of oxygen and unspent fuel, and you can’t expect an accurate measurement of its performance if the parts that you’re using to measure it with (the O2s) are proven faulty. So, when a misfire or O2 code is set, the PCM stops running the cat monitor.

Let’s say a vehicle comes into your shop with a customer complaint of a check engine light and running rough. The technician finds one or more cylinders misfiring and fixes the problem. As we all know, misfires can damage a cat, especially one that has been in service for some time already. Add that to the fact that the PCM stopped testing the cat(s), and it’s very possible the vehicle will darken your door step with a check engine light on once again for cat efficiency. If the original fault is an O2 sensor, then you might not even have a physical symptom to point to (like the misfire) to help explain to the customer that this is a new problem the second time. At the very least, it is important to know this can happen for proper documentation and customer communications on the first visit. It is much easier to explain something like this upfront than apologize for it later.

It’s also important to understand how these monitors arrive at a “pass” or “fail” status. Every monitor has margins of acceptance. In the PCM’s eyes, for the purpose of when to set a fault code, it is OK for an engine to misfire a little bit, or an O2 sensor to be a “little bit bad” or an EGR system to be a little bit restricted or an EVAP system to leak just a little bit. That’s because for every test that a monitor runs, there is a line drawn in the sand (so to speak) that the test results are not allowed to cross. It’s not until that line is crossed that a fault code is set. That line is called the “threshold limit.” The PCM compares the test result, referred to as the “value” against the threshold. The value displayed is the result of the last time the monitor ran/updated. Consider this first screen shot (Figure 1).

As you can see, cylinder #7 was misfiring. This vehicle never set a P0307. Why? Because even though this was enough of a misfire for the customer to feel and complain about, it wasn’t enough to break through that 21.5% threshold. This was considered to be type A (cat damaging) misfire, but it occurred at a rate of less than 5% of the cylinder events tested. So no code was set.

As you know, one condition in one system can effect another system. Let’s say a vehicle has a bad O2 sensor. Let’s also say that it’s not bad enough to cross the threshold to set an O2 slow response code. What might that mean to the catalyst efficiency testing? Consider this next screen shot (Figure 2).

This vehicle set a fault code P0430 (cat efficiency low bank 2). As you can see by CID21 under TID10 (highlighted yellow for ease of locating), the P0430 was set because the rear to front O2 switch ratio exceeded the maximum limit (which is what the code already means). In other words, with the given values shown, the bank 2 downstream O2 sensor crossed the switch point of 0.45 volts (listed in CIDs1 and 2 of TID3) at a rate of 76% of the number of times the upstream bank 2 crossed the 0.45 volt line.

Keep in mind that this is not a tracking of by how much each sensor crossed the 0.45 volt mark. Imagine what might happen if the upstream O2 were faulty and biased lean. If the O2 is biased lean, the PCM will raise the fuel trims within its limits to raise the voltage on the faulty O2.

Such a condition can lower the oxygen in the cat. With less oxygen in the cat, its actual efficiency can be lowered. That can increase the activity on the downstream O2. If that isn’t enough, then factor in some real-world conditions. For example, rain, snow and otherwise cold weather can reduce the cat’s temperature, which can also affect the cat’s actual performance and increase the downstream sensor’s activity. Perhaps the upstream O2 will have failed to cycle past the 0.45-volt switch point a few times whereas the downstream did cross the switch point, that could easily shift the ratios.

It’s quite possible that the P0430 in the above example was actually set by a faulty upstream O2 sensor. At the very least, the technician can see that the bank 2 upstream O2 sensor should be replaced during the repairs, even without an O2 fault code present. Or better yet, the O2 sensor should be replaced first and then driven so that the cat monitor can run and evaluate the new readings on the cat before condemning it. There are real-world cases sited online at websites such as and of where new catalysts were installed to fix a P0430, only to have the vehicle return with a P0430 again due to a weak O2 upstream O2 sensor.

After repairs are made, a technician should drive the vehicle through at least enough drive cycles to complete enough of the monitors to verify the fix. When it comes to setting monitors, there are two types of drive cycles. A “drive cycle” is when enough of the needed conditions are met so that one or some of the monitors are given a chance to run. An OBD II drive cycle is where all of the needed conditions are met so that all of the monitors can run. A P1000 (OBD II readiness checks not complete) will remain in the PCM’s memory until an OBD II drive cycle is completed. The manufacturer of the vehicle doesn’t require that the technician do this, but your local emissions laws might. As you can see by comparison of the next two screen shots (Figure 3) (Figure 4), a drive cycle was performed after a repair, but not an OBD II drive cycle.

The ’97 Explorer in Figure 3 had a fuel economy concern with no codes. Look at CIDs $11 and $21 under TID $01, you can see that both upstream O2 sensors were just barely above passing results. The B1S1 O2 is showing 0.557 volts and the B2S1 O2 is showing 0.691 volts, while the threshold for setting a code is 0.502 volts. The O2 sensors were replaced and the KAM was reset. The vehicle was then driven. The new test results for the O2 sensors are now seen in the next screen shot (Figure 4).

Not only can you see the new O2 sensors’ test results (0.760V and 0.754V), you can also see that some of the monitors had not run (as indicated by a red colored “not complete”). The monitors that have not run are displaying default values in their test sections. Although these default values are well past the threshold limits, this will not set a fault code for these systems.

Notice that some monitors are not used at all by this PCM (not supported). That is because this vehicle does not use those systems. The readiness statuses for the monitors are actually found in Mode $01. This particular scan tool borrows this information from mode $01 and displays it on the same page as the test results for user friendly reasons. The readiness status is not the only thing borrowed from mode $01 here, notice the battery voltage as well in the upper left region of the screen.

Here’s something interesting. Compare CIDs $07 and $08 under TID $51 between the two captures. Notice how cylinders 7 and 8 have no misfires stored in the first capture, yet show over 98% misfires in the second capture. How can that be? Well, first of all those cylinders are missing. When I say “missing,” I mean they really are missing; it’s a V6. It’s not unusual to find TIDs and CIDs available for up to eight cylinders on an older Ford product, even though it may only be a six-cylinder vehicle. It’s up to the programming to determine how to handle those phantom cylinders. Sometimes those cylinders will show an astronomical number of misfires, and sometimes they will show none. What about CID $21 under TID $10 for the bank 2 cat efficiency? Same thing. This particular model only had one downstream O2 located after a Y pipe. Although the TID and CID are displayed, this PCM is programmed to not use it.

Over the years, the information found in these monitor’s test results have grown from nearly nonexistent, (see Figure 5) to overflowing with valuable diagnostic information on CAN systems (see Figures 6) Figure 7and Figure 8 ).

On CAN systems, the term MID is now used. The MID number not only indicates what monitor is referenced, but also what component is referenced and, therefore, also replaced the need for CID. As you also can see, much more in-depth information is available on the CAN systems. Notice how MIDs A1 through A9 have much more misfire information added, including the number of misfire events that occurred over the last 10 drive cycles.

As you can see, being able to access the monitor test results in mode $06 can be very useful for:

  • Diagnosing some driveability issues without fault codes present;

  • Determining the validity some fault codes;

  • Helping to determine the root cause of some fault codes; and

  • Predicting future fault codes.

A technician may use these modes to predict and validate:

  • O2 slow response codes

  • O2 heater circuit codes

  • EGR low flow codes

  • EGR excessive flow codes

  • EVAP leak codes

  • Misfire codes

    The test data in mode $06 is probably the closest we have to a crystal ball. It can benefit any technician who performs driveability work to become proficient in reviewing these test results to apply to them something that the PCM can’t – “reason.”

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