OBD-II Mode 6: Making it part of your diagnostic arsenal

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OBD-II Mode 6: Making it part of your diagnostic arsenal

Craig Truglia is an ASE A6, A8,and L1 certified technician who presently works as a service writer for Patterson Auto Body, a repair facility in Patterson, N.Y. A former shop owner and editor of several automotive repair magazines, Truglia combines his Columbia University education with the real-world experience he sees daily in the automotive repair field. Technicians Truglia and Fred Byron took part in diagnosing the different vehicles in this article.

Mode 6 has been around for many years now. Technicians have often not made much use of it because they find it confusing and often not that informative.

Yet, there are a school of technicians and instructors who think that Mode 6 is the best thing since sliced bread. They boast that it helps quickly diagnose cars and gives us a window into what the software engineers were thinking when they made the criteria for what turns on the Check Engine Light.

For our purposes here, we are going to cover how to read Mode 6 and in the real world, how it helps us diagnose a vehicle. We leave it to you to decide if it is a worthwhile part of your diagnostic arsenal.

Real-world Mode 6

To begin understanding Mode 6, it helps to know exactly what it is: Mode 6 is the criteria that software engineers use to allow us to see the information that causes the Check Engine light to illuminate on a specific vehicle. This means that every single vehicle has different criteria. It is also worth noting that not every vehicle has it, as it only started finding its way into vehicles beginning in the early to mid 2000s.

To help us understand Mode 6, let’s look at a photo capture on a known good 2004 Toyota Prius using the ATS EScan. As we can see, Mode 6 gives us six different sets of data (see Figure 1). The ATS EScan is the only scan tool that not only decodes Mode 6 information, but allows the user to see it all at once. Further, it lets the technicians pick specific Mode 6 tests and update the test results in real-time.

The OBD Monitor ID (OBD MID) tells us what is being tested. The Test ID (TID) tells us what specific test is being run. The Test Value is the actual result of the test. The Min Limit is the minimal allowable test result that will not set a DTC. The Max Limit is the maximum allowable test result that will not set a DTC. So, anything below the Min or above the Max will set a DTC. Units, simply, tells us what unit of measurement is used for the test.

On the Toyota in Figure 1, we can see the Toyota passes all of its tests with flying colors. With the EScan, the Mode 6 Test Values that come close to failing are highlighted in yellow and when they fail, they are red. These are only “ballpark” estimates if something is close to failing, because only the OEM software engineer really knows how close is “real close” for each individual vehicle. It is best only to take a Test Value very seriously if it is above the Max Limit or below the Min Limit.

On most scan tools, you are not going to be able to decode Mode 6. This means you won’t have the TID translated into English for you. If we look at Figure 2, we can see everything, but no TID translations. This is what Mode 6 looks like without the TIDs translated by the EScan.

Keep in mind, most scan tools will neither translate TIDs nor list all of the Mode 6 tests at once. Instead, you will be forced to go into each test in order to get an idea of what the result of each one is. This is so time-consuming, most technicians will just ignore Mode 6 all together. For this reason, if you are serious about wanting to exploit the functionality of Mode 6, it is wise to get a scan tool that makes it quick and usable.

So, when using a regular scan tool, what should you do if you do not know what the TID is? Most of the time you can simply guess. If a MID fails for an oxygen sensor heater, then you know there is some sort of problem with the circuit. Further, if the OBD MID is Cylinder Misfire and you are still counting any misfires since you last deleted the DTCs, it is obvious you did not fix the misfire in that cylinder. Lastly, if you are diagnosing a P0420 and you think you fixed it with a new rear oxygen sensor, you do not need to know what the TID really says in order to know that a low number on Catalyst Monitor Bank 1 is good.

TIDs become most useful when doing an EVAP diagnostic, as they really tell you which part of the EVAP test for the EVAP Monitor failed. But, even then it might not be helpful, because the software engineers do not always tell you exactly what failure will affect the results of which MID.

What can we learn from this? Use your common sense. You can probably figure out what the Mode 6 tells you is failing by looking at the MID alone.

Outside of the misfire counter MID, Mode 6 alone does not tell you everything you need to know anyway, so you will need to use data stream (PIDs) in order to further diagnose the vehicle.


Using Mode 6 in the real world

A 2007 Subaru Forester 2.5L (normally aspirated) came into the shop with a P0420 and a “low power” complaint plus two misfire DTCs (see Figure 3).

Only a few thousand miles before the vehicle had a complete tune-up (spark plugs, ignition wires and an ignition coil). The low power complaint was intermittent, so we knew it was not an issue with exhaust backpressure. Fuel trims were good and these could be confirmed with an exhaust analyzer (see Figure 4). A Lambda of 1.01 is essentially perfect and proves to us the LTFT on the scan tool is an accurate reading. However, the rear oxygen sensor was going up and down, so this reflected a converter that was very inefficient. We started with replacing the catalytic converter. The shop foreman was convinced that the engine “did not feel like a misfire.” However, there were several reasons to doubt this. For one, even after deleting codes in Mode 6, misfires were detected on the misfire counter MIDs. You don’t need to be a rocket scientist to understand Mode 6 (see Figure 5) on a 2007 Subaru. Here, the misfire counter MIDs on the bottom of the screen make it clear that the vehicle’s PCM is detecting a misfire. Further, the fuel trim (the LTFT, specifically) was good on the engine, which reflects that it is neither starving for air or fuel.

Since an ignition coil was recently installed on the vehicle, it was essential that we were totally sure that a misfire was actually occurring. When the vehicle acted up, it was apparent that the engine leaned out (as evidenced by the increase in STFT and A/F ratio sensor voltage). However, during this event volumetric efficiency was good (calculated efficiency was 100%) and the MAF sensor voltage went up when the increased TP demand was 100% due to the driver (here, myself) compensating for the vehicle’s low power. (See Figure 6. Only a misfire can explain the loss of power, misfire code, and normal aspiration into this engine. Only a misfire can explain the engine leaning out during the moment. An intermittent vacuum leak was impossible, because it would seal up during a WOT event. A MAF failure is out of the question because calculated efficiency was good.)

It made the most sense to us to test the primary ignition circuit of the ignition coil to see if the PCM was intermittently dropping out signal to the coil. Using a wiring diagram, we were able to see that internal to the ignition coil was the igniter assembly. Further, we were able to see that pins 18 and 19 grounded the ignition coil, and were used to trigger it in order to control the firing event of each cylinder (see Figure 7. An OE wiring diagram on Mitchell 1 ProDemand assists the technician in knowing where to check primary ignition on this vehicle).

Being that cylinders 1 and 2 were commanded by one driver on the PCM and cylinders 3 and 4 were commanded by the other, it appeared logical to test these wires going to the connector of the coil. If the voltage was dropping out on cylinders 1 and 2, but not on 3 and 4, it would show that there was a break with the wire or an issue with the driver of the PCM. The PCM driver hypothesis would be unlikely, because PCM drivers tend to be bad all the time, so that is important to keep in mind.

When the misfire event occurred again, the test results were unambiguous. The primary ignition voltage did not drop out in cylinders 1 and 2. In fact, it raised quite considerably, and the normal signature of a firing event (a voltage spike and then a slight decline) was replaced by a plateau (see Figure 8. Here, the ATS EScope was the labscope of choice in diagnosing this misfire event. The increase in primary voltage reflects that the coil is getting triggered and the misfire is occurring in the cylinder.) In our article “Waveform Diagnostics: Ignition diagnostics you will actually use” in the October 2012 issue of this magazine, we learned how a misfire will increase the ignition primary circuit voltage.

So, the problem was diagnosed. The ignition coil was getting everything it needed. Being that an intermittently bad spark plug or ignition cable (remember, it is brand new) would be very unlikely, we replaced the ignition coil. After this the problem was gone and the new converter was safe from an intermittent misfire event that would have led to its premature degradation.   ●

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