Within the ever-changing technological advances in the automotive industry, the goals almost always remain the same: increase fuel efficiency, reduce tailpipe emissions, and maintain or improve vehicle performance. Striving to achieve any two of those goals isn't too far-fetched. But to attain all three takes some real effort.

Momentarily disregarding EV propulsion's accomplishments in reducing dependency on hydrocarbon fuels, the internal-combustion engine is still alive and well. With that, so are the challenges driveability technicians face nearly every day in shops across the world.

Step Up to the Plate

These desires to achieve clean-running engines that actually perform well under all operating conditions and still attain relatively good fuel mileage have become an industry-wide reality for many years now, mainly due to the standards put forth by the government and its ever-tightening OBD-II program. Systems driving the components that allow these goals to be carried out are typically scrutinized hard by the ECUs governing their functionality. Just look at the analytic capability these ECUs have via the amount of diagnostic trouble codes available, even in the average PCMs of today.

Providing technical support and training for automotive technicians around the globe is something I do every day of the work week with my company, S.W.A.T. Training and Diagnostics. I interact with technicians with all different levels of understanding and experience. Many times, I'm involved with some of the most difficult diagnostic challenges you could fathom, but in many cases, the prescribed fix is relatively simple to derive. No, it has nothing to do with a lack of intelligence on my clients' behalf. In fact, it's quite the opposite. Many times, they're so intelligent and capable that they simply invest too much time testing everything they can get their hands on and lose sight of what they should be focusing on. They simply get overwhelmed with results to reflect on and weigh out.

Part of that is having to delve into systems experiencing nothing more than symptoms, without DTCs stored. This is always one of the most challenging situations for many technicians because they've grown to rely so heavily on stored DTCs for a starting point rather than focusing on a drivability symptom that may be exhibited.

Crossing the Line

However, most experienced drivability technicians can tell you that a resulting DTC is derived from the conclusion of a test that had failed. And each DTC's failure criteria is defined in the pages of service information, available to anyone who has purchased a subscription to it.

Learning what those failure criteria (or code set criteria) are is the key to replicating the fault. And until you can replicate the fault, testing anything can prove to be futile in many situations. Hence the reason for many of my requests for technical support.

So I said all that really just to ask this question: "Are you spending too much time properly testing all of the components, but under the wrong operating conditions?" Keep that in mind next time you find yourself dug deep into the hole of a diagnostic challenge. Instead, step back and ask a few basic questions, even before getting started.

For instance, consider reading the conditions required for the PCM to perform the test to begin with. Said another way, if the operating conditions required to start the test are not met, even a malfunctioning component or compromised system will not be called out as such.

The Hard Lesson

This situation comes to fruition very often, but the first time I encountered it was embarrassing and took the wind right out of my sails as a young diagnostician. I was faced with a vehicle visiting the shop with the complaint of a poor-running engine, MIL illuminated, and subpar fuel mileage.

Leveraging Mode 03 of my scan tool, I determined the cause of the MIL to be a stored DTC P0131. Little did I know at the time that the cause was a failed exhaust gasket before the primary O2 sensor. The exhaust leak caused the sensor to be biased lean (at lower voltage amplitudes than it should have been).

Replacing the sensor didn't fix the issue. One of the older and more experienced technicians witnessed my struggle and told me he assumed I'd heard the exhaust leak when I pulled the vehicle into the shop. Embarrassed, I admitted I'd heard the leak but hadn't considered its contribution to the fault. He chuckled and said, “Kids!" Back on track and after unnecessarily replacing a likely perfectly good O2 sensor, I was on the maiden voyage post-repair. Everything ran just fine, and I returned the vehicle to the customer. The problem is, just a day or so later, the vehicle returned with the MIL illuminated again.

That sinking feeling in my stomach surfaced when my service manager approached me with that fantastic news. However, I thought I was in the clear because it wasn't another DTC P0131. Nope, this time it was P0420.

I couldn't seem to understand the frustration in my service manager's disposition. After all, how was I supposed to know the catalytic converter was going to fail? You see, that was the entire point. Had I referenced service information, there was plenty there to support the fact that a pending P0420 might be looming in my future.

The code set criteria doesn't even have a chance to come into play here. That's because the conditions for running the DTC test couldn't be met, simply due to the fact that the P0131 was stored. This stands as an automatic "do not run the catalytic converter test."

The Lesson

Where's the lesson in all of this? Had I read the service information provided, I would've learned that I had no accurate way to evaluate the catalytic converter in that condition. I should've left the door open for a catalytic converter that may have sustained damage from hydrocarbon (HC) overload. Had I done that, it would've been the customer's decision to either allow pursuit of testing after the exhaust gasket replacement or omit testing altogether. This way, when the car came back with the MIL illuminated and P0420, I could've said "Told ya so!" and I would've been found innocent on all charges.

System/Component Evaluation

So, let's assume I conducted the catalytic converter evaluation after the replacement of the failed exhaust gasket, as I should have. According to the service information, the failure of the catalytic converter function is indicated if the PCM has determined the catalyst efficiency has degraded below a calibrated threshold. Wonderful. But what does that even mean?

With my now decades of hard-knocks experience, I've grown to learn that a vehicle of this era and "LEV-1" emissions level leveraged a "switch index ratio" strategy to determine the functionality of the catalyst. That is, it conducts a comparative test between the switch rate of both the front O2 sensor and rear HO2 sensor.

This test is used to reflect the ability of the catalyst to store and use oxygen. As the catalyst performance degrades and the switch rate of the rear HO2 sensor begins to mimic the switch rate of the front O2 sensor, the catalyst tends to reach the failure threshold with a comparative O2 sensor signal switch rate of rear to front above 70%. So, the next question is, "How would I know if the catalyst sustained damage or may even be close to failure if it doesn't set a DTC?" Assuming the switch rate didn't exceed 70% but maybe something like 65%, of course a DTC wouldn't set. This is the beauty of the Mode $06 service of OBD2 and how you can leverage it for yourself in the shop.

Mode $06 as a Diagnostic Weapon

The Mode $06 service of the OBD-II program offers valuable insight to the technician looking to know more than just "pass" or "fail" of a component or system. Although it has changed a bit over time (like everything else in our precious industry), on a vehicle of this era (pre-CAN), it was comprised of Monitor IDs, Test IDs, and Component IDs (commonly referred to as MIDs, TIDs, and CIDs). These three aspects of evaluation came together to become the fabric comprising the DTCs they were responsible for setting. It was the ECU's logic that considered these individual health reports to determine if a DTC threshold was achieved. But there's more to it than "pass" or "fail."

Written in hexadecimal format, this base-16 language is comprised of alphanumeric characters that replace the base-10 language we're used to. The reason is that computers speak in binary language (a series of 1s and 0s, or "on" and "off"). The point is, it's easy for computers to speak in "bytes" (like a string of words) consisting of "bits" (letters in each word). A base of 16 allows for many different combinations of these words using only four bits in any combination of numbers (1–10) and letters (A–F). For instance, the byte "FFFF" would be the number 16, multiplied exponentially, four times (F×F×F×F = 16×16×16×16 = 65,536). Understanding how the base-16 hexadecimal numerical system works can make it easy for us to decode, if we take the time to learn.

Well, the Mode $06 service of OBD 2 does that for us! We can see which emissions monitors run which component tests and the results of those tests. What's the point, you ask? Pending failures become evident before they're actually failures, offering you (as the diagnostician) a crystal ball into the future. With these results, we can see just how close to failure a test really is. This, in turn, prevents comebacks or incomplete diagnoses from occurring to begin with!

Breaking the Cipher

Although alien to most of us initially, learning to decode the Mode $06 information is not terribly difficult. Many OBD-II scan tools can help make light work of it by identifying the component test for us and in plain English. You'd still be required to understand how to do it yourself and cross-reference the lists of components and tests in service information.

The Mode $06 data definitions for the O2 sensor, similar to the faulted vehicle I described above, are sourced directly from service information. Indicated here are the specific tests of the specific component being evaluated. The description of the tests is indicated, but also the expected range of function is evaluated in both a decimal range and a hexadecimal range. Although test IDs vary between vehicles, we use this information the same way.

We can get the results from the scan tool and compare them to this chart to see how close to failure the component really is. The results serve as a data snapshot of when the monitor ran its tests. What I choose to view are test values that are closer to the failure thresholds. It's convenient when you have similar TIDs and CIDs for comparison (like misfire data from cylinder to cylinder).

One of my most used tactics is leveraging Mode $06 data for intermittent EVAP system performance faults. It's a frequent occurrence for an EVAP system's purge valve to fail intermittently. The underlying cause of many EVAP system leaks is internal, due to the intermittently failing purge valve. It fails to seat, allowing pressure or vacuum decay within the otherwise sealed system. When the vehicle is in the workshop for evaluation, the fault can be very difficult to recreate. As a result, a pressure- or vacuum-decay test tends to pass with flying colors.

Leveraging Mode $06 data for the EVAP purge monitor would demonstrate the results of the intermittently failing purge valve as a near-failure, with hex numbers nearing the threshold. This points closely to a suspected failure and justifies the time spent repeatedly cycling the purge valve open and closed and forcing the fault to reveal itself as a leak. This makes evasive fault detection easier to uncover, and on the first visit.

Bound by Rules

Keep in mind that the very definition of Mode $06 is the result of non-continuous monitors. To ensure we're seeing accurate and refreshed Mode $06 data test results, it's important to understand how the Mode $06 data updates. The PCM will only refresh the information when its conditions are met. The results will then be updated in the PCM's memory once completed. This will require a specific event to occur or even a full drive cycle.

A drive cycle varies by definition, depending on the vehicle itself. Some of the most challenging tasks techs are tied to include getting the monitors to run to completion with a "pass" result. For this reason, it's crucial to understand the specific drive cycle of the vehicle you're facing.

Adhering to all the specific criteria in the drive cycle allows it to be less of a challenge to complete. Paying attention to cold-soak time, temperature delta, throttle angles, vehicle and engine speed ranges, and key cycles is of the utmost importance. Taking the rules lightly is a recipe for a long and frustrating day. The point being, if the drive cycle doesn't complete, the monitors won't either. And underlying faults are just waiting to jump out and illuminate the MIL. Of course, only after you return the vehicle to the customer. Ouch!

Another Tool in Your Arsenal

I'll be the first to admit that Mode $06 data is not the "be all, end all" for me (like some instructors or technicians claim it is for them). But it’s another avenue I will choose when I'd like supportive evidence of a theory I've hypothesized. After all, having another arrow in the quiver always settles my stomach a bit more. I don't hang my hat on Mode $06 data to make diagnostic decisions, but it's very useful when called upon. I urge you to add it to your repertoire. It's bound to save you some time.