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The evolution of onboard diagnostics

Saturday, June 1, 2019 - 07:00
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Perhaps you have seen manufacturers that are utilizing 7-digit DTCs? Or perhaps a DTC that doesn’t seem to make sense numerically (think "P0A7F"). Some of these diagnostic trouble codes look nothing like we have grown accustomed to seeing over the last twenty years. With this in mind, the Society of Automotive Engineers (SAE) has revised its J-2012 standard to reflect the new classification of DTCs. Understanding how this can help technicians is worth exploring.

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With an evolving list of new technologies found on today’s automobiles, the amount of data and diagnostic information available to technicians has grown. While many technicians and shop owners are overwhelmed by the learning curve of new technology, there is a distinct advantage to more information. Think about no-code diagnostics, for example. Why are they so hard for technicians to fix? Simply, because there is limited data to base our diagnostic strategy upon. With more data at our fingertips one could argue that diagnostics are perhaps getting easier and not harder.

7-digit DTCs such as these Toyota Tundra misfire codes provide an added layer of diagnostic data.

There are several organizations that work alongside automotive manufacturers and engineers to establish best practices, protocols and, ultimately, standards for our industry. The International Organization for Standardization (ISO), the Institute of Electrical and Electronic Engineers (IEEE) and the Society of Automotive Engineers are organizations that technicians should be familiar with, as much of the technology we work with on a daily basis is designed in conjunction with and in consultation of their published standards. These organizations are responsible for the standardized practices we have become accustomed to such as an OBD-II connector that is within a reasonable distance of the driver’s seat, under the dash. The standards published by SAE are publicly available and can be found through the SAE website: www.sae.org. There is a considerable cost to purchasing these documents, however, SAE offers discounts to its members.  SAE offers memberships on an annual and lifetime basis and working technicians should consider joining to stay on the cutting edge of technology. Another approach to accessing SAE documents is through your public library, community college or university library as many often subscribe to electronic SAE databases and make them available with a library card or student ID.  SAE standards are a wealth of information for technicians and I have personally spent many-a-night combing through them while preparing for articles or classes given at the College or through WorldPac.

Society of Automotive Engineers membership is a must for the serious technicians. The International Organization for Standardization works with SAE to align best practices and industry standards for the automotive industry.

Leveling the playing field

In the 1990s, the Society of Automotive Engineers created a standards document to align the naming of diagnostic trouble codes across manufacturer platforms. This naming convention is why we all know that a “P0420” is a catalyst efficiency code and why a “P0300” is a random/multiple misfire, regardless of the vehicle nameplate. The J-2012 standard has been updated over time to reflect new and emerging issues with the naming and classification of DTCs but started as a simple way of cataloguing and standardizing the data across manufacturers.

You are likely familiar with some of these naming conventions and are most likely aware that for many years the three-digit codes we have all come to know are preceded by an alpha-numeric prefix. Such as “Px”, “Bx”, “Cx” or “Ux.” SAE and ISO are responsible for the fact that these represent:

P: Powertrain

B: Body

C: Chassis

U: Network

As an example: B0, B1, B2, B3, C0, C1, C2, C3, P0, P1, P2, P3 and U0, U1, U2, U3

In the early SAE J-2012 publication, a DTCs second digit represented the following:

0: ISO / SAE controlled – This meant these codes were specifically defined by ISO and SAE and have become what we know as “generic” DTCs

1: Manufacturer Controlled – Up to the manufacturer to define within the context of the SAE / ISO standard

2: Manufacturer Controlled - Up to the manufacturer to define within the context of the SAE / ISO standard

3: Reserved by document: This meant SAE had the foresight that they would eventually have more codes that would need to be added as technology progressed.

The third digit (second letter or number) in the DTC sequence provided a classification for the DTC type within the Powertrain Control Module:

            0: Fuel and Air Metering and Auxiliary Emission Controls

            1: Fuel and Air Metering

            2: Fuel and Air Metering

            3: Ignition System or Misfire

4: Auxiliary Emission Controls

5: Vehicle Speed, Idle Control and Auxiliary Inputs

6: Computer and Auxiliary Outputs

7, 8, 9: Transmission

A: Hybrid Propulsion

B, C, D, E, F: Reserved for future publication of DTCs (Planning for the future).

SAE J-2012 also provided four basic categories of DTCs as well as definitions for each:

General Circuit / Open – Fixed Value or no response from the system where specific high or low detection is not feasible or can be used in conjunction with circuit high and low codes where all three circuit conditions can be detected.

Range Performance Problem – Circuit is in the normal operating range but not correct for the current operating conditions, it may be used to indicate stuck or skewed values indicating poor performance of a circuit, component or system.

Circuit Low- Circuit voltage, frequency or other characteristic measured at the control module input terminal or pin that is below the normal operating range

Circuit High- Circuit voltage, frequency or other characteristic measured at the control module input terminal or pin that is above the normal operating range

Understanding these naming conventions as well as DTC classifications makes the identification of system errors easier to diagnose.

As an example, if we are looking at issues with DTCs related to engine coolant temperature we can easily differentiate between a circuit low or high code in which we are looking for an electrical or electronic issue and a range /performance code where we are more likely looking for a mechanical problem such a stuck thermostat.

SAE’s library of standards is a wealth of information for professional technicians seeking to raise their level of knowledge.

When you take a step back from this information for a second and realize there are only four general classifications for codes, diagnosis is really simplified.

Evolving to meet emerging tech

SAE J-2012 has been updated throughout the years as vehicles have become progressively more complex. As microcontrollers improve and continue to come down in price, their capabilities and storage capacity have greatly improved. With new technologies such as advanced driving assist, autonomous driving and vehicle electrification being introduced, the need for a redesign of the J2012 standards was needed. The sheer amount of new diagnostic trouble codes available as a result of technology has essentially created a naming problem for SAE.

In 2016, SAE re-published J-2012 as well as a new digital annex, or Excel spreadsheet, known as J-2012DA. According to SAE, the digital annex will need to be updated frequently to accommodate the new codes being created by manufacturers as new technologies emerge. This new naming convention has also made the original naming conventions somewhat obsolete and a standard for naming has given way to new varieties of alphanumeric combinations.

One of the specific areas that the new standard specifies is a two-digit identifier at the end of the DTC that essentially creates a 7-digit diagnostic trouble code. The technical advances of micro-controllers found on vehicles has allowed manufacturers to more precisely specify a problem that it has self-diagnosed and provide additional information to aid in the repair of the problem.

SAE has multiple membership levels for professionals and students and is a tax-deductible investment in your professional growth.

For example, on a 2019 Toyota Tacoma a P0300 “random multiple misfire” code becomes three different codes:

P030000 – Random / Multiple Cylinder Misfire Detected

P030027 Random / Multiple Cylinder Misfire Detected (Emission) Signal Rate of Change Above Threshold

P030028 – Random / Multiple Misfire (Over Temperature) Signal Rate Above Allowable Range

These new 7-digit DTCs provide an added layer of information for the technician as the detection capabilities of the microcontroller has improved. In the example above, not only is the crankshaft position sensor detecting a variation in crankshaft speed, it is utilizing a software algorithm to determine the estimated weighted moving average (EWMA) of the crank and its perceived effect on emissions and catalyst damage. This allows Toyota to go further than the P0300 by adding two digits that specifically categorize a code within the code so, so-to-speak. The wealth of information this provides for diagnostic purposes allows the technician an additional set of diagnostic data.

To summarize, the complexity of vehicle electronics and microcontrollers has numerically exceeded general code definitions and created a need for new naming conventions. The disadvantage to technicians is we will no longer be able to recognize many codes as we used to such as a “P0171” or “P0400”.  However, we will gain a deeper understanding of the circuit, component or system at fault through data enriched information supported by 7-digit DTCs. While some might complain about the complexity of our new automobiles, we should welcome the additional data that is now available.

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