Diagnostic Dilemmas: Navigating OBD II Scan Tool Diagnostics – UnderhoodService
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Diagnostic Dilemmas: Navigating OBD II Scan Tool Diagnostics

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Although some hybrid OBD I/OBD II versions existed as early as 1994, the current version of OBD II was introduced in 1996. Because OBD II is a scan tool-based diagnostic system, diagnostic techs have become much more reliant upon the scan tool as a primary method of diagnosing malfunction indicator light (MIL) issues. In addition to engine functions, the automatic transmission, anti-lock brakes, air bag and various body control systems may now be controlled via individual computers that must also be diagnosed by using a scan tool.

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These computers or modules may be operated by multiplexed electronics systems. Multiplexed systems use a master computer such as the body control computer, to control and coordinate modules or “nodes” that operate components such as power windows and door locks. These computers and modules communicate with the Powertrain Control Module (PCM) and with each other via a bus communications system linking each of the computers and modules.

Because of the added complexity of networked vehicle operating systems, choosing one or more scan tools to service a range of vehicle makes and models is becoming a daunting task. While most aftermarket scan tools include the most important functions on the most popular vehicles, they don’t include all of the OEM nameplate-specific information and functions due to the cost and sheer immensity of the task.

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For that reason alone, many independent shops are choosing to operate one or more aftermarket and OEM scan tools to diagnose and repair the majority of the day-to-day workflow. In most cases, these choices might include OEM-specific scan tools such as the popular Ford NGS, General Motors Tech 2, and the DaimlerChrysler DRB III.

Of course, even as this is written, Ford and Chrysler are going through generational changes in scan tools in order to accommodate Controller Area Network (CAN) and increased levels of body control technology. Chrysler, for example, requires the DRB III to diagnose 2004 and earlier products while their new Starscan tool is required for 2004 and later product models.

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Consequently, many general repair shops will buy one or more OEM scan tools to cover domestic vehicles while augmenting coverage with several aftermarket tools. If, for example, Asian or European vehicles are a small part of the diagnostic workflow, a single aftermarket scan tool might be able to address the majority of diagnostic work on these vehicles. On the other hand, if Asian or European might comprise a majority of a shop’s diagnostic workflow, a shop might invest in an Asian or Euro-based scan tool or a manufacturer-specific tool to address these needs.

Scan Tool Mythology
Many inexperienced technicians feel that buying a scan tool will give them an advanced diagnostic capability. In most cases, the very act of acquiring a scan tool will expose serious weaknesses in their training and understanding of OBD II operating systems and strategy.

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In addition to many hours of training and experience, beginning diagnostic techs will need a good lab scope, graphing multimeter, digital volt-ohm meter, low-amp current probes, secondary ignition pick-up leads, battery diagnostic equipment, training manuals, databases and many other pieces of peripheral equipment in order to pursue complex diagnostic issues. With that said, let’s look at some basic OBD II scan tool capabilities.

Diagnostic Trouble Codes
I’ll reiterate what’s been said many times in this space: diagnostic trouble codes (DTC) do not indicate which part to replace. The presence of a DTC may instead indicate how a particular component or system is affected by the failure of another.

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A classic example is the P0171, P0174 DTCs found on Ford products equipped with mass airflow (MAF) sensors. The P0171/174 merely indicates that the PCM hasn’t detected the oxygen sensors switching through their normal 0.2 to 0.8-volt range. Inexperienced technicians occasionally replace both oxygen sensors only to discover the reoccurrence of the P0171/174 DTCs. The root cause of the DTCS, of course, is an excessively lean air/fuel (a/f) ratio initiated in most cases by a dirty or defective MAF sensor.

Similarly, the pronounced absence of DTCs doesn’t indicate that the vehicle is in a good state of health. The absence of a DTC may instead indicate that the voltage supply to the PCM has been disrupted. In other words, the DTCs may mysteriously disappear, as was the case recently when I was diagnosing a 460-LE automatic transmission on a Chevy truck with a loose positive battery terminal. The terminal would sometimes cause enough drop in system voltage to erase the PCM’s diagnostic memory. Similarly, a battery with a bad cell may erase DTCs during start-up when the voltage to the PCM drops below the minimum threshold. In most no-code cases, a DIY mechanic has simply attempted to “reset” the PCM by disconnecting a battery cable.

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Last, always be aware that it’s not normal for the diagnostic memory to store more than two or three DTCs. If, for example, the diagnostic memory has stored numerous DTCs, the components and systems involved may have a common power supply or grounding problem. Power supply is a common problem, for example, on General Motors trucks that lose voltage to the automatic transmission controls from a single worn terminal on the ignition switch.

Remember too, that when numerous DTCs are present, a diagnostic chart will indicate which of those DTCs to repair first. This is important, because the failure of a single component may trigger false inputs from other sensors in the OBD II system. If this is the case, the shop manual will say in effect, “Repair this DTC only if no other DTCs are present.”

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Last, never erase a DTC until the repair has been completed and verified. Some technicians advise leaving the DTC in memory, especially when performing emissions repairs because, if the repair is indeed effective, the MIL will go off and the DTC will eventually erase itself.

Freeze-Frame Data
A freeze frame is a “snap shot” describing the vehicle operating parameters under which the DTC was stored. Although a freeze frame retained in the diagnostic memory doesn’t always help, it should always be reviewed for clues to intermittent problems.

To illustrate, if a misfire DTC occurs at low vehicle speeds and high intake manifold vacuum, there’s a very high probability that it’s caused by a vacuum leak rather than an ignition misfire. On the other hand, if a misfire DTC occurs under heavy engine loads, it’s more likely to be caused by a bad spark plug, ignition coil or secondary wiring.

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Data Stream Analysis
Analyzing data streams can be tricky because much depends upon how fast the scan tool updates the information. Keep in mind that the more data lines displayed, the slower their update times.

Checking throttle position (TP) sensor voltage via data stream provides a good example. While the data stream will provide the exact TP voltage at any throttle position, it is not fast enough to catch a momentary voltage drop during a throttle sweep. For that reason, a few scan tools display data in small blocks dedicated to specific systems like oxygen sensor, misfire, fuel control and other update or time-sensitive parameters. A diagnostic tech may be able to increase update speed on some scan tools by selecting the lines of data he wants to appear on the scan tool’s screen.

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Nevertheless, an experienced technician can quickly evaluate a system by looking for irregularities in the data stream. A Ford MAF sensor should produce, for example, about 0.7 volts at idle speed. The calculated barometric pressure of the Ford MAF also should match those found in diagnostic charts. To illustrate, a MAF should produce 159 hertz (Hz) at sea level and 138 Hz at 8,000′ attitude.

Next, the data stream values usually can be verified by a voltage, frequency, temperature, vacuum or pressure test. Indicated coolant temperature, for example, can be verified with a non-contact temperature tester while intake manifold vacuum can be verified with an accurate vacuum gauge. Electrical values also should be tested with a professional-grade DVOM or multimeter. If the electrical value exists at the sensor but not at the appropriate PCM terminal, then the component might be experiencing a circuit fault. Although the reliability of a PCM has increased dramatically during the past decade, an internal failure in the PCM’s circuitry might cause it to misinterpret input data.

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Last, be aware that, when a component such as an oxygen sensor is disconnected, the PCM will substitute a default value into the data stream displayed on the scan tool. While default values are more of an issue on OBD I systems, it’s important to know that, if a data line is static and doesn’t track with engine operating conditions, it may be a default value that merits further investigation.

Scan Tool Graphing
In my opinion, graphing capability is an essential feature of a modern scan tool. Case in point, I had a 1998 Ford Expedition with a poor acceleration and hard shifting complaint. The diagnostic memory yielded the classic P0171, P0174 DTCs, which are characteristic of a bad mass airflow sensor. Keeping in mind that the automatic transmission also uses the MAF to sense engine load, the remaining pieces of the puzzle fell into place.

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Unfortunately, a routine data stream analysis showed me that the idle speed voltage and the barometric readings were correct. By graphing the MAF voltage, I found that the longer the engine ran, the more erratic the MAF’s voltage output. A Ford MAF should produce roughly 1 volt per 1,000 rpm of snap throttle response. The graph indicated that snap throttle voltage varied between 1 and 3 volts on successive snap throttle tests. A new MAF solved the problem.

Although scan tool graphing isn’t equivalent in quality and accuracy to a lab scope reading, it can provide a comparative analysis of the activity in the two, three, four or six oxygen sensors found in most OBD II systems. Similarly, graphing is valuable in testing sensor and relay activity on electronic automatic transmissions.

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Movie Recordings
A “movie” is a recording of the data stream occurring before and after the movie is triggered. I’ve found movies particularly helpful in analyzing road test data concerning transmission electronics and intermittent fuel delivery issues.

Keeping in mind that scan tools and vehicles have differing movie storage capabilities, most scan tools can be armed to record a movie after a specific DTC is stored in the PCM. Alternatively, the scan tool movie might be triggered manually when a specific driveability symptom occurs. In either case, the technician can observe the data or download and print it later.

Troubleshooter Databases
Although many technicians consider scan tool-based “troubleshooters” non-essential, I’ve found them to be particularly valuable when diagnosing unfamiliar systems. For example, the scan tool’s database can quickly link a DTC to a specific technical service bulletin (TSB) about the subject. In addition, scan tool databases might also provide quick service information such as fuel pressure specifications and other information needed to guide the diagnostic process.

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Of course, with that said, only a few scan tools offer this capability. Nevertheless, that’s why it might be important to combine one or more aftermarket scan tools with database capability with the greater diagnostic power of an OE scan tool, especially in a shop with technicians of varied abilities.

Reflash Capability
I won’t dwell too long on reprogramming or reflash capabilities on modern scan tools because there’s a state of flux concerning both the current technology and equipment capabilities. Since OBD II was introduced in 1996, practically all OEMs adopted VIN-specific, “flash” reprogramming of PCMs. To complicate matters further, many other on-board replacement computers and modules must also be reprogrammed in a VIN-specific manner in order so they can communicate with other body control modules contained in the vehicle.

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Of course, all OEM scan tools have full reprogramming capabilities. While aftermarket scan tools have concentrated on PCM or engine management reprogramming, most haven’t yet addressed the issues associated with body control module reprogramming.

Although reprogramming is being promoted by some equipment manufacturers as a potential profit center for independent shops, reprogramming can turn into an assortment of problems.

If, for example, the programming “update” proves to be unsatisfactory, it can’t be reprogrammed back to the original version. Consequently, even though some aftermarket equipment manufacturers are seeking to expand reprogramming through additional software capabilities or through stand-alone reprogramming equipment, using reprogramming as an add-on service may have some unforeseen pitfalls.

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Scope and DVOM Capabilities
Although there are notable exceptions to the rule, most scan tool-based lab scope and volt-ohm meter capabilities are, for the most part, very basic or markedly inferior compared to stand-alone equipment. On the other hand, it’s nice for an individual technician to have those features packaged in a single piece of equipment. The key in this decision-making process is either to be able to demonstrate the tool in day-to-day use or to consult material contained in the archives of the International Automotive Technician’s Network (www.iATN.net).

The Generic Mode
Some scan tools call it the “global” mode, while other euphemisms are used to describe what is commonly known as the “generic” mode. When compared to the enhanced mode, the generic mode allows a technician to use his scan tool without entering VIN information and contains most of the trouble codes, freeze-frame data and basic data stream information needed to solve most OBD II emissions-related issues.

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Although space won’t allow a definitive look at the subject, keep in mind that the generic mode also will allow a technician to enter Mode $06, which is an engineering mode designed to produce predictive failure data. Mode $06 is written in hexadecimal-based numbers which require a scientific calculator to be translated into conventional decimal-based numbers. An OEM or aftermarket chart must be used to translate these numbers into useful information. Currently, several aftermarket scan tools list each test as having passed or failed, which is very helpful in using Mode $06 data. At least one company has developed PC-based software that directly translates Mode $06 data into useful diagnostic information.

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As you can see, the diagnostic capabilities of the modern OBD II-based scan tools have developed far beyond those of the old OBD I scan tools. Since some new scan tools are designed for OBD II only for OBD II diagnostics, it’s important to keep your old scan tools on the shelf for OBD I diagnostics. Whatever your choice in scan tools, make sure the tool meets your requirements in ease of use, service capability and product support.

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