Your Scan Tool's Most Important Functions

The scan tool has sure come a long ways since 1980 when GM provided an Assembly Line Diagnostic Link (ALDL) as a means of checking onboard computer functions before the car rolled off the end of the line. Before long, the service industry quickly adopted this connector as an access point into the computer’s “mind” during vehicle service. The legacy of the scan tool was born.

Here, we take a look at some of the more notable functions that today’s tools offer, which can help prioritize the features you might want to investigate more thoroughly before making your next scan tool purchase.

First Things First

Before we discuss scan tool functions, it’s important to mention that fuel, ignition or mechanical problems can trigger trouble codes or other data presented on a scan tool. This is why you should always check the basics before getting carried away with what might, at first, seem to be a “mysterious problem.” This includes a visual inspection of vacuum hoses, battery cables, wiring and so on, including ground connections.

Code Collection

Retrieving Diagnostic Trouble Codes (DTCs) is the most common use for a scan tool. This is usually related to a condition where the Malfunction Indicator Lamp (MIL) is on. But code chasing has taken on other significance over the last 5 years as more states adopt OBD II checks instead of tailpipe testing for state emissions programs.

Although no one can argue the importance of DTCs and their interpretation, they’re simply an indicator of what’s wrong in a specific area, not the specific fault itself. Always remember that codes don’t tell the whole story - they’re simply a starting place.

In your quest for a scan tool, make sure you check out what kinds of codes can be displayed, as some only display generic OBD II codes, and may not give you all the detailed information you need.

Serial Data

System problems do not always manifest themselves as trouble codes, so just like life, diagnostics are not always black and white. This is where a scan tool’s ability to display serial data, or Parameter Identification Data (PID) comes in handy.

For instance, a vehicle may have a driveability problem that does not set a code, but there is an unmistakable performance problem. By taking a look into the heart and soul of the Powertrain Control Module (PCM), you can learn a lot about whether sensors and their operating ranges fall within specified values. Just as with codes, this type of information tells part of the story, but not all of it.

Mode 6 Data

Mode 6 is an emerging diagnostic mode built into OBD II that performs onboard monitoring of test results for non-continuously monitored systems.

Mode 6 data is live and unrecorded, so it can reveal a lot of real-time information about system faults like ignition misfire, fuel control and so on. Basically, this means that Mode 6 data can reveal a problem before it sets a DTC or lights the MIL.

Though helpful, Mode 6 data may not be available on all scan tools or vehicle systems. Ask your equipment rep whether the scan tool you’re considering can tap into Mode 6 data. Within the last several model years, the carmakers have boosted Mode 6 functionality, and the importance of its data.

Actuator Tests

On some vehicles, scan tools can perform actuator tests. This is where the tool tells the PCM to command an actuator, such as a relay or solenoid, to see whether it’s actually performing its task. These capabilities are also dependent on whether the vehicle manufacturer enables such testing in its system.

Historically, Chrysler has been strong in the use of an Actuator Test Mode (ATM) to check component operation. The main thing to identify is whether the scan tool you’re considering can command this functionality. Actuator testing is valuable because you can clearly see if the component in question is performing accordingly. As with other scan tool functions, actuator testing offers a piece of the puzzle, but not the entire solution.

Bi-Directional Communication

Most scan tools can perform bidirectional communication where the tool can be used to “talk” back and forth with various onboard systems. Some examples include body computers, the Controller Area Network (CAN), entertainment and information systems, fuel control systems, anti-lock brakes (ABS) and transmission control.

Signal Substitution

Some scan tools can substitute values to check the response from the PCM. For example, a vehicle speed sensor (VSS) can affect several systems, like cruise control, speedometer and transmission. By substituting a value, you can quickly emulate different speed conditions that may prove useful during diagnosis.

Onboard Service Information

Current scan tool offerings often include onboard service information, which can be a real time-saver when checking whether a problem may be related to a service bulletin or to access further details from a schematic. Although it’s not a substitute for a service information system, this feature can be a time-saver.

The “Best” Scan Tool?

Questions abound when it comes to choosing a scan tool. Aftermarket-type or OE version? Traditional type or PC-based? The answer depends on your specific needs.

Carefully consider the makes, models and types of vehicles you service regularly, along with those you plan on servicing down the road. Talk over your options with other techs and your equipment rep. Above all, get some hands-on time with whatever scan tool you’re considering. In some cases, multiple scan tools may be necessary to handle the specific population of vehicles that come into your shop.

Also, like it or not, a scan tool is not the single answer for all your diagnostic woes. Onboard diagnostics really help, but they're no substitute for a thorough understanding of how the system works, how to conduct other pinpoint tests, and what other testing equipment might be needed.

Questions about this article? Contact the author, Dave Cappert at