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How to Read Scan Tool Data

When diagnosing an on-board computer control problem, it’s important to distinguish between manufacturer-specific DTC codes and generic codes, so that you correctly interpret the errors and then devise the best solution to the problems. Some scan tools have the capability to distinguish between these DTCs while others do not. 

Some scan tools do not differentiate between generic and manufacturer-specific scan data and do allow them to be queried simultaneously. Other scan tools require you to choose which type of scan data is being queried or what manufacturer made the vehicle in order to show the specific data applicable to the vehicle. Typically, when diagnosing a vehicle, it is very convenient to show generic and manufacturer-specific data at the same time. The manufacturer-specific data typically supplements the generic data when trying to get a good understanding of the current status of the vehicle, especially when looking at real-time data. DTCs are formatted so that you can tell if the trouble code is a generic trouble code or a manufacturer’s. The scan tool interface may or may not differentiate between the two formats, though; so, when in doubt, consult the service manual.


On certain scan tools, the mechanic has to choose what type of scan data to review.

Additional scan tool data can come from controller modules other than the ECM. As the complexity of vehicles increases, more electronic control modules that control complex systems are being added, such as active handling, passive restraint systems, climate systems, etc. These modules have their own diagnostic capabilities and data scanning capabilities that may or may not be separate from the ECM. So the manufacturer has expanded the scan-data toolset to include specific information from the other modules. 

Obviously, these control modules vary among vehicles and manufacturers, and aren’t covered by the generic scan-data protocols. Therefore, when diagnosing these systems, a manufacturer-specific scan tool is often required, or a robust scan tool that supports the manufacturer-specific data, as well as technical documentation on the various control modules. Some scan tools contain a data dictionary for manufacturer-specific code, so consult the scan tool manual for details.

The issue with scan tool data is whether or not the interface supports some or all of the manufacturer-specific data sets. Unfortunately, many of the scan tools do not support the entire set of manufacturer-specific data. Also there are very few scan tools, except for the very-high-end scan tools, that support the entire set of every manufacturer’s specific codes. Because the manufacturers are constantly modifying these data sets, some scan tool vendors have online update capabilities to allow mechanics to keep up with the changing technology. These updates/upgrades can be free or they may be available on a monthly/yearly paid subscription service.

scannerconnectorWhen a scanner is connected to the system, some ECMs do not correctly identify the make and model of the vehicle. In these cases, the mechanic needs to enter the VIN into the scanner, so that manufacturer-specific codes and the codes available for the particular vehicle can be determined.

Written Keith McCord and posted with permission of CarTech Books


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