What Is a Scan Tool?

Beyond code readers

A scan tool is a car diagnostic tool that can exhibit a range of features. Scan tools typically include a code reader, the ability to view and sort live data, and some type of knowledge base. Professional scan tools include extensive knowledge bases, diagnostic procedures, and sometimes have built-in scopes, multimeters, and other diagnostic tools.

What Can a Scan Tool Do?

Scan tools are designed to interface with a car's onboard diagnostic system to facilitate the diagnostic process. In that way, they are similar to car code readers. They can be plugged into an OBD-I or OBD-II socket, read and clear codes, and view data readouts from various sensors. However, scan tools go beyond that basic functionality.

Two people looking at a car scan tool.

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In addition to reading and clearing codes, a scan tool may be able to:

  • Store and play back live data
  • Graph data
  • Read generic and manufacturer-specific trouble codes
  • Display pending codes
  • Provide trouble code definitions
  • Provide troubleshooting procedures or tips

Although the ability to read and clear codes is important, the additional functionality provided by a good scan tool can be helpful in diagnosing a problem. OBD-II vehicles, in particular, can provide an enormous amount of data from a variety of sensors, which is why many scanners can store and play back live data. This allows you to test drive the vehicle and then view a recording of specific sensor output readings during operation.

How to Use a Scan Tool

The exact procedure for using a scan tool varies depending on whether your vehicle has OBD-I or OBD-II. Some vehicles with OBD-I also have unique steps or procedures, and some aren't designed to work with scan tools at all.

Here are the basic steps involved in using a scan tool:

  1. If the scan tool has multiple connector options, identify the correct connector and attach it to the tool.

  2. Plug the connector for the scanner into the OBD-I or OBD-II port on the vehicle.

    OBD-II ports are usually found under the driver's side of the dashboard or in the center console, and they are sometimes hidden by a trim piece. OBD-I connectors are usually found under the driver's side of the dashboard or in the engine compartment.

  3. Insert the key into the ignition, and turn the ignition to the accessory position.

  4. If the scan tool doesn't turn on automatically, consult the user's manual and turn it on.

  5. If necessary, enter the vehicle identification number (VIN) into the scan tool.

  6. Look for a scan option on the scan tool. There may be a physical button, or you may need to scroll through on-screen menu options.

  7. Wait for the scan to complete, and make note of any trouble codes.

  8. Use the scan tool's built-in knowledge base, or search the internet for more information on the trouble codes.

  9. Depending on your scan tool, you may also be able to view stored data and pending codes for more helpful diagnostic information.

  10. After you have diagnosed and fixed the problem, use the scan tool to clear the trouble codes. You will then need to drive the vehicle until the scan tool reports that all of the readiness monitors run.

The Importance of Scan Tool Knowledge Bases

In addition to interfacing with the onboard diagnostic system of a vehicle, the other important thing a scan tool can do is provide you with some type of knowledge base. The specific information contained in the knowledge base varies from one manufacturer to another, but it's almost impossible to overstate the importance of good troubleshooting information.

Unless you have relevant prior experience with the specific problem that you're dealing with, a good knowledge base can save you time. Scanners that include troubleshooting information and other tips typically provide you with the definition of a code, the potential faults that can lead to that code being sent, and the tests you can do to track down the cause of the problem.

Premium Scan Tool Features

The best scan tools essentially provide you with a one-stop-shop that includes code and data reading functionality, advanced knowledge bases and troubleshooting tips, and a built-in scope and other meters. Some of these tools, like the professional-grade Snap-On MODIS, are expensive, but they provide all of the tools you need to identify, test, and diagnose failed components.

Premium Scan Tool Alternatives

Although professional-grade scan tools can be prohibitively expensive, you can achieve much of the same functionality without breaking the bank. Some of the things you might want to have in your toolbox include:

  • A good consumer-grade scan tool or ELM327 scanner
  • A multimeter and, if possible, scope
  • Access to the internet

While resources on the internet don't provide a direct 1:1 replacement for the type of troubleshooting information you get with a professional-grade scan tool, this is a more affordable way to go.

A good consumer-grade scan tool (or an ELM327 scanner and the right software) will get you on the right track, and you can supplement the information that your tool provides with an online OBD-II code chart and troubleshooting information. After you've tracked down the likely culprit, tools like a multimeter and scope will help you determine whether or not specific components are bad.

  • Why won't my car start even though the lights work?

    If your car won’t start but the lights work fine, it could be one of several problems, including a dead battery. Check the fuses, fusible links, starter, and ignition switch.

  • Why does my car battery keep dying?

    Common causes of a dead car battery include corroded or loose battery connections, extremely hot or cold temperatures, charging system problems, headlights or dome lights left on​ overnight, and other parasitic drains in the electrical system.

  • Why is my ABS light on?

    Possible reasons why the ABS light is on include low brake fluid and damaged wheel speed sensors. Driving when the ABS light is on can be dangerous, so get it checked out immediately.

  • What are some other diagnostic tools for DIY car repair?

    In addition to scan tools and code readers, other essential tools for DIY auto repair include a 12v test light, mechanic’s stethoscope, timing light, vacuum gauge, vacuum pump, and fuel pressure gauge.

  • How do you reset an ABS light without a scan tool?

    You need a fused jumper wire that works with the Data Link Connector (DLC). Find the DLC in your car, connect the jumper wire correctly, ignite the car without starting the engine, and pump the brake eight times. The ABS light will turn off if you're successful. Always exercise caution when using jumper wires on a car.

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