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Scan Tools

Trouble code reading scan tools

Courtesy I-CAR

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At a glance...

  • Basics of scan tools
  • Standards
  • Selection
  • Use
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This material first appeared in the I-CAR Advantage Online, which is published and distributed free of charge. I-CAR, the Inter-Industry Conference on Auto Collision Repair, is a not-for-profit international training organization that researches and develops quality technical education programs related to collision repair. To learn more about I-CAR, and to subscribe to the free publication, visit

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When vehicle makers began using electronics to control certain functions, it led to vehicles being equipped with on-board diagnostic (OBD) systems to monitor and diagnose problems. Computer-controlled systems are found on every vehicle built today.

With the implementation of OBDII [in the US] in 1994, the number of systems capable of being accessed using a scan tool has increased. At the same time, the amount of diagnostic information available to technicians has also increased. Because of this, it has become more important for technicians to have equipment capable of accessing this information and thus, the advancement of the handheld scan tool.

The Need for Scan Tools

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With technological development in today’s vehicles, the handheld scan tool has become a necessary tool for proper diagnosis and repair. A scan tool may be required after simply disconnecting and reconnecting the battery on some vehicles. Some primary functions of a scan tool include accessing and/or clearing diagnostic trouble codes (DTCs), monitoring and/or activating system parts, uploading information, and recording data. A scan tool gives a technician the ability to communicate with computer-controlled systems, including the anti-lock brake system (ABS) and passive restraint system.


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To ensure vehicle and equipment compatibility for such things as the connector and communication procedures, the Society of Automotive Engineers (SAE) developed standard requirements for vehicle, equipment, and toolmakers. These standards included scan tool operation. Standard emission diagnostic regulations, known as OBDII, were implemented [in the US] on January 1, 1996. Compliance to the OBDII regulations requires standard 16-pin data link connectors (DLC), communication protocols for scan tools, and DTCs.

There are [at least] three OBDII communications protocols being used by vehicle makers. When using a scan tool, be sure it works with the protocol of the vehicle. Typically, the three scan tool protocols and the vehicles they support are:

  • SAE J1850VPW (Variable Pulse Width Modulation) (GM)

  • SAE J1850PWM (Pulse Width Modulation) (Ford)

  • ISO 9141 (Chrysler group, most European, most Asian)

On vehicles built since January 1, 1996, scan tools link to a vehicle diagnostic system through a universal connection port, which is typically located just below the instrument panel on the driver's side. As a requirement of OBDII, the DLC must be located within 91 cm (36") of the steering wheel, and not require any tools to access. Some DLCs may be covered to protect the pins against damage. Depending on the scan tool being used, hooking up to the DLC may provide the necessary power for the scan tool to operate. Some vehicles may have connectors in other locations in addition to the required DLC under the instrument panel.

Selecting a Scan Tool

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Scan tools can be purchased either from the original equipment manufacturer (OEM) or an aftermarket supplier.

OEM scan tools include, but are not limited to:

  • Tech II (GM)

  • DRB III (DaimlerChrysler)

  • STAR (Ford)

  • CONSULT (Nissan)

  • VAG 1551 (VW, Audi)

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Aftermarket scan tools include, but are not limited to:

  • Sunpro

  • Snap-on

  • OTC

  • Pro Link 9000

  • Master Tec

  • Palm Handheld

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Some diagnostic tool providers offer diagnostic software for use with the Palm handheld Personal Digital Assistant or a laptop PC.

Scan tool operations, capabilities, and display configurations will vary. Because all scan tools are different, it is very important to have the user’s manual for the tool that you will be using. To support changes in vehicle technology some scan tools have the capability of being upgraded.

When using scan tools and reading repair manuals there may be acronyms used. These acronyms have been developed by the automotive industry and standardized by the SAE (document number J1930). Understanding the meaning of common acronyms will help the technician with quick and complete understanding of electronic information.

Scan Tool Procedure

When accessing information from a vehicle computer using a scan tool:

  1. Connect the scan tool to the diagnostic connector.

  2. Retrieve the DTCs.

  3. Record the data that is contained within the vehicle computer. This will help the technician retain the DTC information in the event the data gets lost or erased from the computer memory.

After the diagnostic information has been gathered:

  1. Access the appropriate repair information.

  2. Repair the affected systems as required.

  3. Test-drive the vehicle to verify that the original complaint is corrected.

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The repairs to a vehicle, the current DTCs have to be cleared. Typically vehicles built after June 1995 will have the DTCs cleared using a scan tool or by doing a test-drive.

Test drives may be required to allow the vehicle computer to verify that the problem no longer exists. Doing the test drive also allows the technician to verify that the problem is corrected. After the computer does a self-test during a test drive, the current code should clear.

A test drive can be done with a scan tool connected. Do not attempt to monitor the scan tool display while driving. Have a second technician ride along to monitor the scan tool, or use a scan tool with data storage capability, which allows accessing the information after the test drive is completed and the vehicle is no longer in motion.

Additional Functions

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Depending on the vehicle and ability of the scan tool being used, a technician may be able to monitor some systems, or turn some electrical systems on and off.

Some vehicle systems that may be tested include the fuel pump, ABS, engine cooling fans (both HI and LOW speed), and vehicle computer output tests. Using a scan tool and other equipment, a technician can perform various tests including fuel and vacuum leak tests and electrical tests of solenoids and sensors.

Even though some tests can be done on electronic sensors using a multimeter and other simple test equipment, this random-type testing is not recommended. There may be hundreds of possible problem areas. The most efficient method of identifying a faulty sensor, or a fault in a sensor circuit, is by connecting a scan tool to the OBD connector and retrieving a DTC that leads to the area of the problem. The DTC does not identify a failed sensor, but a problem circuit that may or may not lead to a faulty sensor. Then, following a flowchart in a service manual, the problem can be repaired. The steps in the flowchart usually require simple tests on the problem circuit. Many of these tests can be done with the use of the scan tool.

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To make sure that the vehicle has been fully tested through all possible scenarios, a drive cycle can be performed. A drive cycle may also be done with a scan tool connected while the drive cycle is performed. Some scan tools will even guide the technician through the steps of a drive cycle for the particular vehicle.

After the drive cycles have been performed, if the malfunction indicator lamp (MIL) is on, a scan tool is the best tool to determine why the lamp is lit. The drive cycle may or may not set a DTC, turning a MIL on.

Scan tools may also help with record keeping. Scan tool data can be printed out and can serve as documentation as part of the repair record.


In review, with vehicles of today being equipped with multiple computer controlled systems, having scan tools available and knowing how to use them allows for an efficient repair process. In some situations, repairs may not be possible without the use of a scan tool.

For more information on DIY scan tools, see: Reading Your Car's Brain - Part 1, Reading Your Car's Brain - Part 2 and Reading Your Car's Brain - Part 3.

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