What to Do When Your OBD2 Scanner Not Connect to Your Car

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    When it comes to diagnosing vehicle issues, one of the first steps is to use an On-Board Diagnostics (OBD)  scanner. This tool allows you to retrieve error codes and other information from your car’s computer system, helping pinpoint the source of any problems. However, what happens when your OBD2 scanner doesn’t communicate with your vehicle?

    What is the OBD2 System and How It Work?

    The OBD2 (On-Board Diagnostics II) system is an advanced technology embedded in vehicles, designed to monitor and report on various aspects of the vehicle’s performance. This system is essential for diagnosing problems related to the engine, emissions, and other crucial components. Since 1996, all vehicles sold in the United States are required to have an OBD2 system.

    The OBD2system continuously monitors the vehicle's performance and checks for any anomalies. It uses a standardized diagnostic connector, usually located under the dashboard, to communicate with an OBD2scanner. This scanner reads the data and diagnostic trouble codes (DTCs) generated by the vehicle's electronic control unit (ECU). These codes help identify specific issues, allowing for precise troubleshooting and repairs.

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    Possible Causes

    There are a few reasons why your OBD II scanner may not be able to communicate with your vehicle.

    Faulty Scanner

    The first possibility is that your OBD II scanner itself is faulty. This could be due to a hardware or software issue. It’s essential to verify that your scanner is working correctly before concluding that the problem lies with your vehicle.

    How to Test:

    • Try using the scanner on another vehicle to see if it communicates properly.

    • Ensure the scanner’s firmware is up-to-date.

    • Check the scanner’s power source and replace batteries if needed.

    Broken OBD II Port

    Another potential cause is that the port where you plug in your scanner is damaged or malfunctioning. The OBD II port must be securely connected to the scanner for accurate data transmission.

    How to Check:

    • Ensure the connector is firmly plugged into the OBD2 port.

    • Inspect the connector for any visible damage or debris.

    • Clean the connector and port if necessary.

    Disconnected Wires

    The wires connecting the OBD II port to the vehicle’s computer may be disconnected or damaged, preventing communication.

    Steps to Diagnose:

    • Inspect the wiring behind the OBD2 port for any signs of damage or disconnection.

    • Use a wiring diagram to ensure all connections are intact.
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    Dead Battery

    If your car battery is dead or low, the OBD2 scanner may not be able to establish a connection.

    How to Check:

    • Test the vehicle’s battery using a voltmeter. A healthy battery should read around 12.6 volts when the vehicle is off.

    • Replace the battery if it’s weak or failing.

    • Many professional auto mechanics use the ANCEL BST600,an adavanced battery tester, to check the condition of their car's battery for only $139.99.

    Blown Fuses

    Blown fuses can also prevent your OBD II scanner from communicating with the vehicle. The OBD2 system is powered by a fuse, and if this fuse is blown, the scanner won't receive power.

    How to Check:

    • Locate the fuse box in your vehicle and identify the fuse related to the OBD II system.

    • Check the fuse to see if it’s blown and replace it if necessary.

    Troubleshooting Steps

    To diagnose and resolve OBD II communication issues, you can perform a series of tests using a Digital Multimeter (DMM).

    Test 1: Check Ground Voltage

    Step-by-Step Instructions:

    1. Connect the positive lead of the DMM to DLC pin 4 and the negative lead to the negative terminal of the battery. If needed, use jumper wires to extend the leads.

    2. With the ignition turned on, the voltage drop across the ground terminal should be 0.1 Volts or less.

    Test 2: Repeat Ground Voltage Check

    Step-by-Step Instructions:

    1. Repeat the procedure for DLC pin 5.

    2. Note: If there is an open circuit or high ground resistance, the PCM will not enter diagnostic mode.

    Test 3: Voltage Check on DLC Pin 2

    Step-by-Step Instructions:

    1. Perform a voltage check on DLC pin 2. (Note: This does not apply to a Mazda MPV.)

    2. Connect the positive lead of the DMM to the class 2 Serial Data pin, and the negative lead to either pin 4, pin 5, or a known good ground.

    3. With the ignition on, the voltage on the class 2 serial data pin should be 5V or fluctuate between 3.5V and 5.0V. If the circuit is open, the PCM will be unable to transmit data to the scan tool.

    Test 4: Check ECM Voltage Supply

    Step-by-Step Instructions:

    1. Connect the positive lead of the DMM to the positive terminal of the DLC battery.

    2. Connect the negative lead of the DMM to the ground terminal of the battery.

    3. The DMM should display the battery voltage. This test is performed to check the voltage supply to the ECM, as a lack of power could be due to a blown lighter fuse.

    Test 5: Check DLC Bias Terminals

    Step-by-Step Instructions:

    1. Connect the positive lead of the DMM to either of the DLC bias terminals.

    2. Connect the negative lead of the DMM to the ground terminal.

    3. With the ignition key turned ON and no bus activity, the BUS+ should read 0V and the BUS- should read 5V.

    4. With the ignition key turned ON and bus activity present, the voltage reading should vary from 0 to 5V, depending on the level of bus activity.

    Additional Notes

    Open Circuits and Ground Issues

    An open circuit or high ground resistance means that the PCM will never enter diagnostic mode. Ensure all circuits are intact and grounds are properly connected.

    PCM Power Supply

    The PCM receives battery voltage (B+) through a fuse or fusible link. Check for an open circuit. An open ground or power (B+) circuit on a fuel-injected engine removes power from the ECM and prevents the engine from starting.

    Replacing the PCM

    If all else fails, the PCM might be faulty. Replacing the PCM can resolve persistent communication issues. Ensure the new PCM is compatible with your vehicle and properly installed.

    Preventive Measures

    Regular Maintenance

    Maintaining your vehicle’s health can prevent many OBD II communication issues.

    • Regularly check and clean the OBD II port.

    • Keep your vehicle’s software and firmware updated.

    • Perform routine checks on the vehicle’s battery and electrical system.

    Proper Use of OBD II Scanner

    Using your OBD2 scanner correctly can prolong its life and ensure accurate communication.

    • Follow the manufacturer’s instructions for connecting and using the scanner.

    • Store the scanner in a safe, dry place when not in use.


    Dealing with an OBD II scanner that doesn’t communicate with your vehicle can be challenging, but by following the steps outlined in this guide, you can diagnose and resolve the issue effectively. Whether it’s checking for power issues, ensuring proper connections, or updating software, taking a systematic approach can help restore communication between your scanner and vehicle.

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