OBD2 Scanner Not Communicating? Common Causes and How to Fix It

On-Board Diagnostics (OBD) II scanners are indispensable tools for anyone looking to understand their vehicle’s health. These devices plug into your car’s OBD2 port to read diagnostic trouble codes (DTCs) and access a wealth of data from the engine control unit (ECU), helping you diagnose issues efficiently.

However, frustration arises when your OBD2 scanner fails to communicate with your vehicle. A “no communication” error can leave you in the dark, unable to read codes or understand what’s wrong. This guide will explore the common reasons why your OBD2 scanner might not be connecting and provide troubleshooting steps to resolve these issues, ensuring you can get back to diagnosing your car problems.

Diagnosing OBD2 Communication Failures: Possible Causes

Several factors can prevent your OBD2 scanner from establishing a connection with your vehicle’s computer system. Let’s delve into the most frequent culprits:

Faulty OBD2 Scanner: The simplest explanation is that the problem lies within the scanner itself. Like any electronic device, OBD2 scanners can malfunction due to hardware failures or software glitches. Internal components may break down, or the software could become corrupted, leading to communication errors.
Damaged OBD2 Port: The OBD2 port, also known as the Diagnostic Link Connector (DLC), is your scanner’s gateway to your car’s computer. This port can suffer physical damage. Bent or broken pins inside the port can prevent proper contact with the scanner’s connector, interrupting the communication pathway. Corrosion or debris buildup within the port can also hinder connectivity.
Wiring Problems in the OBD2 Circuit: The OBD2 port isn’t directly connected to the ECU in isolation. Wiring links the port to the vehicle’s computer system, and these wires can be the source of communication breakdowns. Disconnected, frayed, or short-circuited wires between the OBD2 port and the ECU will disrupt the data flow, preventing scanner communication.
Vehicle Battery Issues: The OBD2 system relies on a stable power supply to function correctly. A dead or significantly low car battery can prevent the OBD2 port and the ECU from powering up adequately. Without sufficient power, the scanner won’t be able to establish a connection.
ECU/PCM Malfunctions: While less common, issues with the vehicle’s Engine Control Unit (ECU) or Powertrain Control Module (PCM) can also be responsible for communication failures. If the ECU/PCM itself is faulty, unresponsive, or has power supply issues, it won’t be able to communicate with the OBD2 scanner.
Blown Fuses: The OBD2 port and ECU are often protected by fuses. A blown fuse in the OBD2 circuit or the ECU power supply can cut off power to these components, leading to a “no communication” scenario.

Step-by-Step Troubleshooting for OBD2 Communication Problems

When your OBD2 scanner won’t connect, systematic troubleshooting is crucial. Here are tests you can perform using a Digital Multimeter (DMM) to pinpoint the issue:

Test 1: Ground Circuit Integrity Check (DLC Pins 4 & 5)

This test verifies the ground connections to the OBD2 port, which are essential for proper circuit function.

Setup: Set your DMM to measure DC Voltage. Connect the positive (red) lead of the DMM to pin 4 of the DLC (OBD2 port). Connect the negative (black) lead directly to the negative terminal of your car’s battery. Important: Do not connect to chassis ground; connect directly to the battery negative terminal. Use jumper wires if needed to reach the battery terminal.
Procedure: Turn the ignition key to the “ON” position (engine off).
Reading: Observe the voltage reading on the DMM. The voltage drop should be 0.1 Volts or less.
Interpretation: A reading higher than 0.1V indicates excessive resistance in the ground circuit for pin 4.

Repeat the same procedure for DLC pin 5. Both pins 4 and 5 should have low resistance to ground.

Note: High ground resistance or an open ground circuit on these pins can prevent the PCM from entering diagnostic mode, thus blocking communication.

Test 2: Data Line Voltage Check (DLC Pin 2)

This test checks if the PCM is transmitting data signals on the designated data pin. Note: Pin 2 is often associated with SAE J1850 PWM and VPW protocols and may not be applicable in all vehicles, particularly those using CAN protocols on different pins (6 & 14). Refer to your vehicle’s wiring diagram for protocol and pin assignments.

Setup: Keep the DMM set to DC Voltage. Connect the positive (red) lead to DLC pin 2. Connect the negative (black) lead to a known good ground – you can use DLC pin 4, pin 5, or the battery negative terminal.
Procedure: Turn the ignition key to the “ON” position (engine off).
Reading: Observe the voltage reading. You should ideally see 5V, or a fluctuating voltage between 3.5V and 5.0V.
Interpretation:
- No Voltage or Very Low Voltage (close to 0V): Indicates an open circuit or a problem with the PCM’s ability to transmit data on this pin. The PCM might not be sending data to the scan tool.
- Voltage within the 3.5V-5.0V range or a steady 5V: Suggests the PCM is likely attempting to communicate on this data line.

Test 3: Power Supply Voltage Check (DLC Pin 16)

This test verifies that the OBD2 port is receiving power from the vehicle’s battery, which is essential for scanner operation and communication.

Setup: DMM remains on DC Voltage. Connect the positive (red) lead to DLC pin 16 (Battery Positive Terminal). Connect the negative (black) lead to a known good ground (DLC pin 4, 5, or battery negative terminal).
Procedure: Turn the ignition key to the “ON” position (engine off).
Reading: The DMM should display battery voltage (approximately 12V-14.5V depending on battery state and if the engine is running).
Interpretation:
- No Voltage or Very Low Voltage: Indicates a lack of power supply to the OBD2 port. This could be due to a blown fuse, wiring issue, or a problem in the power supply circuit. Check the vehicle’s fuse box for fuses related to the diagnostic port or ECU and inspect for any blown fuses.

Test 4: Bias Voltage Check (DLC Pins 6 & 14 or 7 & 15 – CAN Bus)

This test is more relevant for vehicles using CAN (Controller Area Network) communication protocols, which are common in modern cars. CAN typically uses pins 6 & 14 (High and Low CAN). Refer to your vehicle’s service information to confirm CAN bus pin assignments if applicable.

Setup: DMM on DC Voltage. Connect the positive (red) lead to DLC pin 6 (CAN High) or DLC pin 7 (ISO 9141-2 K-Line) or DLC pin 14 (CAN Low) or DLC pin 15 (ISO 9141-2 L-Line) depending on your vehicle’s protocol. Connect the negative (black) lead to a known good ground.
Procedure: Turn the ignition key to the “ON” position (engine off).
Reading and Interpretation:
- Ignition ON, No Bus Activity:
  - CAN High (Pin 6): Should read approximately 2.5V (common mode voltage).
  - CAN Low (Pin 14): Should read approximately 2.5V (common mode voltage).
  - ISO 9141-2 K-Line (Pin 7) or L-Line (Pin 15): Should read approximately 5V (idle state).
- Ignition ON, Bus Activity Present (Scanner Connected and Attempting Communication): Voltage readings on CAN High and CAN Low pins will fluctuate around the 2.5V common mode voltage as data is transmitted. On ISO 9141-2 lines, voltage will fluctuate between 0V and 5V.

If any of these voltage tests reveal abnormal readings, it indicates a problem in the corresponding circuit that needs further investigation.

Additional Tips for Resolving OBD2 Communication Issues

Check Fuses: Always start by checking the vehicle’s fuse box for any blown fuses related to the OBD2 port, ECU, or cigarette lighter (sometimes the OBD2 port shares a fuse with the lighter). Replace any blown fuses with the correct amperage rating.
Inspect the OBD2 Port: Visually examine the OBD2 port for bent or damaged pins, corrosion, or debris. Gently clean the port if necessary.
Try a Different OBD2 Scanner: If possible, test with another OBD2 scanner to rule out a faulty scanner as the cause.
Check Battery Voltage: Ensure your car battery is adequately charged. A low battery can cause communication problems.
Consult Vehicle-Specific Information: Refer to your vehicle’s repair manual or online resources for wiring diagrams and specific troubleshooting steps related to the OBD2 system for your make and model.
Professional Diagnosis: If you are unable to resolve the communication issue after these steps, it’s advisable to seek professional diagnosis from a qualified automotive technician. They have advanced tools and expertise to diagnose complex electrical and computer system problems.

By systematically troubleshooting and understanding the potential causes, you can effectively address OBD2 communication failures and get your diagnostic process back on track.