An OBD II scanner is an essential tool for modern car diagnostics. It plugs into your vehicle’s diagnostic port and retrieves trouble codes, giving you a crucial insight into what might be going wrong under the hood. But what do you do when your OBD scanner refuses to read these codes? It’s a frustrating situation, but often resolvable with a bit of troubleshooting. This guide will walk you through the common reasons why your OBD scanner might not be communicating with your car and provide step-by-step tests you can perform yourself.
Common Reasons Why Your OBD Scanner Might Fail
Several factors can prevent your OBD scanner from reading diagnostic trouble codes. Let’s explore the most frequent culprits:
1. Faulty OBD-II Scanner
The simplest explanation is sometimes the correct one: your scanner itself might be malfunctioning. Like any electronic device, OBD scanners can experience hardware or software failures. This is especially true for cheaper, lower-quality scanners.
2. Damaged OBD-II Port
The OBD-II port, also known as the Diagnostic Link Connector (DLC), is your car’s interface with the scanner. This port can be physically damaged – bent pins, corrosion, or general wear and tear can disrupt the connection.
3. Wiring Issues
The OBD-II port isn’t directly connected to your car’s computer; it’s linked by a network of wires. These wires can become disconnected, frayed, or damaged over time, interrupting the communication pathway between the scanner and your vehicle’s computer system (PCM or ECM).
4. Dead or Weak Car Battery
A healthy car battery is crucial for all your vehicle’s electrical systems, including the diagnostic system. If your car battery is dead or significantly weakened, it might not provide enough power for the OBD-II port and scanner to establish a connection.
5. Blown Fuse
A blown fuse in the circuit that powers the OBD-II port can also cut off communication. Often, the OBD-II port shares a fuse with other components, like the cigarette lighter or accessory power outlets.
6. PCM/ECM Problems
In more serious cases, the issue might lie with your car’s Powertrain Control Module (PCM) or Engine Control Module (ECM). These are the computers that manage your engine and transmit diagnostic data. If the PCM/ECM is faulty, it may not respond to the OBD scanner’s requests.
Step-by-Step Troubleshooting Guide
Before assuming the worst, follow these troubleshooting steps to pinpoint the problem. You’ll need a Digital Multimeter (DMM) for these tests.
Preliminary Checks
- Try a Different Scanner: If possible, test your vehicle with another OBD scanner. If a different scanner works, your original scanner is likely the problem.
- Visually Inspect the OBD-II Port: Look closely at your car’s OBD-II port. Are any pins bent or damaged? Is there any visible corrosion or debris inside the port?
Test 1: Ground Circuit Test (Pin 4 & 5)
This test checks the ground connections of the OBD-II port, which are essential for proper communication.
- Set up your DMM: Set your DMM to measure DC Voltage.
- Connect to DLC Pins: Connect the positive (red) lead of your DMM to pin 4 of the DLC and the negative (black) lead to the negative terminal of your car battery. Do not connect to the chassis ground; connect directly to the battery terminal. You may need jumper wires to reach the battery terminal.
- Ignition ON: Turn your car’s ignition to the “ON” position (do not start the engine).
- Read Voltage Drop: The voltage drop should be 0.1 Volts or less.
- Repeat for Pin 5: Repeat steps 2-4, but connect the positive DMM lead to pin 5 instead of pin 4.
Note: High voltage drop or no voltage indicates a ground circuit issue, preventing the PCM from entering diagnostic mode.
Test 2: Data Line Voltage Check (Pin 2)
This test verifies if the PCM is transmitting data on the designated data line. This test is generally for older systems and may not apply to all modern vehicles, especially those using CAN bus. Refer to your vehicle’s wiring diagrams. The original article mentions this test is not applicable to Mazda MPV, indicating it targets older protocols.
- Set up your DMM: Set your DMM to measure DC Voltage.
- Connect to DLC Pins: Connect the positive (red) lead of your DMM to pin 2 (Class 2 Serial Data pin) and the negative (black) lead to pin 4 or pin 5 (ground) or a known good ground.
- Ignition ON: Turn your car’s ignition to the “ON” position (do not start the engine).
- Read Voltage: The voltage should be approximately 5V or fluctuate between 3.5V and 5.0V.
Note: No voltage or a reading significantly outside this range suggests an issue with the PCM’s data transmission or an open circuit in the data line.
Test 3: Power Supply to ECM/PCM (Pin 16)
This test checks if the OBD-II port is receiving power from the car’s electrical system.
- Set up your DMM: Set your DMM to measure DC Voltage.
- Connect to DLC Pins: Connect the positive (red) lead of your DMM to pin 16 (Battery + terminal) and the negative (black) lead to pin 4 (ground) or the negative battery terminal.
- Ignition OFF or ON: This test can be performed with the ignition either ON or OFF.
- Read Voltage: The DMM should display battery voltage (approximately 12V-14V depending on battery charge and ignition state).
Note: No voltage reading indicates a lack of power to the OBD-II port, possibly due to a blown fuse or wiring issue. Check your car’s fuse box for the OBD-II port fuse (consult your owner’s manual for fuse box diagrams and locations). A common culprit is the cigarette lighter or accessory power fuse.
Test 4: Bias Voltage Check (Pins 6 & 14 or 7 & 15)
This test is relevant for vehicles using CAN (Controller Area Network) bus (pins 6 & 14) or ISO 9141 or ISO 14230-4 (pins 7 & 15). It checks for proper bias voltage on the communication lines. Consult your vehicle’s service manual to determine which pins are used for communication.
- Set up your DMM: Set your DMM to measure DC Voltage.
- Connect to DLC Pins (CAN example – pins 6 & 14): Connect the positive (red) lead of your DMM to pin 6 (CAN High) or pin 14 (CAN Low). Connect the negative (black) lead to pin 4 (ground) or the negative battery terminal.
- Ignition ON: Turn your car’s ignition to the “ON” position (do not start the engine).
- Read Voltage (No Bus Activity): With no bus activity (scanner not connected and attempting communication), BUS+ (CAN High – pin 6) should read approximately 2.5V (ideally, for CAN High and CAN Low both). BUS- (CAN Low – pin 14) should also read approximately 2.5V. Some systems may show slightly different bias voltages, but they should be roughly in the middle of the voltage range.
- Read Voltage (Bus Activity): When the scanner is connected and actively requesting data, the voltage on both CAN High and CAN Low lines should fluctuate. The differential voltage between CAN High and CAN Low is what carries the data.
Note: Incorrect or no bias voltage can indicate problems with the PCM, wiring, or termination resistors in the CAN bus system.
Additional Tips and Considerations
Check the Fuse Box
As mentioned earlier, a blown fuse is a common reason for OBD scanner communication failure. Locate your car’s fuse box (usually under the dashboard or in the engine compartment) and consult your owner’s manual for the fuse layout. Check the fuses related to the OBD-II port, accessory power, or cigarette lighter. Replace any blown fuses with the correct amperage rating.
PCM/ECM Issues
If you’ve gone through all the troubleshooting steps and your OBD scanner still won’t read codes, and the voltage tests indicate problems with the data lines or bias voltages, there’s a possibility of a faulty PCM or ECM. This is a more complex issue that might require professional diagnosis and repair.
Figure: OBD scanner screen displaying a “No Communication” error, a common symptom when the PCM or ECM is faulty and preventing vehicle diagnostics.
Note: A faulty PCM/ECM can sometimes be caused by voltage spikes or electrical issues. It’s crucial to address any underlying electrical problems before replacing the PCM/ECM.
Conclusion
When your OBD scanner won’t read codes, it can be a roadblock to diagnosing car problems. However, by systematically checking the potential causes – from a faulty scanner to wiring issues, blown fuses, and even PCM/ECM problems – you can often pinpoint the issue. These DIY troubleshooting steps, especially the voltage tests, can help you narrow down the problem and potentially save you a trip to the mechanic. If you are uncomfortable performing these tests or the issue persists, it’s always best to consult a qualified automotive technician for further diagnosis and repair.
