OBD2 Scanner Won't Read? Troubleshoot the Communication Issues

Using an OBD2 scan tool to check your vehicle’s computer system should be a simple process. You plug it in, turn the key, and get the data you need. However, sometimes you might encounter a frustrating issue: your OBD2 scanner won’t read, displaying a “No Communication” error. This can be perplexing, especially when you’re trying to diagnose a car problem.

This article will explore the common reasons why your OBD2 scanner might fail to connect and communicate with your car’s computer (ECU). We’ll provide troubleshooting steps to help you pinpoint the problem and get your scanner working, so you can get back to diagnosing and fixing your vehicle.

Understanding OBD2 Communication Issues

In most instances, connecting an OBD2 scanner is indeed straightforward. Vehicles manufactured from 1996 onwards in the US (and similar dates in other regions) are mandated to use the standardized OBD2 protocol for emissions and engine diagnostics. This standardization is designed to make it easy for technicians and car owners to access important vehicle data.

However, the “OBD2 won’t read” problem is more common than many people realize. Several factors can disrupt the communication pathway between your scan tool and your car’s computer. These issues can range from simple oversights to more complex technical problems within the vehicle or the scan tool itself.

Some of the typical culprits for OBD2 communication failures include:

Incorrect ignition key position: The car’s ignition needs to be in the correct “Run” position for the OBD2 port to be active.
Voltage problems at the OBD2 connector: The scanner needs power from the car’s battery through the OBD2 port to function.
Compatibility issues: While OBD2 is standardized, older or some specialized vehicles might use different protocols, or the scan tool might not be fully compatible.
ECM/PCM (Engine Control Module/Powertrain Control Module) issues: The car’s computer itself might be in a state where it’s not responding to communication requests.
Data problems: Issues with how the scanner requests and interprets data can also lead to communication errors.
Software glitches: Both in the car’s computer and the scan tool, software problems can sometimes cause communication breakdowns.

Let’s delve into each of these potential reasons and explore how to troubleshoot them when your OBD2 scanner won’t read.

Common Reasons Why Your OBD2 Scanner Won’t Read

OBD2 Scan Tool Compatibility

While the OBD2 standard aims for universality, compatibility issues can still arise.

Protocol Mismatches: The most fundamental compatibility problem is a protocol mismatch. While most modern cars use standardized OBD2 protocols, older vehicles or those from specific manufacturers might utilize proprietary or pre-OBD2 protocols like ALDL, OBD1, MOBD, or MUTT. A standard OBD2 scanner is designed for OBD2 and will not be able to communicate with these older systems.

If you are working on a vehicle manufactured before the mid-1990s or are unsure about its OBD protocol, you may need a specialized scan tool. Professional-grade multi-system diagnostic scanners often come with broader protocol coverage, including OBD1 and various manufacturer-specific protocols. These tools are designed to handle a wider range of vehicles and systems beyond just the standard OBD2 engine and emissions diagnostics.

Alt text: A professional-grade OBD1 and OBD2 diagnostic scanner kit with multiple connectors, designed for broad vehicle compatibility.

Software and Database Limitations: Even with OBD2 compliant vehicles, some basic OBD2 scanners might have limitations in their software or vehicle database. They might not be updated to recognize very new models or specific vehicle systems. For comprehensive diagnostics, especially on newer cars with advanced electronics, a more advanced scan tool with regular software updates is recommended.

Before assuming your scanner is faulty, double-check your vehicle’s year, make, and model to confirm OBD2 compliance. If you suspect a protocol mismatch, consult your vehicle’s service manual or consider using a multi-system scanner.

Ignition Key not in Run Position

This might seem obvious, but it’s a very common oversight: the ignition key must be in the “Run” position for the OBD2 port to be fully powered and for the car’s computer systems to be active and ready to communicate. Simply having the key in the “Accessory” position might not be sufficient.

Correct Ignition Procedure:

Insert the key into the ignition.
Turn the key to the “Run” position. This is usually the position just before you engage the starter to crank the engine. In the “Run” position, the dashboard lights should illuminate, and the car’s systems are powered on.
Do NOT start the engine yet. For initial OBD2 connection, it’s generally recommended to have the ignition in the “Run” position, but the engine off, unless your scan tool specifically instructs otherwise.

System Boot-Up Time: Modern vehicles are complex networks of electronic modules. When you turn the ignition to “Run,” these modules need time to power up and initialize their systems. This “boot-up” process can take several seconds. If you plug in your OBD2 scanner and try to connect immediately, the modules might not be fully online and ready for communication.

Wait for System Initialization: A good practice is to wait a short period after turning the key to “Run” before attempting to connect with your scan tool. Wait until:

Dashboard chimes or warning sounds have stopped.
Most of the initial dashboard lights have stabilized (some might remain on depending on vehicle condition).
Any initial system checks or displays on the instrument cluster have completed.

This waiting period ensures that the vehicle’s computer systems, including the ECM/PCM, are fully powered and ready to communicate with your OBD2 scanner. If you’re still having trouble, double-check your scan tool’s instructions, as some might have specific recommendations regarding ignition position and timing.

No Voltage or Low Voltage on Connector

OBD2 scanners rely on power supplied through the OBD2 port in your vehicle. According to the OBD2 standard, pin 16 of the OBD2 connector must provide 12-volt power, and pins 4 and 5 are designated for ground. If there’s no power or insufficient voltage at pin 16, your scan tool won’t be able to power on or communicate.

Checking for Voltage at the OBD2 Port:

You will need a voltmeter to perform this check.

Set your voltmeter to measure DC voltage. The 20V DC range is usually appropriate for automotive voltage checks.
Turn the ignition key to the “Run” position (engine off).
Locate the OBD2 port in your vehicle (typically under the dashboard on the driver’s side).
Identify pin 16 and pins 4/5 on the OBD2 connector. Pinout diagrams for OBD2 ports are readily available online if you’re unsure.
Connect the RED (positive) lead of your voltmeter to pin 16.
Connect the BLACK (negative) lead of your voltmeter to pin 4 or pin 5.
Read the voltage displayed on the voltmeter.

Interpreting Voltage Readings:

Normal Reading: You should ideally see a reading of 12 volts DC or very close to it (e.g., 11.5V to 14.5V, depending on battery charge and vehicle system state). This indicates that power is being supplied to the OBD2 port as expected.
Low Voltage: If the reading is significantly below 11 volts, it suggests a low voltage issue. Some scanners are very sensitive to voltage and may not function reliably below a certain threshold. Possible causes of low voltage include:
- Weak Battery: Check your car’s battery condition. A low battery can cause voltage drops throughout the vehicle’s electrical system.
- Charging System Problems: If the vehicle’s charging system (alternator, voltage regulator) is malfunctioning, it might not be adequately charging the battery, leading to low voltage.
No Voltage (0 volts): A reading of 0 volts indicates a complete lack of power to pin 16. The most likely cause of this is a blown fuse.

Troubleshooting Voltage Issues:

Check Fuses: The power supply to the OBD2 port is usually protected by a fuse. This fuse often also powers other accessory circuits in the vehicle.
1. Locate the fuse panel(s) in your vehicle. Refer to your owner’s manual for fuse box locations and diagrams.
2. Consult your owner’s manual or fuse box diagram to identify the fuse that powers the accessory circuits or the OBD2 port (it might be labeled “Accessory,” “Cigar Lighter,” “OBD,” or similar).
3. Inspect the fuse. A blown fuse will typically have a broken filament inside.
4. Replace the blown fuse with a new fuse of the SAME AMPERAGE RATING. Never use a fuse with a higher amperage rating, as this can damage electrical components.
5. Re-check voltage at pin 16 of the OBD2 port after replacing the fuse.
6. Try connecting your OBD2 scanner again.

Alt text: A typical automotive fuse box with fuses of various colors and amperage ratings, highlighting the importance of checking and replacing fuses for electrical troubleshooting.

Inspect Battery and Charging System: If the voltage is low but not zero, and the fuses are okay, have your car’s battery and charging system tested by a professional mechanic or auto parts store. A weak battery or charging system problem can cause a wide range of electrical issues, including OBD2 communication failures.

ECM Communications Hung Up

Occasionally, the car’s Engine Control Module (ECM) or Powertrain Control Module (PCM) can get into a state where it’s functioning enough to keep the engine running, but it becomes unresponsive to external communication requests, including those from an OBD2 scanner. This is sometimes referred to as the ECM being “hung up” or in a “communication lock-up.”

Symptoms of an ECM Communication Hang-Up:

Engine runs normally: The vehicle may drive and operate without any obvious performance problems.
“OBD2 won’t read” error: Your scan tool fails to establish a communication link with the ECM and displays a “No Communication” message.
All other checks are normal: You’ve verified ignition position, voltage at the OBD2 port, and scan tool compatibility, but still cannot connect.

Rebooting the ECM to Restore Communication:

Just like computers, car ECUs can sometimes benefit from a reboot. Disconnecting and reconnecting the car’s battery is a common method to reset the ECM and potentially clear a communication hang-up.

ECM Reboot Procedure (Battery Disconnect):

Important Safety Precautions:

Consult your vehicle’s owner’s manual BEFORE disconnecting the battery. Some vehicles have specific procedures for battery disconnection to avoid damaging electronic systems or losing important settings (like radio presets or security system codes).
Make sure the ignition is turned OFF and the keys are removed from the ignition.
Locate the car battery, usually under the hood or sometimes in the trunk or under the rear seat.
Identify the negative (-) and positive (+) battery terminals.
Use a wrench to loosen the nut on the NEGATIVE (-) battery terminal clamp first.
Carefully remove the negative terminal clamp from the battery post. Make sure it’s fully disconnected and not touching the battery post.
Next, loosen and remove the POSITIVE (+) battery terminal clamp.
Wait for a few minutes (around 5-10 minutes) to allow any residual electrical charge in the system to dissipate.
Optional: Press and hold the brake pedal for about 30 seconds. This helps to discharge any capacitors in the vehicle’s electrical system more quickly.
Reconnect the POSITIVE (+) battery terminal clamp first. Tighten the nut securely.
Reconnect the NEGATIVE (-) battery terminal clamp. Tighten the nut securely.
Ensure both terminal clamps are firmly attached and make good contact.

After Reconnecting the Battery:

Turn the ignition to the “Run” position (engine off).
Try connecting your OBD2 scanner again.

In many cases, rebooting the ECM by disconnecting the battery can resolve a communication hang-up and allow your OBD2 scanner to connect successfully. If this doesn’t work, or if the problem recurs frequently, there might be a more underlying issue with the ECM or vehicle’s electrical system that requires further professional diagnosis.

Missing Data or Invalid Data

Even when an OBD2 scanner successfully connects to the vehicle, you might still encounter issues related to data retrieval. Sometimes, the scanner might report “No Data” for certain parameters (PIDs – Parameter IDs) or display obviously incorrect or illogical data values. This can also be perceived as “OBD2 won’t read” in terms of getting meaningful information.

PID Scanning and Vehicle-Specific Data:

Modern vehicles have a vast number of sensors and systems, and the data available through the OBD2 port (or enhanced diagnostic protocols) is extensive. To manage this complexity, OBD2 scanners typically need to “query” the vehicle’s ECM to determine which specific PIDs are supported and valid for that particular vehicle. This process is known as PID scanning or PID discovery.

Potential Data-Related Issues:

Incorrect PID List: Some basic OBD2 scanners might assume that the last vehicle they connected to is still the current vehicle. If you switch to a different vehicle, the scanner might be using an outdated PID list from the previous car. This can lead to “No Data” or invalid data readings because the scanner is requesting PIDs that are not supported by the current vehicle’s ECM.
Failure to Scan PIDs: Some simpler scan tools might not automatically perform a PID scan every time they connect. They might rely on VIN (Vehicle Identification Number) input or pre-loaded generic PID lists, which might not be completely accurate for every vehicle.
Software Glitches: Occasional software glitches in the scan tool or ECM can also lead to data transmission errors or misinterpretations, resulting in invalid data displays.

Resolving Data Issues:

Force PID Scan: Most OBD2 scanners, especially mid-range and professional models, have a function to manually initiate a PID scan or “Query PIDs” process. Look for this option in your scanner’s menu (it might be under “Setup,” “Vehicle Info,” or similar). Running a PID scan ensures that the scanner obtains the correct list of supported PIDs directly from the vehicle’s ECM for the current session. This process might take a minute or two to complete.
Verify Vehicle Selection: Double-check that you have correctly selected the year, make, and model of your vehicle in the scan tool’s settings. Incorrect vehicle selection can lead to the scanner using the wrong PID database.
Software Updates: Ensure that your OBD2 scanner’s software is up-to-date. Updates often include improved vehicle coverage, more accurate PID databases, and bug fixes that can address data-related issues.

Importance of Accurate Data: Taking the time to perform a PID scan or ensure proper vehicle selection is crucial for obtaining accurate and reliable diagnostic data from your OBD2 scanner. It’s a worthwhile step to ensure that the scanner is reporting the correct information provided by the vehicle’s ECM.

Advanced Troubleshooting and When to Seek Professional Help

While the troubleshooting steps outlined above can resolve many common “OBD2 won’t read” scenarios, there are situations where the problem might be more complex and require advanced diagnostics or professional assistance.

Beyond Basic OBD2: It’s important to remember that basic OBD2 diagnostics primarily focuses on engine and emissions-related systems. While very useful for many issues, OBD2 has limitations. For problems related to other vehicle systems like the transmission, ABS, SRS (airbags), chassis, or body electronics, a basic OBD2 scanner might not provide comprehensive access or troubleshooting capabilities. Advanced diagnostic scan tools and professional-level equipment are often needed for in-depth analysis of these systems.

When to Consider Professional Help:

Persistent “No Communication” after Troubleshooting: If you have diligently followed all the troubleshooting steps (ignition position, voltage checks, ECM reboot, compatibility checks) and your OBD2 scanner still consistently fails to connect, there might be a more serious underlying problem. This could involve:
- Faulty OBD2 Port Wiring: Damage to the wiring or connector pins of the OBD2 port itself.
- ECM/PCM Malfunction: A more severe issue with the car’s computer that goes beyond a simple “hang-up.”
- CAN Bus or Communication Network Problems: Modern vehicles use complex communication networks (like CAN bus) to allow various modules to communicate. Problems in these networks can disrupt OBD2 communication.
Intermittent Communication Issues: If the OBD2 scanner connects sometimes but frequently loses connection or provides unreliable data, this can also indicate a wiring problem, loose connections, or a failing module.
Unfamiliar or Complex Symptoms: If you are not comfortable with automotive electrical troubleshooting or are facing complex or unusual vehicle symptoms, it’s always best to consult a qualified mechanic or automotive diagnostic specialist.

The Value of Expertise and Advanced Tools: Diagnosing complex “OBD2 won’t read” issues or problems beyond basic engine/emissions often requires:

Specialized Diagnostic Equipment: Professional technicians use advanced scan tools, oscilloscopes, circuit testers, and other equipment that go far beyond basic OBD2 scanners.
Technical Knowledge and Experience: Understanding vehicle electrical systems, communication protocols, and diagnostic procedures is crucial for accurately pinpointing and resolving complex issues.
Access to Vehicle-Specific Information: Professional technicians have access to service manuals, wiring diagrams, technical service bulletins, and other resources that are essential for in-depth diagnostics.

Choosing the Right Diagnostic Support: Just as choosing a reputable scan tool supplier is important, having access to reliable diagnostic support and professional expertise is invaluable when you encounter challenging vehicle problems. When basic troubleshooting fails, seeking help from qualified professionals ensures accurate diagnosis and effective repairs.

Conclusion

Encountering an “OBD2 won’t read” error can be frustrating, but by systematically troubleshooting the common causes, you can often identify and resolve the issue. Remember to check OBD2 compatibility, ignition key position, voltage at the OBD2 port, and consider rebooting the ECM. For data-related problems, ensure you perform a PID scan and verify vehicle selection in your scan tool.

While OBD2 scanners are powerful tools for vehicle diagnostics, they are not a magic bullet. Advanced troubleshooting and professional expertise are sometimes necessary for complex issues. Choosing reputable suppliers for your diagnostic equipment and seeking professional help when needed are key to effectively maintaining and repairing modern vehicles.

OBD2 Scanner Won’t Read? Troubleshoot the Communication Issues