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How to Read OBD2 Codes: A Simple Guide for Beginners

Updated at 11 Mar, 2024

— Unlock the secrets of your vehicle's health with our step-by-step guide on how to read OBD2 codes, making vehicle diagnostics accessible for everyone.

How to Read OBD2 Codes: A Simple Guide for Beginners

Understanding your vehicle's health is crucial, and the On-Board Diagnostics system version 2 (OBD2) is your gateway to mastering this skill. This guide empowers you to read OBD2 codes, offering insights into maintaining and troubleshooting vehicle issues efficiently.

Understanding OBD2

OBD2 systems monitor your vehicle's performance, ensuring it operates efficiently while adhering to emissions standards. For a deep dive into OBD2’s functionality and its pivotal role in modern vehicles, explore our comprehensive blog post.

The Tools You'll Need

Embarking on the journey of reading and interpreting OBD2 codes requires not just knowledge and patience but also the right set of tools. Here's a rundown of what you'll need to get started:

AutoPi TMU CM4

AutoPi device products beside each other

The AutoPi TMU CM4 stands out as a premier tool in vehicle diagnostics and telematics. It's not just an OBD2 scanner; it's a comprehensive vehicle management device that offers features such as real-time data monitoring, GPS tracking, and even remote control capabilities. Its compatibility with a wide range of vehicles and easy-to-use interface makes it a top choice for professionals and enthusiasts alike.

AutoPi Cloud

The AutoPi cloud platform shown as widgets

An integral part of the AutoPi ecosystem is the AutoPi Cloud. This cloud-based platform allows users to access the data collected by the AutoPi TMU CM4 in an intuitive, user-friendly environment. From diagnostics to real-time vehicle monitoring, the AutoPi Cloud provides a centralized location for all your vehicle management needs. It enables detailed analysis of OBD2 codes, offering insights into vehicle health and performance that are invaluable for proactive maintenance and troubleshooting.

OBD2 Extension Cable

The OBD2 extension cable product

While the AutoPi TMU CM4 plugs directly into your vehicle's OBD2 port, certain situations may necessitate the use of an OBD2 extension cable. This cable offers flexibility in device placement, preventing potential interference with the driver's space or comfort. It also reduces wear and tear on the vehicle’s OBD2 port by minimizing the need to repeatedly plug and unplug the device, ensuring a secure connection every time.

Power Cable

The OBD2 power cable product

The AutoPi TMU CM4 is powered directly from your vehicle's OBD2 port, but in some cases, you might need an external OBD2 power source. A dedicated power cable allows you to connect the device to an alternative power source, ensuring continuous operation even when the vehicle is turned off. This is particularly useful for running diagnostics, performing software updates, or using the AutoPi TMU CM4's features without draining the vehicle's battery.

Reading OBD2 Codes - Step by Step

Reading OBD2 codes is a straightforward process that can provide a wealth of information about your vehicle's condition. Here's how to do it, using the AutoPi TMU CM4 as our example scanner:

  1. Locate the OBD2 Port: The OBD2 port is typically located under the dashboard, near the steering column. Look for a 16-pin connector that may be covered by a protective cap. Find it here.

  2. Connect the AutoPi TMU CM4: Insert the AutoPi TMU CM4 device into the OBD2 port until it's securely connected. Ensure the connection is firm to avoid data transmission errors.

  3. Start Your Vehicle: Turn on the engine to power up the AutoPi device. The AutoPi TMU CM4 uses the vehicle’s power, so the engine must be running for the device to operate.

  4. Access AutoPi Cloud: Using a computer or smartphone, log in to your AutoPi Cloud account. This platform is where all the diagnostic data from your vehicle will be accessible.

  5. Interpret the Codes: After logging in to your AutoPi Cloud account, click on 'Vehicles' in the navigation bar on the left. Select the device that is connected to the vehicle for which you want to check diagnostics. In the new navigation bar that appears, click on 'Diagnostics'. Here, you'll see a list of OBD2 codes retrieved from your vehicle, along with their definitions and suggested corrective actions. This feature allows you to not just identify but understand the issues your vehicle is facing. Try the AutoPi Demo for free.

  6. Address the Issues: Based on the diagnostics, take the necessary actions to resolve the issues identified. This might involve simple fixes you can do yourself or require professional mechanic services.

  7. Clear the Codes: After resolving the issues, you can clear the diagnostic codes directly from the AutoPi Cloud dashboard. It’s important to ensure that the underlying problems are fully addressed before clearing the codes to prevent recurrence.

Take a look at our getting started guide.

Deciphering OBD2 Codes

OBD2 codes are standardized and begin with a letter followed by four numbers, e.g., P0301. The first letter indicates the system related to the issue (e.g., P for Powertrain). The numbers identify the specific issue, such as a misfire in cylinder 1 in the case of P0301.

Here's a list of common OBD2 codes you might encounter:

P0300 Powertrain Random/Multiple Cylinder Misfire Detected Indicates multiple cylinders misfiring, affecting engine performance.
P0420 Powertrain Catalyst System Efficiency Below Threshold Suggests the catalytic converter isn't operating efficiently, possibly due to wear or damage.
P0171 Powertrain System Too Lean (Bank 1) The air-fuel mixture is too lean, indicating an imbalance in the engine's operation.
P0128 Powertrain Coolant Temperature Below Thermostat Regulating Temperature The engine's coolant is not reaching the required temperature, possibly due to a faulty thermostat.
P0442 Powertrain Evaporative Emission Control System Leak Detected (Small Leak) A small leak in the EVAP system, which controls fuel vapor emissions.
C0035 Chassis Left Rear Wheel Speed Sensor Circuit A problem with the wheel speed sensor can affect the ABS system and safety.
C1214 Chassis Brake Control Relay Contact Circuit Open Indicates a malfunction in the brake control relay, potentially affecting braking performance.
C0036 Chassis Right Front Wheel Speed Sensor Circuit A fault in the speed sensor that can lead to inaccuracies in ABS functioning and vehicle stability.
C0561 Chassis ABS Brake Control Module System Suggests an issue within the ABS system, affecting its ability to properly control braking.
C1210 Chassis Brake Fluid Pressure Sensor Circuit A problem detected in the brake fluid pressure sensor, critical for braking accuracy.
B0020 Body Front Passenger Side Deployment Loop Resistance High Indicates a high resistance in the passenger side airbag deployment loop, potentially affecting its deployment.
B1000 Body Electronic Frontal Sensor Data Malfunction in the electronic frontal sensors, affecting airbag systems and safety.
B1200 Body Climate Control Push Button Circuit Open A fault in the climate control buttons, affecting the system's operability.
B1325 Body Oil Pressure Sensor Circuit Indicates a malfunction in the oil pressure sensor, critical for engine health monitoring.
B1422 Body Seat Belt Pretensioner Deployment Control Circuit A fault detected in the seat belt pretensioner circuit, affecting its readiness in crashes.
U0100 Network Lost Communication with ECM/PCM A Communication issues with the Engine Control Module, affecting overall vehicle management.
U0121 Network Lost Communication with Anti-lock Brake System (ABS) Control Module Communication problems with the ABS module, potentially impacting braking safety.
U0073 Network Control Module Communication Bus A Off Indicates a general communication failure in the vehicle's network, affecting multiple systems.
U0140 Network Lost Communication with Body Control Module A lack of communication with the Body Control Module, which manages various body-related functions.
U0401 Network Invalid Data Received from Engine Control Module (ECM) Incorrect data being received from the ECM, possibly leading to issues in engine performance.

From Codes to Action - What Next?

Not all codes signify immediate problems, but they do provide vital clues. Use online forums, repair manuals, or professional advice to determine the urgency of the issue and whether you can tackle it yourself or need expert intervention.

The Dos and Don'ts of OBD2 Diagnostics

Navigating the world of OBD2 diagnostics can be both empowering and daunting. To ensure you get the most out of your diagnostic efforts while safeguarding your vehicle, here are some essential dos and don'ts:


  • Do Regular Checks: Make it a habit to periodically check your vehicle with your OBD2 scanner. Early detection of issues can save time and money on repairs.

  • Do Keep a Log: Maintain a log of the codes and their corresponding issues and resolutions. This record can be invaluable for troubleshooting future problems or when consulting with a professional mechanic.

  • Do Update Your Tools: Ensure your diagnostic tools, especially devices like the AutoPi TMU CM4, are updated with the latest software. Updates can enhance functionality, improve accuracy, and introduce new features.

  • Do Research Thoroughly: When you encounter an OBD2 code, use resources like AutoPi Cloud to research it comprehensively. Understanding the broader context of a problem can lead to more effective solutions.

  • Do Prioritize Safety: If an OBD2 scan reveals issues that could compromise vehicle safety (e.g., braking system faults, airbag issues), address them immediately. Safety should always be your top priority.


  • Don’t Ignore Codes: Even if the vehicle seems to be running fine, do not ignore OBD2 codes. They are an early warning system for potential issues.

  • Don’t Clear Codes Prematurely: Resist the temptation to clear codes without addressing the underlying problem. Clearing codes does not fix the issue and can make diagnosing recurring problems more challenging.

  • Don’t Overlook Basic Maintenance: Regular maintenance, such as oil changes and tire rotations, can prevent many issues that might trigger OBD2 codes. Don’t rely solely on technology to keep your vehicle in good shape.

  • Don’t Guess and Replace Parts: Avoid the costly mistake of replacing parts based on guesswork. Use the diagnostic information to investigate issues thoroughly before deciding on replacements.

  • Don’t Disregard Professional Help: While OBD2 tools empower you to understand and fix many issues, some problems require professional expertise. Recognizing when to seek help is crucial to prevent causing further damage to your vehicle.

Final Advice

Armed with the right approach and tools, OBD2 diagnostics can significantly enhance your vehicle maintenance routine. By adhering to these dos and don'ts, you’ll ensure that you’re not only effectively diagnosing issues but also contributing to the longevity and safety of your vehicle.

Key Takeaway

The journey through OBD2 diagnostics leads us to one indispensable tool: the AutoPi TMU CM4. More than just a scanner, it revolutionizes vehicle management with advanced telematics, real-time monitoring, and the AutoPi Cloud platform for unparalleled insights into your vehicle's health.

With the TMU CM4, proactive maintenance and detailed diagnostics are in your hands, enhancing vehicle performance and longevity. It's an investment in smarter vehicle management.

Discover the AutoPi TMU CM4's full capabilities and how it can transform your approach to vehicle diagnostics in our shop. Elevate your diagnostic game with AutoPi — where technology meets automotive excellence.

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