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OBD2 Trouble Codes

How to read OBD2 Diagnostic Trouble Codes (DTC)

Nowadays, cars are essentially mobile computers. And just like your PC, your vehicle also doesn’t always run smoothly. Modern cars have up to 80 control units built-in which regulate all the electronic parts and processes. When your car presents problems, they are stored in the form of OBD2 codes, which can then be read out to understand your car’s health. There are several tools and ways to diagnose your car. This first option is going to a mechanic’s garage and having the codes read out by car technicians. But a quicker and more affordable alternative is to read out the codes yourself! To do this, all you will need is an OBD2 scanner and suitable software. If you use the Carly OBD2 scanner, you just need to plug it into your car’s OBD port and open the Carly app (aka the software). After running the diagnostics check with your phone, you can see all the OBD2 codes inside your Carly app. It’s that easy! Additionally, with the OBD2 scanner, you will get more insights about your car’s health, and depending on your car model you can even code your car or carry out your own maintenance! Let’s start off by understanding how to take better care of your car by reading out OBD2 codes. We will explain to you what error codes are, how to read them out, and the best ways to do this. It is like having your own mechanic!

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What are OBD2 Trouble Codes?

Problems and malfunctions can occur in your car from time to time. In many cases, the control units can fix them on their own. Every malfunction is recorded in the form of an OBD2 code, which is also called an error, trouble, or fault code. Issues that cannot be fixed immediately by the car’s control units are often shown to the driver in the form of indicator lights on the dashboard. Depending on the vehicle model, a flashing light may indicate a very serious fault. In such a case, you should immediately turn off the engine to prevent further damage. There are different types of OBD2 codes and most of them follow the same logic and structure. Each error code consists of five digits. Each digit has a specific meaning. All codes – of which there are about 10,000 – have been standardized by the SAE J2012 and ISO 15031-6 standards.

What Does the First Digit of the OBD2 Fault Code Mean?

The first digit indicates the area of the car where the error comes from. These letters represent each area:

    P (Powertrain) = Engine and transmission

    C (Chassis) = Chassis

    B (Body) = Body

    U (User Network) = Network

What Does the Second Digit of the OBD2 Fault Code Mean?

The second digit tells you whether it is a standardized or manufacturer-specific code:

    0 = Standard code

    1 = Manufacturer specific code

What Does the Third Digit of the OBD2 Fault Code Mean?

The third digit represents the subsystem that is related to the code:

    0, 1, and 2: air and fuel metering control

    3: Ignition system

    4: Additional emission control

    5: Engine idle control

    6: On-board computer and auxiliary outputs

    7, 8, and 9: Transmission control system

    A, B, and C: Hybrid drive

What Do the Fourth and Fifth Digits of the OBD2 Fault Code Mean?

The last two digits (00-99) indicate the actual error. However, don’t worry about learning all of these error codes. The app (or the workshop) will explain the errors to you in detail.

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Free OBD2 Codes List: Meaning, Causes, Symptoms, & Fixes [EXPLAINED]

If you are dealing with manufacturer-specific faults, only specialized workshops and apps can help. Here are some examples of what error codes look like and what they mean.

TOP 10 most common OBD2 trouble codes

Other common OBD2 trouble codes

Which OBD2 Codes are Recorded?

Errors can occur once or repeatedly in different areas of your car. If the control unit of your car can correct the error itself, it is registered in the error memory, but the driver is not necessarily informed. However, there are errors that cannot be corrected by the control units since the car needs manual repair. A simple example of an OBD2 code that cannot be solved by the car itself is the oil level. If the oil level falls below a certain mark, the driver receives a warning and the error is recorded. It works the same for the brake fluid or the wiper water. There are also more complex errors that the driver isn’t immediately alerted to, but that will need to be addressed eventually. For instance, many electronics of the car are exposed to strong external influences such as high temperatures, moisture, vibrations, wear, or corrosion, so eventually, a part may need replacement. Therefore, it is recommended to read your car’s errors regularly to learn more about its health status. You might be wondering if many errors occur in your car? And if so, which errors occur, and how often? With the help of an OBD2 scanner and its mobile app, you can easily get all the answers to your questions. Having your own OBD2 scanner allows you to read out your car’s codes as often as you want without having to pay for it every time – which is not the case if you go to a repair shop. With the Carly scanner and app, you get back more control over your car, beyond reading and deleting error codes,! But let’s dive into that later.

How Do Electronic Control Units (ECUs) Work?

ECUs are like small computers that are often equipped with their own chips (up to quad-core processors). They control all the vehicle’s electronic systems and work according to the EVA principle: input–processing–output. ECUs receive information (input) via sensors, e.g. about the amount of gasoline injected. This value is compared with a target value. If there is a deviation between the target value and the actual value, the control unit takes corrective action. For every case, a fault code is generated and stored in the car’s system. If the control unit cannot correct the error itself, a warning light is often activated to inform the driver about the error. Moreover, all ECUs are connected via a data highway (CAN bus) and can be addressed via the so-called OBD interface using a scanner.

What Is the OBD Interface?

OBD means on-board diagnostics and was originally developed only for exhaust gas-relevant systems, in order to ensure environmentally compatible exhaust gas values. All the important data about the exhaust gas was stored in the car’s system so that car technicians could see where possible problems were. OBD2 allows the storage of the car’s health data throughout its entire lifetime in order to know the state of your car. In very concrete terms, OBD2 has the following tasks:

    Continuous monitoring of all exhaust-related components, such as the lambda sensor or the mass airflow sensor.

    Recording and reporting of all emission increases (error codes)

    Protection of the catalytic converter

    Saving data and errors in the event of faults occurring

    Provision of an interface for reading out stored and real-time operating data

    Display of errors via indicator lights

Nowadays, through the current OBD2 standard (which has been installed in all new vehicles since approx. 2001), your vehicle’s data can be easily accessed using a mobile app.

Nowadays, through the current OBD2 standard (which has been installed in all new vehicles since approx. 2001), your vehicle’s data can be easily accessed using a mobile app.

What Is an OBD2 Scanner?

An OBD2 scanner is a device that allows you to access your vehicle data. Carly’s OBD2 scanner establishes a connection between your car and a Carly mobile app. Once the connection is established, you can easily access all of your car’s data from your phone.

What Is an OBD2 Splitter?

An OBD2 splitter is a device that expands a single OBD2 port into two, allowing two products to be simultaneously plugged into the OBD2 (OBD-II) port.

What Is OBD3?

In the USA, experts working are already working on the new OBD3 standard. With OBD2, all emission-related values are recorded, warnings are displayed and errors are recorded. With this standard, however, if the driver decides not to fix a fault code, they can drive around with an “environmental fault” until their next emissions test. With OBD3, this would no longer be possible, since errors and problematic values would be transmitted directly to the responsible authority via SIM or satellite. The driver would then have a certain period of time in which they can fix the problem in order to avoid the corresponding fine.

How Does Reading Out the OBD2 Codes with an App Work?

If you choose the Carly app, reading out your car is very simple. You simply order the OBD2 scanner on the website. As soon as you have received the scanner, just plug it into your car’s OBD2 socket. Usually, you will find the socket in the vehicle’s interior below the steering column. The app connects to your vehicle via the scanner which uses Bluetooth. Your data can then be read and processed. In four very simple steps, you can read out, analyze, and also delete the error codes:

    Order the OBD2 scanner (called “Carly Universal OBD2 Scanner”) on our website

    Download the Carly app from the App Store or Google Play

    Plug the scanner into the OBD2 socket of your vehicle

    Open the Carly app (it automatically connects to the scanner) and you are done!

The app is designed to help even beginners to read out their cars. The error codes can be read out in an average of 2-3 minutes. In some cases, it may take up to 20 minutes the first time you connect and scan your car. Afterward, the errors will be displayed and explained, and safely stored inside the app. Typically, after running a scan, you will then delete the fault memory of the vehicle, in order to see which are old faults, and which are ongoing issues. After you delete the memory, you can choose to read out single ECUs again or run an entirely new scan. With the free version of the Carly app you can read out the standard OBD codes. If you want to have access to all the premium functions, you will need to purchase an app license, either for one car brand or for all brands.

Why Is It Important to Read the OBD2 Codes Regularly?

Reading out the error codes regularly is important in order to detect emerging issues before they can harm your car and avoid potentially expensive repairs. If an error appears over and over again, there must be something wrong with the system. Before a system fails, you can have it checked by a specialist in a workshop. Furthermore, problems are often interconnected. Let’s assume there are repeated errors in the lambda sensor (the sensor responsible for measuring the correct ratio of combustion air to fuel). If it is defective, this can lead to extremely high fuel consumption and higher CO2 emissions. While you have the lambda sensor fault code, you also find an error regarding the mass airflow sensor. This can tell that actually, the lambda sensor is likely fine, but there may be a leak in the intake tract for the air, caused by corrosion. Repairing this leakage is much cheaper than replacing the lambda sensor (which could be perfectly functional even though it generates an error code).

How to Clear Error Codes

After you read out the error codes and fixed them, you can delete your errors. Generally, you can also delete codes without fixing them, as it may be something that fixes itself. In fact, many errors are deleted automatically from the control unit's error memory in regular intervals. These errors can occur, because of temporary conditions or communication errors. But sometimes a code doesn’t delete itself, so you can do it manually. For example, let’s say in January an error code is stored inside your transmission. When you run diagnostics in January, you could clear your error memory already. If you then run diagnostics again a couple of days later and the same error appears again, you should definitely keep an eye on this issue. If this error occurs again the week after, you should consider taking a closer look since it might need repair. The time between your first and second diagnostics sessions depends on the type and severity of the error. Engine errors require immediate attention, whereas multimedia errors may have no immediate effect on your car's condition.

Can I Delete a Service Light Using a Diagnostics Tool?

Your oil service is due and the service reminder is annoying you on your display. Sound familiar? Typically, service entries are not stored inside your fault memory and therefore will not be read during diagnostics. However, if you want to reset your service interval or see logged maintenance data regarding your car, you can use the Carly Service function. This helps you to do maintenance services yourself and save a lot of time and money. The availability of this function always depends on your brand and model. Please check if the service function is available also for your model.

What Else Can You Do With Carly?

With Carly, you have access to other functions besides reading the OBD2 codes. Here are a few examples of Carly’s features. Keep in mind, the functions available always depend on your brand and model. Please check if your preferred function is available also for your model.


Modern vehicles are basically drivable computers with up to 80 control units. Using Carly you can customize up to hundreds of configurations in your control units in order to give your car an individual appeal. For example, you can change the logos on your screen, deactivate the start/stop function, activate acoustical or visual signals (Welcome lights) when you close or open your car, or enable watching videos while driving.

Vehicle Diagnostics

Reading out OBD2 codes is only one part of the extensive diagnostics that you can perform with the Carly app. Carly helps you not only to read out generic OBD code but also provides diagnostics on the manufacturer level. After diagnosing your error codes, Carly evaluates the severity of the issue, provides helpful links and repair guides to fix your car. The app is designed to be user-friendly and helps even non-professionals to interpret and understand errors.

Additional Maintenance Services

Carly also helps you with other maintenance tasks. For example, you can also register new batteries or switch your electronic parking brake between driving and service mode. Another maintenance feature is the diesel particulate filter (DPF) reset. Check out the critical values of your DPF, request a DPF regeneration, and track the entire process. For cars with automatic transmission, you can also reset learning values. After buying a used car, this helps to adapt the transmission to your personal driving style.

Used Car Check

Up to 30% of all used vehicles have the mileage tampered with to try and fetch a higher price. Typically, the mileage is only tampered with in your tacho, but there are other control units that are storing the actual mileage. Carly has a special function to find the actual mileage of your vehicle by reading out several control units and comparing the mileage values. This helps to identify manipulation and discrepancies. Detecting mileage manipulation can save you a lot of money and trouble.

Digital Garage

All of your Carly data is collected, and safely stored in your Carly cloud. This helps you to have your data available for further analysis. You can also download PDF reports and send them by email (e.g. to a new buyer or to a garage). Now you are fully equipped to understand how OBD2 codes work, and with your personal OBD2 scanner you can keep your car’s health under control! It’s like having your own mechanic!

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