What Is the Purpose of OBD2? Unlocking Your Car's Diagnostic Secrets

Ever wondered what that mysterious port under your dashboard is for? It’s likely your car’s OBD2 port, and it’s a gateway to understanding what’s really going on under the hood. In this comprehensive guide, we’ll dive into the purpose of OBD2 (On-Board Diagnostics II), explaining everything from its basic function to its advanced applications in modern vehicles.

Think of OBD2 as your car’s built-in doctor. It’s a standardized system that continuously monitors your vehicle’s vital signs. When something goes wrong, OBD2 is there to record it, helping mechanics and car owners diagnose issues quickly and efficiently.

This article provides a practical and SEO-optimized introduction to OBD2, tailored for an English-speaking audience. We’ll cover the OBD2 connector, Parameter IDs (PIDs), its connection to the CAN bus, and most importantly, the core purpose behind this essential automotive technology.

Decoding OBD2: Your Car’s Diagnostic System

OBD2, or On-Board Diagnostics II, is essentially your vehicle’s self-diagnostic and reporting system. Its primary purpose is to provide you and automotive technicians with access to your vehicle’s health information. This is achieved through a standardized digital communications port, the OBD2 connector, which allows external devices like scan tools to interface with the car’s computer.

You’ve probably already encountered OBD2 indirectly. That “check engine light” or “malfunction indicator light (MIL)” on your dashboard? That’s OBD2 in action. It’s alerting you to a potential problem detected by the car’s internal monitoring systems.

When you take your car to a mechanic because of this light, they use an OBD2 scanner. This tool plugs into the 16-pin OBD2 connector, usually located near the steering wheel. The scanner sends standardized “OBD2 requests” to the car’s computer. The car then responds with “OBD2 responses” containing valuable data. This data can include things like engine speed, fuel level, and, crucially, Diagnostic Trouble Codes (DTCs). These DTCs are essentially error codes that pinpoint specific problems within the vehicle, making troubleshooting much faster and more accurate.

Understanding the Malfunction Indicator Light (MIL) as part of the On-Board Diagnostics system.

Is My Car OBD2 Compliant?

The good news is, if you own a relatively recent car, it almost certainly supports OBD2.

OBD2 has become a standard in the automotive industry. For gasoline cars manufactured after 2001 in Europe and 1996 in the USA, OBD2 compliance is generally mandatory. For diesel cars in Europe, the requirement came into effect in 2004. While older cars might have a 16-pin connector that looks like OBD2, it may not actually support the OBD2 protocol.

To quickly check if your car is OBD2 compliant, consider these guidelines based on where and when it was originally purchased:

Timeline illustrating OBD2 compliance mandates in the US and EU regions.

If your car was bought new in the USA in 1996 or later (for cars and light trucks) or in Europe in 2001 or later (for gasoline cars) and 2004 or later (for diesel cars), it’s highly likely OBD2 compliant. Medium and heavy-duty vehicles in the US became OBD2 compliant later, in 2005 and 2010 respectively.

The Origins and Evolution of OBD2: A History of Emission Control and Diagnostics

The story of OBD2 starts in California, driven by the need for stricter emission control. The California Air Resources Board (CARB) mandated OBD for all new cars sold in California from 1991 onwards, primarily to monitor and reduce vehicle emissions.

The Society of Automotive Engineers (SAE) played a crucial role in standardizing OBD, leading to the OBD2 standard. This standardization included Diagnostic Trouble Codes (DTCs) and the physical OBD connector itself, ensuring consistency across different vehicle manufacturers. The SAE J1962 standard defined the now-familiar OBD2 connector.

The OBD2 standard was implemented gradually across the globe:

1996: OBD2 becomes mandatory in the USA for cars and light trucks.
2001: The European Union mandates OBD2 for gasoline vehicles (EOBD – European On-Board Diagnostics).
2004: EOBD is extended to diesel vehicles in the EU.
2005: OBD2 is required for medium-duty vehicles in the US.
2008: US vehicles are required to use ISO 15765-4 (CAN bus) as the communication basis for OBD2.
2010: OBD2 compliance extends to heavy-duty vehicles in the US.

Visual representation of OBD2 history emphasizing its role in emission control and data access via CAN bus.

Timeline summarizing the key milestones in OBD2 history and its evolution.

Conceptual illustration of OBD3 and the future trends in remote diagnostics and emissions testing.

Initially focused on emissions, the purpose of OBD2 has expanded significantly over time. It’s become a vital tool for general vehicle diagnostics, performance monitoring, and even for aftermarket telematics and data logging applications.

The Future of OBD2: Trends and Challenges

OBD2 is not static; it continues to evolve alongside automotive technology. Here are some important trends shaping the future of OBD2:

While legislated OBD2 was initially designed for emissions testing, electric vehicles (EVs) present a shift. Interestingly, most modern EVs are not required to support OBD2 in its standard form. In practice, very few EVs fully implement standard OBD2 requests. Instead, many use manufacturer-specific protocols like UDS (Unified Diagnostic Services). This makes accessing data from EVs more challenging, often requiring reverse engineering to decode proprietary communication rules. However, there are successful case studies in accessing data from EVs like Tesla, Hyundai/Kia, Nissan, and VW/Skoda, often through reverse engineering and OEM-specific knowledge.

Recognizing the limitations of traditional OBD2 in data richness and communication protocols, newer, enhanced standards are emerging. WWH-OBD (World-Wide Harmonized OBD) and OBDonUDS (OBD on UDS) are designed to improve OBD communication by using the UDS protocol as a foundation. These aim to standardize and enhance diagnostic capabilities for modern vehicles.

In our increasingly connected world, manual OBD2 testing for emissions feels outdated. The future points towards OBD3 and telematics integration. Imagine cars equipped with a small radio transponder, similar to those used for toll roads. OBD3 envisions cars automatically transmitting their Vehicle Identification Number (VIN) and DTCs wirelessly to a central server for remote monitoring and diagnostics.

Technologies already exist to facilitate wireless data transfer from vehicles. Devices like the CANedge2 WiFi CAN logger and CANedge3 3G/4G CAN logger can transmit CAN or OBD2 data via WiFi or cellular networks.

While offering convenience and cost savings, OBD3 raises privacy concerns related to surveillance, which pose political and societal challenges.

Originally designed for stationary emission checks, OBD2’s purpose has expanded to real-time data generation for third-party services. OBD2 dongles, CAN loggers, and other devices are widely used to access vehicle data for various applications. However, some automotive manufacturers are seeking to limit this third-party access. The argument is that OBD2 was intended for repair shops, not for building a data-driven economy around vehicle data access.

Proposals have emerged to “turn off” OBD2 functionality while driving, centralizing data collection within manufacturer-controlled servers. This would give manufacturers control over “automotive big data,” raising concerns about data ownership and access for independent service providers and consumers. While security concerns, such as car hacking risks, are cited, many see this as a commercially motivated move to control valuable vehicle data. Whether this trend will prevail remains to be seen, but it could significantly disrupt the market for third-party OBD2-based services.

Illustration depicting the trend of electric vehicles potentially limiting access to data via the OBD2 connector.

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Understanding OBD2 Standards: The Framework for Vehicle Diagnostics

On-board diagnostics, OBD2, operates as a higher-layer protocol – a language for communication with your car’s computer. CAN (Controller Area Network) bus, on the other hand, is the communication method, like a phone line. This places OBD2 alongside other CAN-based higher-layer protocols like J1939 (used in trucks and buses), CANopen (for industrial automation), and NMEA 2000 (for marine electronics).

OBD2 standards comprehensively define the OBD2 connector, the lower-layer communication protocols used, OBD2 Parameter IDs (PIDs), and much more.

These standards can be visualized within the 7-layer OSI model, a conceptual framework for network communication. Notably, both SAE (Society of Automotive Engineers, primarily US standards) and ISO (International Organization for Standardization, global standards) contribute to different layers of the OBD2 framework. This reflects the parallel development and harmonization of OBD standards in the US (SAE) and Europe (ISO). Many SAE and ISO standards are technically very similar, such as SAE J1979 and ISO 15031-5 (defining diagnostic services) and SAE J1962 and ISO 15031-3 (defining the OBD2 connector).

OSI Model illustration comparing OBD2 and CAN Bus standards, highlighting ISO 15765 and ISO 11898.

Detailed pinout diagram of a Type A OBD2 connector socket.

The OBD2 Connector: Your Physical Access Point [SAE J1962 / ISO 15031-3]

The 16-pin OBD2 connector is the physical interface that enables you to easily access data from your vehicle. It’s formally specified in the SAE J1962 standard and ISO 15031-3.

The illustration shows a typical Type A OBD2 connector pinout, also known as the Data Link Connector (DLC).

Key features of the OBD2 connector:

Location: Usually near the steering wheel, but sometimes hidden under a panel.
Pin 16: Provides battery power, often even when the ignition is off, allowing scanners to operate.
Pinout Variability: The specific pins used depend on the communication protocol employed by the vehicle.
CAN Bus Dominance: CAN bus is the most common lower-layer protocol, meaning pins 6 (CAN-High) and 14 (CAN-Low) are frequently connected.

OBD2 Connector Types: A vs. B

In practice, you might encounter two main types of OBD2 connectors: Type A and Type B. Type A is predominantly found in cars and light vehicles, while Type B is more common in medium and heavy-duty vehicles.

While both types share a similar pinout for data communication, they differ in their power supply outputs. Type A typically provides 12V power, while Type B provides 24V. Baud rates can also vary; cars often use 500 Kbps, while heavy-duty vehicles might use 250 Kbps (though 500 Kbps is becoming more common).

Visually, Type B connectors have a distinguishing interrupted groove in the middle, helping to differentiate them from Type A. Interestingly, a Type B OBD2 adapter cable is generally compatible with both Type A and Type B sockets, whereas a Type A adapter will only fit into a Type A socket.

Illustration comparing OBD2 Connector Type A and Type B, highlighting voltage differences and applications.

Diagram showing the relationship between OBD2 and CAN bus according to ISO 15765 standards.

OBD2 and CAN Bus Integration [ISO 15765-4]: Communication Foundation

Since 2008, CAN bus (Controller Area Network) has become the mandatory lower-layer protocol for OBD2 in all cars sold in the US, as mandated by ISO 15765.

ISO 15765-4, often called Diagnostics over CAN (DoCAN), defines a specific set of rules and restrictions applied to the CAN standard (ISO 11898) for OBD2 communication.

Specifically, ISO 15765-4 standardizes the CAN interface for diagnostic equipment, focusing on the physical layer, data link layer, and network layer. Key aspects include:

Bit-rate: CAN bus communication for OBD2 must use either 250 Kbps or 500 Kbps.
CAN IDs: Both 11-bit (standard) and 29-bit (extended) CAN identifiers are permitted.
CAN ID Allocation: Specific CAN IDs are reserved for OBD requests and responses to ensure standardized communication.
Data Frame Length: Diagnostic CAN frames must have a data length of 8 bytes.
Cable Length Limit: The OBD2 adapter cable should not exceed 5 meters in length to maintain signal integrity.

OBD2 CAN Identifiers: 11-bit and 29-bit Addressing

OBD2 communication relies on a request-response message structure.

In most cars, 11-bit CAN IDs are used for OBD2. Functional addressing is achieved using CAN ID 0x7DF. This ID acts as a broadcast request, asking all OBD2-compliant Electronic Control Units (ECUs) if they have data to report for the requested parameter (as defined in ISO 15765-4). Physical addressing, using CAN IDs in the range 0x7E0-0x7E7, allows for requests to be sent to specific ECUs, although this is less commonly used in generic OBD2 scanning.

ECUs respond using 11-bit CAN IDs in the range 0x7E8-0x7EF. The most common response ID is 0x7E8, typically originating from the Engine Control Module (ECM). 0x7E9, often from the Transmission Control Module (TCM), is also frequently seen.

In some vehicles, particularly vans and medium to heavy-duty vehicles, 29-bit extended CAN identifiers might be used for OBD2 communication instead of the 11-bit standard IDs.

For 29-bit CAN, the functional addressing CAN ID is 0x18DB33F1.

Responses in 29-bit CAN OBD2 use CAN IDs ranging from 0x18DAF100 to 0x18DAF1FF, with typical examples being 0x18DAF110 and 0x18DAF11E. These response IDs are sometimes represented in the J1939 PGN (Parameter Group Number) format. Specifically, PGN 0xDA00 (55808) is reserved for ISO 15765-2 within the J1939-71 standard.

Diagram illustrating the request-response frame structure in OBD2 protocol, showing PID and data parameters.

Conceptual image contrasting OBD2 standardized CAN bus data with OEM-specific proprietary CAN bus data.

OBD2 vs. Proprietary CAN Protocols: Understanding the Difference

It’s crucial to understand that your car’s ECUs rely on OEM-specific proprietary CAN protocols for their core functions, not OBD2. Each vehicle manufacturer develops its own unique CAN protocols, which can vary significantly between brands, models, and even model years. Unless you are the OEM and have access to their proprietary specifications, interpreting this raw CAN data directly is generally not possible without reverse engineering.

If you connect a CAN bus data logger to your car’s OBD2 port, you might observe this OEM-specific CAN data alongside OBD2 communication. This proprietary data is often broadcast at high rates (1000-2000 frames per second). However, in many newer vehicles, a ‘gateway’ module acts as a filter, blocking access to the OEM-specific CAN data through the OBD2 port and only allowing standardized OBD2 communication.

Think of OBD2 as an “add-on” higher-layer protocol that operates parallel to the vehicle’s essential OEM-specific communication network. OBD2’s purpose is primarily for diagnostics and standardized data access, not for the vehicle’s fundamental operation.

Bit-rate and ID Validation: Ensuring Correct Communication

As OBD2 can use either of two bit-rates (250 Kbps or 500 Kbps) and two CAN ID lengths (11-bit or 29-bit), there are four potential communication configurations. Modern cars commonly use 500 Kbps and 11-bit IDs, but diagnostic tools need to systematically determine the correct combination for each vehicle.

ISO 15765-4 provides a recommended initialization sequence to automatically identify the correct configuration. This process leverages the fact that OBD2-compliant vehicles must respond to a mandatory OBD2 request (specifically, Mode 0x01 PID 0x00, which we’ll discuss later). Incorrect bit-rate settings will result in CAN error frames, which can be detected by diagnostic tools.

Newer versions of ISO 15765-4 also account for vehicles that may support OBD communication via OBDonUDS (OBD on Unified Diagnostic Services) instead of the more traditional OBDonEDS (OBD on Emission Diagnostic Service). While this article primarily focuses on OBD2/OBDonEDS (as defined in ISO 15031-5/SAE J1979), WWH-OBD/OBDonUDS (defined in ISO 14229, ISO 27145-3/SAE J1979-2) is becoming more relevant, particularly in European trucks and buses.

To differentiate between OBDonEDS and OBDonUDS, diagnostic tools can send additional UDS requests using 11-bit or 29-bit functional address IDs for service 0x22 and Data Identifier (DID) 0xF810 (protocol identification). Vehicles supporting OBDonUDS should respond to this DID.

In practice, OBDonEDS (also known as OBD2, SAE OBD, EOBD, or ISO OBD) is prevalent in most non-electric vehicles today, while WWH-OBD is mainly found in EU trucks and buses.

Flowchart illustrating the OBD2 bit-rate and CAN ID validation process as per ISO 15765-4 standards.

Diagram showing the five lower-layer OBD2 protocols, including CAN (ISO 15765), KWP2000, SAE J1850, and ISO9141.

Five Lower-Layer OBD2 Protocols: Beyond CAN

While CAN bus (ISO 15765) is now the dominant lower-layer protocol for OBD2, especially in vehicles manufactured after 2008, older cars might use other protocols. Understanding these legacy protocols can be helpful when working with older vehicles. The OBD2 connector pinout itself can sometimes provide clues about the protocol in use.

Besides CAN, the four other lower-layer protocols historically used for OBD2 are:

ISO 15765 (CAN bus): Mandatory in US cars since 2008 and currently the most widely used.
ISO 14230-4 (KWP2000): Keyword Protocol 2000, common in 2003+ vehicles, particularly in Asia.
ISO 9141-2: Used in European, Chrysler, and Asian cars in the early 2000s (around 2000-2004).
SAE J1850 (VPW – Variable Pulse Width Modulation): Primarily used in older General Motors (GM) vehicles.
SAE J1850 (PWM – Pulse Width Modulation): Mostly found in older Ford vehicles.

ISO-TP: Transporting OBD2 Messages [ISO 15765-2]

All OBD2 communication over CAN bus relies on a transport protocol called ISO-TP (ISO 15765-2). ISO-TP is essential because it allows for the transmission of data payloads larger than the 8-byte limit of a standard CAN frame. This is necessary in OBD2 for tasks like retrieving the Vehicle Identification Number (VIN) or Diagnostic Trouble Codes (DTCs), which often require more than 8 bytes of data. ISO-TP handles data segmentation, flow control, and reassembly of these larger messages.

However, many common OBD2 data requests and responses fit within a single CAN frame. In these cases, ISO 15765-2 specifies the use of a ‘Single Frame’ (SF) format. In a Single Frame, the first data byte (the PCI field – Protocol Control Information) indicates the payload length (excluding padding), leaving 7 bytes available for OBD2-related communication.

Diagram illustrating different ISO-TP frame types for OBD2 communication, including Single Frame, First Frame, Consecutive Frame and Flow Control Frame.

The OBD2 Diagnostic Message Structure [SAE J1979, ISO 15031-5]

To understand OBD2 communication at a deeper level, let’s examine the structure of a raw ‘Single Frame’ OBD2 CAN message. In simplified terms, an OBD2 message consists of a CAN identifier, a data length indicator (PCI field), and the actual data payload. The data payload itself is further structured into:

Mode (Service): Defines the type of diagnostic request or response.
Parameter ID (PID): Specifies the particular data parameter being requested or reported (e.g., engine speed, coolant temperature).
Data Bytes: Contain the actual data values for the requested parameter.

Breakdown of an OBD2 message structure, highlighting Mode, Parameter ID (PID), and Data Bytes within a CAN frame.

Example: OBD2 Request and Response Sequence

Let’s illustrate with a practical example: requesting ‘Vehicle Speed’.

To get the vehicle speed, an external tool sends a request message to the car. This request uses CAN ID 0x7DF and a 2-byte payload: Mode 0x01 and PID 0x0D. Mode 0x01 signifies “Show current data,” and PID 0x0D is the identifier for Vehicle Speed.

The car responds with a message using CAN ID 0x7E8. The response payload contains 3 bytes. The first byte indicates the response mode (0x41 – which is 0x01 + 0x40, a standard response code). The following byte is the original PID (0x0D), confirming the response is for vehicle speed. The third byte, 0x32 (hexadecimal), represents the vehicle speed value. Converting 0x32 from hexadecimal to decimal gives 50.

By consulting the OBD2 PID documentation for PID 0x0D, we find the decoding rule: the value represents speed in km/h. Therefore, 0x32 corresponds to a vehicle speed of 50 km/h.

Example of an OBD2 request (CAN ID 7DF) and response (CAN ID 7E8) sequence for Vehicle Speed.

Detailed explanation of OBD2 PID 0x0D (Vehicle Speed) including request and response parameters.

Overview of the 10 OBD2 service modes (or modes) and their primary functions.

The 10 OBD2 Services (Modes): Diagnostic Functions

OBD2 defines 10 standardized diagnostic services, also referred to as ‘modes’. Each mode serves a specific purpose in accessing diagnostic information.

Mode 0x01: Show current data: Requests real-time data parameters like speed, RPM, temperature, etc.
Mode 0x02: Show freeze frame data: Retrieves data captured when a DTC was set.
Mode 0x03: Show stored Diagnostic Trouble Codes (DTCs): Requests currently active DTCs.
Mode 0x04: Clear Diagnostic Trouble Codes and stored values: Clears DTCs and resets related system values.
Mode 0x05: Oxygen sensor monitoring test results: Retrieves results from oxygen sensor tests.
Mode 0x06: On-board monitoring test results for non-continuously monitored systems: Accesses test results for specific systems.
Mode 0x07: Show pending DTCs (detected during current or last driving cycle): Displays DTCs that haven’t yet triggered the MIL.
Mode 0x08: Control operation of on-board system, test or component: Allows for commanding certain system operations (less commonly used in generic OBD2).
Mode 0x09: Request vehicle information: Retrieves vehicle-specific data like VIN, calibration IDs, etc.
Mode 0x0A: Show permanent DTCs: Displays DTCs that cannot be cleared by simply clearing codes; require fault correction.

Vehicles are not required to support all 10 OBD2 modes. Furthermore, manufacturers can implement additional, OEM-specific OBD2 modes beyond these 10 standard ones for more advanced diagnostics.

In OBD2 messages, the mode is indicated in the second byte of the data payload. In a request message, the mode is included directly (e.g., 0x01 for Mode 01). In a response message, 0x40 is added to the requested mode to indicate a positive response (e.g., 0x41 for a response to Mode 01).

OBD2 Parameter IDs (PIDs): Accessing Specific Data

Within each OBD2 mode, Parameter IDs (PIDs) are used to specify which data parameter is being requested or reported.

For instance, Mode 0x01 (“Show current data”) contains approximately 200 standardized PIDs. These PIDs cover a wide range of real-time data points, including speed, RPM, engine load, fuel level, temperatures, and many more. However, vehicles typically only support a subset of these standardized PIDs. The actual PIDs supported vary depending on the vehicle’s make, model, and year.

One PID stands out as particularly important for OBD2 compatibility testing: PID 0x00 in Mode 0x01. If an emissions-related ECU supports any OBD2 services, it must support Mode 0x01 PID 0x00. When requested, this PID returns a bitmask indicating which PIDs in the range 0x01-0x20 are supported by that ECU. This makes PID 0x00 a fundamental way to check for basic OBD2 compliance and discover supported PIDs. Similarly, PIDs 0x20, 0x40, 0x60, 0x80, 0xA0, and 0xC0 are used to determine support for subsequent ranges of Mode 0x01 PIDs (0x21-0x40, 0x41-0x60, and so on).

Diagram reiterating the OBD2 request-response frame structure, emphasizing PID and data parameter fields.

Screenshot of an OBD2 PID overview tool, illustrating service 01 and available PIDs.

OBD2 PID Overview Tool: Your Decoding Aid

The appendices of the SAE J1979 and ISO 15031-5 standards provide essential scaling and decoding information for standard OBD2 PIDs. This information is crucial for converting the raw data bytes received from the vehicle into meaningful physical values (like km/h, degrees Celsius, etc.).

To simplify PID lookup and request construction, consider using an OBD2 PID overview tool. This tool helps you easily find PIDs, construct OBD2 request frames, and dynamically decode OBD2 responses, streamlining your diagnostic or data logging tasks.

OBD2 PID overview tool

Practical Guide: Logging and Decoding OBD2 Data

Let’s walk through a practical example of logging OBD2 data using a CANedge CAN bus data logger. The CANedge is configurable to transmit custom CAN frames, making it suitable for sending OBD2 requests and logging responses.

Connecting the CANedge to your vehicle is straightforward using an OBD2-DB9 adapter cable.

Diagram illustrating OBD2 data logging setup with requests (7DF) and responses (7E8) using a data logger.

Screenshot showing bit-rate validation for OBD2 communication.

Screenshot from asammdf software showing responses to ‘Supported PIDs’ test.

Step #1: Bit-rate, ID, and Supported PID Validation

As discussed earlier, ISO 15765-4 outlines a process for determining the correct bit-rate and CAN IDs for OBD2 communication with a specific vehicle. Here’s how you can use a CANedge (or similar tool) to perform this validation:

Bit-rate Test: Attempt to send a CAN frame at 500 Kbps. Check if the transmission is successful (e.g., by monitoring for CAN error frames). If unsuccessful, try 250 Kbps.
Bit-rate Confirmation: Use the successfully identified bit-rate for all subsequent OBD2 communication.
Supported PIDs Discovery: Send multiple ‘Supported PIDs’ requests (Mode 0x01 PID 0x00, and subsequent PIDs like 0x20, 0x40, etc.) and analyze the responses.
CAN ID Type Detection: Based on the response CAN IDs (e.g., 0x7E8 range for 11-bit, 0x18DAF1xx range for 29-bit), determine whether the vehicle uses 11-bit or 29-bit CAN identifiers for OBD2.
Supported PID List: Analyze the response data to identify the specific PIDs supported by the vehicle’s ECUs.

Pre-configured CANedge configurations are available to streamline these initial tests.

Most post-2008 non-EV cars typically support 40-80 PIDs using a 500 Kbps bit-rate, 11-bit CAN IDs, and the OBD2/OBDonEDS protocol.

As illustrated in the asammdf GUI screenshot, you might observe multiple responses to a single OBD request, particularly for PID 0x00. This is because using the functional address ID 0x7DF broadcasts the request to all OBD2-compliant ECUs. Since all such ECUs must support Mode 0x01 PID 0x00, multiple responses are common. Note that as you request PIDs in higher ranges (0x20, 0x40, etc.), fewer ECUs may respond, indicating varying PID support across different modules. Interestingly, the ECM (Engine Control Module, responding with ID 0x7E8) often supports all PIDs supported by other ECUs in the vehicle. To reduce bus load and simplify data, you can switch to ‘Physical Addressing’ using CAN ID 0x7E0 to directly target the ECM for subsequent PID requests.

Step #2: Configuring OBD2 PID Requests for Data Logging

Once you’ve determined the supported PIDs, bit-rate, and CAN ID type for your vehicle, you can configure your data logger to request the specific PIDs you want to monitor.

Consider these points when setting up your OBD2 PID requests:

CAN Addressing: Switch to ‘Physical Addressing’ request IDs (e.g., 0x7E0 for the ECM) to minimize redundant responses and bus traffic.
Request Spacing: Introduce a delay of 300-500 milliseconds between consecutive OBD2 requests. Overly rapid requests can overwhelm ECUs and lead to dropped responses.
Power Management: Implement triggers to stop PID requests when the vehicle is inactive (ignition off) to prevent battery drain and avoid unnecessary ECU “wake-up”.
Data Filtering: Configure filters to record only OBD2 response messages, especially if your vehicle also broadcasts OEM-specific CAN data, to keep your logs focused.

With these configurations in place, your CANedge (or similar logger) is ready to record raw OBD2 data from your vehicle.

Example configuration showing a list of OBD2 PID requests set up in CANedge.

Screenshot from asammdf software showing decoded and visualized OBD2 data, plotted using a DBC file.

Step #3: DBC Decoding of Raw OBD2 Data

To analyze and visualize your logged OBD2 data effectively, you need to decode the raw data bytes into human-readable ‘physical values’ (e.g., speed in km/h, temperature in °C).

The necessary decoding formulas and scaling factors are defined in the ISO 15031-5/SAE J1979 standards (and often summarized on resources like Wikipedia). To simplify this process, a free OBD2 DBC file is available. This DBC file allows you to easily decode raw OBD2 data within most CAN bus analysis software tools that support DBC files.

Decoding OBD2 data is slightly more complex than decoding typical CAN signals. This is because different OBD2 PIDs are transmitted using the same CAN ID (e.g., all Mode 01 responses from the ECM use CAN ID 0x7E8). Therefore, the CAN ID alone is not sufficient to identify the specific signal within the payload.

To correctly decode OBD2 data, you need to consider both the CAN ID, the OBD2 mode, and the OBD2 PID. This technique is a form of multiplexing known as ‘extended multiplexing.‘ It can be implemented in DBC files using multiplexer definitions.

CANedge: Your OBD2 Data Logging Solution

The CANedge CAN bus data logger provides a user-friendly solution for recording OBD2 data. It logs data directly to an 8-32GB SD card, making it easy to capture data from vehicles, trucks, and other OBD2-compliant systems. Simply connect the CANedge, configure your desired PIDs, and start logging. Free software and APIs are available for data analysis, along with the OBD2 DBC file to streamline decoding.

OBD2 logger intro
CANedge product details

OBD2 Multi-Frame Communication Examples [ISO-TP]

While many OBD2 interactions use single-frame CAN messages, some operations, like retrieving VIN or DTCs, require multi-frame communication due to larger data payloads. As mentioned earlier, ISO-TP (ISO 15765-2) handles this multi-frame transport. Multi-frame OBD2 communication involves flow control frames to manage the data exchange (refer to the UDS introduction for more details on flow control). In practice, a static flow control frame can be transmitted approximately 50 milliseconds after the initial request frame to facilitate multi-frame responses.

Software tools and APIs that support ISO-TP are necessary for handling multi-frame OBD2 responses, such as the CANedge MF4 decoders.

Example of a multi-frame request message for Vehicle Identification Number (VIN) retrieval via OBD2.

Example 1: Retrieving Vehicle Identification Number (VIN) via OBD2

The Vehicle Identification Number (VIN) is crucial for vehicle identification in telematics, diagnostics, and other applications. To retrieve the VIN using OBD2, you use Mode 0x09 (Request vehicle information) and PID 0x02 (Vehicle Identification Number).

The diagnostic tool sends a Single Frame request with PCI field 0x02, service identifier 0x09 (Mode 09), and PID 0x02 (PID 02).

The vehicle responds with a First Frame. This frame contains the PCI field, the total length of the multi-frame message (0x014 = 20 bytes), mode 0x49 (response to mode 0x09), and PID 0x02. Following the PID is byte 0x01, representing the Number Of Data Items (NODI), which is 1 in this case. The remaining 17 bytes contain the VIN itself, encoded in hexadecimal ASCII representation. These bytes can be converted to the standard alphanumeric VIN string.

Example 2: Multi-PID Request (Requesting 6 PIDs at Once)

OBD2 allows diagnostic tools to request up to 6 Mode 0x01 PIDs in a single request frame. The vehicle’s ECU should respond with data for all supported PIDs from the request (omitting data for unsupported PIDs in the response). If necessary, the response can span multiple frames using ISO-TP.

This multi-PID request feature can improve data acquisition efficiency and frequency. However, it also makes data decoding more complex, as the signal encoding becomes specific to the request structure, potentially limiting the use of generic OBD2 DBC files. Generally, this method is not recommended for routine data logging due to the decoding complexities.

The multi-frame response format is similar to the VIN example, but the payload includes both the requested PIDs and their corresponding data values. In practice, the PIDs in the response are often ordered similarly to their order in the request, although this is not strictly mandated by the ISO 15031-5 standard.

Decoding multi-PID responses using generic DBC files becomes very challenging. Therefore, this approach is generally not recommended for practical data logging unless you are using tools specifically designed to handle multi-PID requests and responses. Effectively, you’re dealing with a complex form of extended multiplexing, where the payload structure depends on the specific PIDs requested, making it difficult to create generalized DBC files that work across different vehicles and PID combinations. While custom scripts and logging of both request and response messages could potentially address this, managing such solutions at scale can be cumbersome.

Example 3: Retrieving Diagnostic Trouble Codes (DTCs)

OBD2 Mode 0x03 (“Show stored Diagnostic Trouble Codes”) is used to request emissions-related Diagnostic Trouble Codes (DTCs). The request message for Mode 0x03 does not include a PID. The targeted ECU(s) will respond with the number of DTCs currently stored (which could be zero if no DTCs are active). Each DTC is represented by 2 data bytes. Multi-frame responses are necessary when more than two DTCs are stored due to the 8-byte CAN frame limit.

The 2-byte DTC value is structured according to ISO 15031-5 and ISO 15031-6. The first two bits indicate the DTC ‘category’ (e.g., Powertrain, Chassis, Body, Network), and the remaining 14 bits define a 4-digit code (displayed in hexadecimal). Decoded DTC values can be looked up in online OBD2 DTC lookup tools like repairpal.com or similar databases to understand the specific fault indicated by the code.

Example of OBD2 DTC decoding, showing the structure and interpretation of DTC bytes.

The example trace shows a request to an ECU that has 6 DTCs stored, resulting in a multi-frame response to accommodate all the DTC data.

Real-World Applications: Use Cases for OBD2 Data Logging

OBD2 data from cars and light trucks has a wide array of practical applications across various industries and user needs:

Vehicle Data Logging

OBD2 data logging in cars can be used for diverse purposes such as optimizing fuel efficiency, improving driving habits, testing prototype automotive components, and usage-based insurance programs that reward safe driving.

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Real-time Vehicle Diagnostics

OBD2 interfaces enable real-time streaming of vehicle data in a human-readable format. This is invaluable for mechanics and technicians diagnosing vehicle issues, monitoring engine performance, and pinpointing faults quickly.

Discover OBD2 streaming interfaces

Predictive Maintenance

For fleet management and vehicle maintenance, IoT-enabled OBD2 loggers can continuously monitor vehicle health data in the cloud. This data analysis can facilitate predictive maintenance, anticipating potential breakdowns and scheduling proactive repairs, reducing downtime and costs.

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Vehicle ‘Black Box’ Recording

An OBD2 logger can serve as a ‘black box’ for vehicles or equipment, continuously recording driving data. This data can be crucial for incident analysis, resolving disputes, accident reconstruction, and warranty validation.

Explore CAN bus black box loggers

Do you have a specific OBD2 data logging application in mind? Contact us for expert guidance and free consultation to explore how OBD2 and CAN bus technology can meet your needs.

For more in-depth introductions to CAN bus and related technologies, explore our comprehensive guides section or download our ‘Ultimate Guide’ PDF.

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Recommended Resources

OBD2 DATA LOGGER: EASILY LOG & CONVERT OBD2 DATA

CANEDGE2 – PRO CAN IoT LOGGER

CAN BUS INTERFACE: STREAMING OBD2 DATA WITH SAVVYCAN

What Is the Purpose of OBD2? Unlocking Your Car’s Diagnostic Secrets