Decoding DTC OBD2: Your Guide to Diagnostic Trouble Codes for Vehicle Repair

Understanding your vehicle’s health is crucial, and the OBD2 (On-Board Diagnostics II) system is a powerful tool for this purpose. As a car owner or automotive professional, deciphering Dtc Obd2 (Diagnostic Trouble Codes) is essential for efficient vehicle repair and maintenance. This guide provides a comprehensive introduction to DTC OBD2, exceeding the original article in depth and SEO optimization for an English-speaking audience.

What are DTC OBD2 Codes?

DTC OBD2 codes are essentially your car’s way of communicating when something is wrong. They are standardized codes that your vehicle’s computer system generates when it detects a malfunction in any of its monitored systems. Think of the malfunction indicator light (MIL), commonly known as the “check engine light,” as the initial alert. When this light illuminates, it signifies that a DTC OBD2 code has been stored in your car’s computer memory.

Mechanics and car enthusiasts alike use OBD2 scanners to retrieve these DTC OBD2 codes. By connecting an OBD2 scanner to the OBD2 16-pin connector, typically located under the dashboard near the steering wheel, you can access a wealth of diagnostic information. The scanner sends requests, and your car responds with data, including DTC OBD2 codes, allowing for quicker and more accurate troubleshooting.

Is My Car OBD2 Compliant for DTC Reading?

The good news is that if you own a relatively modern vehicle, it almost certainly supports OBD2 and DTC OBD2 code reading. For gasoline cars in the USA, OBD2 became mandatory in 1996, and for light trucks in the same year. The European Union followed suit, mandating OBD2 for gasoline cars in 2001 and diesel cars in 2004 (EOBD).

While a 16-pin OBD2 connector is a strong indicator, some older vehicles might have the connector without full OBD2 compliance. To be sure, consider the vehicle’s origin and year of purchase:

The Evolution of OBD2 and DTCs

The history of OBD2 and DTC OBD2 codes is rooted in emission control. The California Air Resources Board (CARB) pioneered the standardization by requiring OBD in new cars from 1991 onwards for emission monitoring.

The Society of Automotive Engineers (SAE) played a crucial role in standardizing DTC OBD2 codes and the OBD connector, leading to SAE J1962. This standardization ensured that DTC OBD2 codes would be consistent across different vehicle manufacturers, making diagnostics much more accessible.

The OBD2 mandate expanded over time:

1996: OBD2 mandatory in the USA for cars and light trucks.
2001: Required in the EU for gasoline vehicles.
2004: Required in the EU for diesel vehicles (EOBD).
2005: OBD2 required in the US for medium-duty vehicles.
2008: US vehicles mandated to use ISO 15765-4 (CAN) as the communication basis for OBD2 and DTC OBD2 transmission.
2010: OBD2 required for heavy-duty vehicles in the US.

The Future of DTC OBD2 and Vehicle Diagnostics

While OBD2 remains relevant for accessing DTC OBD2 codes, the automotive landscape is evolving. Electric vehicles (EVs), for instance, are not legally obliged to support OBD2 in the traditional sense, primarily because emissions control is less of a concern. Consequently, most modern EVs do not fully support standard OBD2 requests for DTCs. They often use OEM-specific UDS communication protocols, making DTC OBD2 retrieval more challenging unless decoding methods are reverse-engineered, as seen in case studies for electric cars including Tesla, Hyundai/Kia, Nissan and VW/Skoda EVs.

Emerging standards like WWH-OBD (World Wide Harmonized OBD) and OBDonUDS (OBD on UDS) aim to improve OBD communication, including DTC OBD2 reporting, by utilizing the UDS protocol. For a deeper dive into these, refer to our intro to UDS.

The future may also see the rise of OBD3, which envisions integrating telematics into all vehicles. OBD3 could involve a radio transponder in cars to wirelessly transmit the vehicle identification number (VIN) and DTC OBD2 codes to a central server for remote diagnostics. Devices like the CANedge2 WiFi CAN logger and CANedge3 3G/4G CAN logger already facilitate data transfer via WiFi/cellular networks.

However, concerns about data privacy and manufacturer control over vehicle data are also growing, potentially impacting the future accessibility of DTC OBD2 data for third-party services.

Deep Dive into OBD2 Standards and DTCs

OBD2 functions as a higher-layer protocol built upon communication methods like CAN. Similar to protocols like J1939, CANopen, and NMEA 2000, OBD2 standards define the connector, communication protocols, and importantly for DTC OBD2, the parameter IDs (PIDs) used to request and receive diagnostic information, including DTCs.

The OBD2 standards are structured across the 7-layer OSI model, with both SAE and ISO standards playing a role, reflecting US (SAE) and EU (ISO) standardization efforts. SAE J1979 and ISO 15031-5 are technically similar standards for diagnostic services, including DTC OBD2 codes, while SAE J1962 and ISO 15031-3 are similar for the OBD connector specifications.

Understanding the OBD2 Connector for DTC Access [SAE J1962]

The SAE J1962 / ISO 15031-3 standard specifies the 16-pin OBD2 connector, your physical interface for accessing DTC OBD2 codes and other data. This connector, also known as the Data Link Connector (DLC), is typically a Type A connector found in most cars.

Key features of the OBD2 connector:

Location: Usually near the steering wheel, though it can be somewhat hidden.
Pin 16: Provides battery power, even when the ignition is off.
Pinout: Varies based on the communication protocol used by the vehicle.
CAN Bus: For vehicles using CAN bus, pins 6 (CAN-H) and 14 (CAN-L) are utilized.

OBD2 Connector Types: A vs. B

While Type A is common in cars, Type B OBD2 connectors are often found in medium and heavy-duty vehicles. Both share similar pinouts for DTC OBD2 access, but Type B provides 24V power supply (Type A is 12V). Baud rates can also differ, with cars often using 500K and heavy-duty vehicles frequently using 250K (with increasing support for 500K).

Visually, Type B connectors have an interrupted groove in the middle, distinguishing them from Type A. Type B OBD2 adapter cables are compatible with both Type A and Type B sockets, while Type A adapters only fit Type A sockets.

DTC OBD2 Communication via CAN Bus [ISO 15765-4]

Since 2008, ISO 15765 has mandated CAN bus as the underlying communication protocol for OBD2 in US vehicles. ISO 15765-4, also known as Diagnostics over CAN (DoCAN), specifies how OBD2, including DTC OBD2 requests and responses, operates over CAN. It sets constraints on the CAN standard (ISO 11898) concerning the physical, data link, and network layers:

CAN Bit-rate: Must be 250K or 500K.
CAN IDs: Can be 11-bit or 29-bit.
CAN IDs for OBD2: Specific IDs are reserved for OBD2 requests and responses, including DTC OBD2 related communication.
CAN Frame Data Length: Limited to 8 bytes for diagnostic frames.
OBD2 Adapter Cable Length: Maximum 5 meters.

CAN Identifiers for DTC OBD2 Requests and Responses (11-bit, 29-bit)

OBD2 communication, including DTC OBD2 retrieval, is based on request and response messages. In most cars, 11-bit CAN IDs are used. Functional addressing, using ID 0x7DF, is common, broadcasting requests to all OBD2-compliant ECUs to check for data on the requested parameter (ISO 15765-4). Physical addressing, using IDs 0x7E0-0x7E7, allows requests to be sent to specific ECUs, but is less frequently used for DTC OBD2 queries.

ECUs respond with 11-bit IDs in the range 0x7E8-0x7EF. 0x7E8 (ECM – Engine Control Module) is the most common response ID, followed by 0x7E9 (TCM – Transmission Control Module).

Some vehicles, especially vans and medium/heavy-duty vehicles, may utilize 29-bit extended CAN identifiers for DTC OBD2 communication. In these cases, the functional addressing CAN ID is 0x18DB33F1. Responses are typically found with CAN IDs ranging from 0x18DAF100 to 0x18DAF1FF (often 18DAF110 and 18DAF11E). These response IDs are sometimes represented in the ‘J1939 PGN’ format, specifically PGN 0xDA00 (55808), which is reserved for ISO 15765-2 in the J1939-71 standard.

OBD2 vs. OEM-Specific CAN Protocols and DTCs

It’s important to understand that OBD2 and DTC OBD2 are not the primary communication systems for your vehicle’s ECUs. Car manufacturers (OEMs) employ their own proprietary CAN protocols for core vehicle functions. These protocols are often brand, model, and year-specific. Interpreting this OEM-specific data requires reverse engineering, as described in our guide to CAN bus sniffing and reverse engineering.

When you connect a CAN bus data logger to the OBD2 port, you might observe OEM-specific CAN data alongside OBD2 data, typically broadcast at high rates (1000-2000 frames/second). However, many newer vehicles utilize a ‘gateway’ that restricts access to OEM CAN data through the OBD2 port, allowing only OBD2 communication, including DTC OBD2 access.

Think of OBD2 as an additional higher-layer protocol that operates in parallel with the OEM-specific protocol. This is crucial to remember when diagnosing DTC OBD2 codes – they represent standardized diagnostic information, not the entirety of the vehicle’s internal communication.

Bit-rate and ID Validation for DTC OBD2 Access

OBD2 can use two bit-rates (250K, 500K) and two CAN ID lengths (11-bit, 29-bit), resulting in four possible combinations. Modern cars commonly use 500K with 11-bit IDs, but diagnostic tools should systematically verify this.

ISO 15765-4 provides a sequence for automatic bit-rate and ID validation. This process relies on the fact that OBD2-compliant vehicles must respond to a specific mandatory OBD2 request (see the OBD2 PID section) and that incorrect bit-rate transmission will cause CAN error frames.

Newer versions of ISO 15765-4 also consider OBDonUDS, but OBD2/OBDonEDS (OBD on Emission Diagnostic Service per ISO 15031-5/SAE J1979) remains prevalent in non-EV cars. WWH-OBD is primarily used in EU trucks and buses. To differentiate between OBDonEDS and OBDonUDS, diagnostic tools can send UDS requests with 11-bit/29-bit functional address IDs for service 0x22 and data identifier (DID) 0xF810 (protocol identification). OBDonUDS compliant vehicles must respond to this DID.

Legacy OBD2 Protocols Beyond CAN

While CAN is dominant today (ISO 15765), older vehicles might use other lower-layer protocols for OBD2 and DTC OBD2 communication. Knowing these is helpful when working with pre-2008 vehicles. Pinouts can help identify the protocol:

ISO 15765 (CAN bus): Mandatory in US cars since 2008, now widely used.
ISO14230-4 (KWP2000): Common in 2003+ Asian cars.
ISO 9141-2: Used in EU, Chrysler, and Asian cars (2000-2004).
SAE J1850 (VPW): Primarily in older GM vehicles.
SAE J1850 (PWM): Primarily in older Ford vehicles.

ISO-TP for Extended OBD2 Messages Including DTCs [ISO 15765-2]

OBD2 data, including potentially lengthy DTC OBD2 responses, is transmitted over CAN using ISO-TP (ISO 15765-2), a transport protocol that allows for payloads exceeding the 8-byte CAN frame limit. This is crucial for transmitting Vehicle Identification Numbers (VINs) and collections of Diagnostic Trouble Codes (DTCs). ISO 15765-2 handles segmentation, flow control, and reassembly of these larger messages. For more information, see our UDS introduction.

For smaller OBD2 data, a ‘Single Frame’ (SF) format is used within ISO-TP. The first data byte (PCI field) indicates the payload length, leaving 7 bytes for OBD2 communication.

Decoding the OBD2 Diagnostic Message Structure for DTCs [SAE J1979, ISO 15031-5]

A simplified OBD2 CAN message consists of an identifier, data length (PCI field), and data. The data field is further structured into Mode, parameter ID (PID), and data bytes.

OBD2 Request/Response Example for DTCs

Let’s examine a request and response example, this time focusing on retrieving DTC OBD2 codes.

To request stored DTCs (Mode 0x03), an external tool sends a request message with CAN ID 0x7DF and two payload bytes: Mode 0x03 and PID 0x00 (PID 0x00 is often used as a general request within a mode). The car responds via CAN ID 0x7E8. The response payload will vary depending on whether DTCs are present, and if so, how many. Multi-frame responses are common for DTC retrieval if multiple codes are stored.

Next, we will delve into OBD2 modes and PIDs, particularly as they relate to DTC OBD2 codes.

The 10 Standard OBD2 Services (Modes) and DTCs

OBD2 defines 10 diagnostic services or modes. Mode 0x03, “Show stored Diagnostic Trouble Codes,” is specifically used to retrieve DTC OBD2 codes. Other modes are used for real-time data (Mode 0x01), clearing DTCs (Mode 0x04), and accessing freeze frame data (Mode 0x02), which captures sensor data when a DTC is set.

Not all vehicles support all 10 OBD2 modes, and manufacturers can implement OEM-specific modes beyond the standard set. In OBD2 messages, the mode is indicated in the second byte. In a request, the mode is directly included (e.g., 0x03 for DTC request). In the response, 0x40 is added to the mode (e.g., 0x43 in response to a DTC request).

OBD2 Parameter IDs (PIDs) and DTCs

Within each OBD2 mode, Parameter IDs (PIDs) further specify the data being requested or provided. For DTC OBD2 retrieval (Mode 0x03), the PID is often 0x00 or omitted, as the mode itself indicates the request for DTCs.

Mode 0x01, which provides real-time data, contains approximately 200 standardized PIDs for parameters like speed, RPM, and fuel level. However, vehicles typically support only a subset of these PIDs.

A crucial PID for OBD2 compatibility testing is Mode 0x01 PID 0x00. If an emissions-related ECU supports OBD2, it must support this PID. Responding to PID 0x00 indicates which PIDs in the 0x01-0x20 range are supported. PIDs 0x20, 0x40, 0x60, etc., serve a similar function for subsequent PID ranges within Mode 0x01.

OBD2 PID Overview Tools for DTC Interpretation

SAE J1979 and ISO 15031-5 appendices detail scaling information for standard OBD2 PIDs, enabling conversion of raw data to physical values. Our OBD2 PID overview tool helps in constructing OBD2 requests and dynamically decoding responses, aiding in DTC OBD2 interpretation and diagnostics.

Practical Guide: Logging and Decoding DTC OBD2 Data

Let’s illustrate how to log DTC OBD2 data using a CANedge CAN bus data logger. The CANedge allows custom CAN frame transmission, making it suitable for OBD2 and DTC OBD2 logging. Connect it to your vehicle via an OBD2-DB9 adapter cable.

Reviewing ‘Supported PIDs’ responses in asammdf

Step 1: Bit-rate, ID, and Supported PID Validation for DTC OBD2

As per ISO 15765-4, determine the vehicle’s bit-rate and CAN IDs. Use CANedge for these tests (see CANedge Intro):

Test 500K bit-rate by sending a CAN frame; if unsuccessful, try 250K.
Use the validated bit-rate for further communication.
Send ‘Supported PIDs’ requests to identify supported PIDs, including those relevant to DTC OBD2 codes.
Response IDs indicate 11-bit or 29-bit CAN ID usage.
Response data reveals supported PIDs and DTC OBD2 capabilities.

Plug-and-play configurations for these tests are available. Most post-2008 non-EV cars support 40-80 PIDs via 500K, 11-bit CAN IDs, and OBD2/OBDonEDS. Multiple responses to a single OBD request (using 0x7DF) are common as all ECUs respond. Using ‘Physical Addressing’ (e.g., 0x7E0) can reduce bus load by targeting specific ECUs for subsequent requests.

Step 2: Configuring OBD2 PID Requests for DTCs

Once you know your vehicle’s supported PIDs and communication parameters, configure your transmit list, focusing on DTC OBD2 related PIDs or mode 0x03 for DTC retrieval.

Consider these factors:

CAN IDs: Use ‘Physical Addressing’ request IDs (e.g., 0x7E0) to avoid multiple responses.
Spacing: Add 300-500 ms between OBD2 requests to prevent ECU overload.
Battery Drain: Use triggers to stop transmissions when the vehicle is inactive.
Filters: Filter for OBD2 responses if OEM-specific CAN data is also present.

With configuration complete, the device is ready to log raw DTC OBD2 data.

Example CANedge OBD2 PID request list

Visualizing decoded OBD2 data in asammdf using a DBC file

Step 3: DBC Decoding of Raw DTC OBD2 Data

To analyze logged DTC OBD2 data, you need to decode the raw data into readable values. Decoding information is in ISO 15031-5/SAE J1979 and online resources. Our free OBD2 DBC file simplifies DBC decoding in CAN bus software tools.

Decoding DTC OBD2 data is more complex than standard CAN signals because different OBD2 PIDs, including DTC responses, share the same CAN ID (e.g., 0x7E8). The CAN ID alone isn’t enough to identify the signal. Therefore, decoding requires considering the CAN ID, OBD2 mode, and OBD2 PID – a form of multiplexing called ‘extended multiplexing‘, which can be implemented in DBC files.

CANedge: Your DTC OBD2 Data Logger

The CANedge is ideal for logging DTC OBD2 data to SD cards (8-32 GB). Connect it to your vehicle to start logging, and utilize free software/APIs and our OBD2 DBC for data decoding.

OBD2 logger intro CANedge

Multi-Frame Examples for DTC OBD2 and Extended Data [ISO-TP]

OBD2 communication, including DTC OBD2 responses, relies on ISO-TP (ISO 15765-2). While many examples are single-frame, multi-frame communication is essential for larger data sets like VIN and extensive DTC lists. Multi-frame communication involves flow control frames (see UDS intro). CANedge configurations can include static flow control frames transmitted shortly after the initial request. CAN software/API tools supporting ISO-TP, like CANedge MF4 decoders, are needed for multi-frame DTC OBD2 responses.

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

The VIN is crucial for telematics and diagnostics. To retrieve it using OBD2, mode 0x09 and PID 0x02 are used:

The tool sends a Single Frame request with PCI (0x02), service ID (0x09), and PID (0x02). The vehicle responds with a First Frame containing PCI, length (0x014 = 20 bytes), mode (0x49), and PID (0x02). Byte 0x01 indicates one data item (NODI), followed by the 17-byte VIN (HEX to ASCII conversion needed).

Example 3: Diagnostic Trouble Codes (DTCs) in Detail

Mode 0x03, “Show stored Diagnostic Trouble Codes,” is used to request emissions-related DTC OBD2 codes. No PID is needed in the request. ECUs respond with the number of stored DTCs (possibly zero), with each DTC using 2 bytes. Multi-frame responses are necessary if more than two DTCs are stored.

The 2-byte DTC value is structured (ISO 15031-5/ISO 15031-6). The first 2 bits define the category, and the remaining 14 bits form a 4-digit hexadecimal code. Decoded DTC values can be looked up using OBD2 DTC lookup tools like repairpal.com.

The example below shows a request to an ECU with six stored DTCs.

Use Cases for DTC OBD2 Data Logging

DTC OBD2 data logging has diverse applications:

Car Data Logging for Performance and Efficiency

DTC OBD2 data helps analyze fuel consumption, improve driving habits, test parts, and inform insurance policies.

obd2 logger

Real-time Vehicle Diagnostics and DTC Monitoring

OBD2 interfaces enable real-time DTC OBD2 data streaming for immediate issue diagnosis.

obd2 streaming

Predictive Maintenance and DTC-Based Alerts

IoT OBD2 loggers can monitor vehicle health and DTC OBD2 codes in the cloud to predict and prevent breakdowns.

predictive maintenance

Vehicle Black Box Recording including DTC Events

OBD2 loggers act as ‘black boxes’ for vehicles, recording data and DTC OBD2 events for dispute resolution and diagnostics.

can bus blackbox

Have a DTC OBD2 data logging use case? Contact us for expert advice!

Explore our guides or download the ‘Ultimate Guide’ PDF for more insights.

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