Editor’s note: This article is based on information that was originally relevant in 2013. While OBD-II standards are maintained, always refer to the latest documentation for the most current practices.
The evolution of automotive technology has been remarkable. Many seasoned mechanics fondly recall the simplicity of older vehicles – distributors, carburetors, and road draft tubes. The aroma of those classic engines often evokes a sense of nostalgia. However, it’s also crucial to acknowledge the significant strides made in environmental protection since those times. Imagine the current air quality if vehicles still operated with 1960s technology.
Driven by the severe smog issues in areas like the Los Angeles basin, California initiated emission control requirements for 1966 model cars. This pioneering effort was expanded nationwide by the federal government in 1968. The landmark Clean Air Act of 1970 further solidified these efforts and led to the establishment of the Environmental Protection Agency (EPA).
Many in the automotive repair industry remember the era of On-Board Diagnostics (OBD-I). This early system was characterized by a lack of standardization, with each manufacturer employing their proprietary methods. However, in 1988, the Society of Automotive Engineers (SAE) introduced a pivotal change by setting a standard for the Diagnostic Link Connector (DLC) and establishing a uniform list of fault codes. The EPA largely adopted these SAE recommendations, paving the way for a more unified approach. OBD-II emerged as an enhanced and expanded set of standards and practices, again developed by the SAE and adopted by the EPA and the California Air Resources Board (CARB). Implementation of OBD-II became mandatory on January 1, 1996.
Reflecting back to 1996, some automotive technicians expressed concerns about the complexities of these new, computer-controlled vehicles. Some even opted for career changes, seeking simpler professions. Conversely, many experienced technicians embraced the technological advancements, underwent training, and emerged as more skilled professionals, capable of tackling the new diagnostic challenges. This begs the question: would you prefer to work on pre-OBD-II vehicles or those equipped with OBD-II technology?
As we delve into the 10 modes of OBD-II, it’s essential to remember that the system’s primary design is for emissions monitoring, not as a comprehensive diagnostic system for every vehicle function. OBD-II standards specifically apply to emissions-related aspects of the vehicle, including the engine, transmission, and drivetrain. While systems like body controls, anti-lock brakes, airbags, and lighting are often computer-controlled, they fall outside OBD-II jurisdiction and remain manufacturer-specific. One of the most significant benefits of the OBD-II emissions program is the standardization of the diagnostic connection and communication protocols. For emissions-related repairs, a technician can effectively use a global OBD-II scan tool. This tool provides access to the engine and transmission data necessary for diagnosing issues that trigger the Check Engine light.
Understanding the OBD II Port: This standardized port is key to accessing vehicle diagnostic information.
Exploring the 10 Modes of OBD-II
At first glance, 10 different modes within Global OBD-II might seem intricate. It’s more than simply connecting a scan tool to retrieve codes and replace parts to resolve a check engine light. The OBD-II emissions program is a constantly evolving system, governed by numerous regulations and driven by ongoing research and development.
However, understanding the 10 modes demystifies the system. Many technicians already utilize several of these modes daily. For those new to OBD-II, grasping these modes opens up new avenues for diagnostic precision. Let’s examine each mode individually.
Mode 1: Request Current Powertrain Diagnostic Data
Mode 1 is designed to provide access to real-time, live powertrain data values. Crucially, the data presented must be actual sensor readings, not default or substituted data that a manufacturer might use in enhanced datastreams. This ensures accurate and reliable information for diagnosing current operating conditions.
Mode 2: Request Freeze Frame Information
Mode 2 allows technicians to access emissions-related data that was recorded at the precise moment a related Diagnostic Trouble Code (DTC) was set. While the standards are primarily focused on emissions data, manufacturers are permitted to expand upon these requirements to include manufacturer-specific data. A prime example is the enhanced freeze frame and failure records provided by General Motors, offering a more detailed snapshot of the conditions when a fault occurred.
Example of Freeze Frame Data: This data captures the conditions present when a DTC is triggered, aiding in diagnosis.
Mode 3: Request Emissions-Related Diagnostic Trouble Codes
Mode 3 is fundamental for retrieving emissions-related Diagnostic Trouble Codes (DTCs) stored within emissions-related modules. These are the standardized “P” codes that trigger the Malfunction Indicator Lamp (MIL), commonly known as the Check Engine light. These codes indicate issues that have been identified and confirmed as “matured” according to OBD-II standards, signifying a persistent problem.
Mode 4: Clear/Reset Emissions-Related Diagnostic Information
Mode 4 provides the function to clear emissions-related diagnostic information from the modules where it’s stored. This action not only clears the DTCs but also erases freeze frame data, stored test results, and resets all emissions monitors, effectively turning off the Check Engine light. It’s important to note that clearing codes without addressing the underlying issue will only result in the light illuminating again.
Mode 5: Request Oxygen Sensor Monitoring Test Results
Mode 5 is specifically for accessing the engine control module’s (ECM) oxygen sensor monitoring test results. This mode offers insights into the performance and efficiency of oxygen sensors. However, it’s important to note that Mode 5 data is not available on vehicles utilizing the Controller Area Network (CAN) system. For CAN-equipped vehicles, Mode 6 provides the necessary oxygen sensor information.
Mode 6: Request On-Board Monitoring Test Results for Specific Monitored Systems
Mode 6 is a more comprehensive mode that allows access to test results for on-board diagnostic monitoring of specific components. This includes both continuously monitored systems (like misfire detection) and non-continuously monitored systems. A critical aspect of Mode 6 is the lack of standardization across vehicle makes and models. Interpreting Mode 6 data requires either a sophisticated scan tool that can decode the data or referencing service information to understand the specific test identifiers and component IDs.
Understanding Mode 6 Data: This mode provides detailed test results for specific monitored systems, requiring careful interpretation.
Mode 7: Request Emission-Related Diagnostic Trouble Codes Detected During Current or Last Completed Driving Cycle
Mode 7 is designed to access DTCs that are stored after the first drive cycle following an ECM reset. These are often referred to as “pending codes” in scan tool menus. Pending codes indicate potential issues that the system has detected but not yet confirmed as a persistent fault requiring the MIL to illuminate. Mode 7 is valuable for identifying intermittent problems or issues in the early stages of development.
Mode 8: Request Control of On-Board System, Test or Component
Mode 8 enables bidirectional control of on-board systems or components through a scan tool. Currently, its application is often limited to evaporative emissions systems, allowing technicians to seal the system for leak testing. This active control capability is expanding, offering potential for more interactive diagnostics in the future.
Mode 9: Request Vehicle Information
Mode 9 provides access to essential vehicle information, including the Vehicle Identification Number (VIN) and calibration numbers from all emissions-related electronic modules. This mode is crucial for verifying vehicle identity and software versions, especially when performing reprogramming or module replacements.
Mode 10: Request Emissions-Related Diagnostic Trouble Codes with Permanent Status
Mode 10 is used to retrieve DTCs that are classified as “permanent codes.” These codes are unique because they can only be cleared by the vehicle’s computer itself after it confirms the fault is no longer present through its own system tests. Even after a successful repair and clearing codes using Mode 4, permanent codes remain until the system verifies the repair, ensuring long-term emission system integrity.
It’s important to remember that OBD-II is a dynamic standard that has evolved since its inception and continues to be refined. For example, Mode 5 (oxygen sensor monitoring test results) might not be available on older OBD-II vehicles, particularly those from 1998 or earlier. As OBD-II standards evolve, so does its implementation across different vehicle models and years.
Real-World OBD-II Application
Understanding the 10 modes of OBD-II translates directly into more effective diagnostics. Many experienced technicians already leverage several of these modes intuitively. However, a deeper understanding of each mode maximizes the diagnostic potential of scan tools and improves troubleshooting efficiency.
Consider a diagnostic scenario: a 2002 Subaru Outback with 168,000 miles and a “check engine light on” complaint. The vehicle, equipped with an automatic transmission and a 2.5-liter engine, exhibits no drivability issues other than the illuminated MIL. Connecting a scan tool reveals a P0420 code (Catalyst System Efficiency Below Threshold).
The single P0420 code simplifies the initial diagnostic path. A visual inspection of emission and vacuum hoses is a good starting point, followed by checking oxygen sensor operation and inspecting for exhaust leaks. If these checks are satisfactory, a catalytic converter replacement might seem like the next step.
However, leveraging OBD-II’s diagnostic capabilities can provide more conclusive evidence. Since OBD-II’s primary function is emissions monitoring, the illuminated check engine light for a P0420 code indicates that tailpipe emissions are exceeding 1.5 times the federal test procedure (FTP) certification limit, specifically pointing towards a catalytic converter with reduced oxygen storage capacity.
Starting with Mode 2 (freeze frame information), we can examine the conditions when the P0420 code was set. Key parameters include closed loop operation status, fuel trim levels (short and long term, ideally within 10 percent total), engine coolant temperature (within normal range), and other PIDs to ensure the engine was operating within normal parameters. In this Subaru example, the freeze frame data shows no anomalies.
Moving to Mode 1 (current diagnostic data) allows real-time monitoring of the front and rear oxygen sensors. Knowing that the P0420 test relies on these sensors, their live data is crucial. In this case, the front sensor is a wide-band air-fuel ratio sensor. While Mode 5 (oxygen sensor monitoring test results) is not functional on this particular vehicle, live sensor data and fuel trim information from Mode 1 are sufficient. Data logging during a test drive reveals no fuel control issues.
Exhaust and vacuum leak checks are essential next steps, as leaks can skew sensor readings and impact test results. In this case, no leaks are found.
Mode 6 information becomes the next critical diagnostic step. Service information indicates that Test ID (TID) 01 and Component ID (CID) 01 correspond to catalytic converter test results. Mode 6 data reveals a maximum test value of 180, while the actual test result is 205. While these numbers are initially cryptic, consulting service information or using a scan tool with Mode 6 data interpretation clarifies that the catalytic converter is indeed failing the efficiency test.
Finally, checking Mode 9 information for the PCM calibration ID and comparing it against the Subaru programming website reveals a software update, but not one relevant to the P0420 code.
With all OBD-II mode data analyzed – no leaks, proper fuel control, functional oxygen sensors, and a failing Mode 6 catalytic converter test – the diagnosis points definitively to a faulty catalytic converter. OBD-II provides significant diagnostic power directly from the driver’s seat, streamlining the troubleshooting process.
Understanding and utilizing the 10 modes of OBD-II is an invaluable skill for any automotive technician. It moves beyond simple code reading, offering a deeper dive into vehicle system data and enhancing diagnostic accuracy and efficiency. By mastering these modes, technicians can confidently tackle complex emission-related issues and ensure vehicles are running cleanly and efficiently.
