Editor's note: This article was originally published March 18, 2013. Some of the information may no longer be relevant, so please use it at your discretion.
I can remember back a few years before the advent of computers when the automobile seemed to be simple. A distributor, a carburetor, road draft tubes for crankcase breathers — oh, yes, those were the days when things were simple. Every time I walk past an idling classic car, I remember those days and can smell them too. I can only imagine the air quality now if vehicles had stayed with the technology of the 1960s.
To combat the smog problem in the Los Angeles basin, the state of California started requiring emission control systems on 1966 model cars. The federal government extended these controls nationwide in 1968. In 1970, Congress passed the Clean Air Act and established the Environmental Protection Agency (EPA).
Many of us remember the days of on-board diagnostics (OBD-I). There were few standards and each manufacturer had their own way of doing things. In 1988, the Society of Automotive Engineers (SAE) set a standard for the Diagnostic Link Connector (DLC) and developed a standard list of fault codes. The EPA adapted most of these standards, based on the SAE recommendations. OBD-II is an expanded set of standards and practices developed by SAE and adopted by the EPA and California Air Resources Board (CARB) for implementation by Jan. 1, 1996.
Thinking back to 1996, I can remember some auto technicians complaining how hard it was going to be to work on those new, totally computer-controlled cars, and I also saw several of those technicians abandon the trade for simpler things to do. I saw many older technicians get some training, embrace the change in technology and come out the other end better technicians who were able to take on the challenge. I guess I would ask, would you guys and gals rather work on cars from the pre-OBD-II days, or the cars that support OBD-II technology?
As we discuss the 10 modes of OBD-II, keep in mind that the OBD-II system is designed as an emissions program and not a diagnostic system. The OBD-II standards are applicable only to the emissions-related functions of the vehicle, like the engine, transmission and drivetrain components. Body controls, antilock brakes, airbags and lighting, while they also might be computer controlled, do not fall under OBD-II jurisdiction and are manufacturer-specific items. Many good things have come from the OBD-II emissions program, in particular that we all enjoy is the standardized diagnostic connection. Communication protocols also were standardized. As long as you are only doing emissions-related repairs, a technician can get by with a global OBD-II scan tool. Global OBD-II provides technicians the ability to access the engine and transmission data that is needed to analyze problems that result in an illuminated Check Engine light.
What are the 10 modes?
With 10 different modes, Global OBD-II might seem a little complicated. There’s more to it than just plugging in a scan tool, pulling some codes and installing a part or two to fix that check engine light. The OBD-II emissions program is an ongoing program that is in a state of constant change. This program also is governed by many rules, which are subjected to extensive research and development so that we have a working and functioning program.
Once you understand what the 10 modes are, however, you’ll see that it isn’t too involved. Some of you already are used to and use them every day. Others, while new, will open new diagnostic doorways for you once you understand them. Let’s go through them one at a time.
- Request current powertrain diagnostic data- The purpose of Mode 1 data is to have access to current live powertrain data values. What’s neat is that this sensor data must be actual readings and not default or substitute data like a manufacturer might use in their “enhanced” datastream.
- Request freeze frame information- The purpose of Mode 2 is to allow access to emissions-related data that is stored at the time the related code was set. The rules allow some expansion to meet manufacturer-specific requirements that will exceed the requirements of OBD-II. One example of this would be the General Motors freeze frame and failure records.
- Request emissions-related diagnostic trouble codes- The purpose of Mode 3 is for the external test equipment (scan tool) to have access to emissions-related Diagnostic Trouble Codes (DTCs) that are stored in emissions-related modules. These are the “P” codes that turned on the Malfunction Indicator Lamp (MIL) and have “matured” as defined by OBD-II standards.
- Clear/reset emissions-related diagnostic information- The purpose of Mode 4 is to clear the emissions-related diagnostic information from the modules that have stored this information. This function includes clearing not only the DTCs; but also the freeze-frame data, all stored test data, and it resets all monitors and turns off the check engine light.
- Request oxygen sensor monitoring test results- The purpose of this mode is to allow access to the engine control module’s oxygen sensor monitoring test results. The same information can be obtained by the use of Mode 6. The Mode 5 information is not available on vehicles using the Controller Area Network (CAN) system. For those cars, you’ll need to go directly to Mode 6.
- Request on-board monitoring test results for specific monitored systems- The purpose of Mode 6 is to allow access to the test results for on-board diagnostic monitoring tests of specific components that are both continuously monitored (misfire monitoring) and non-continuously monitored systems. The Mode 6 test information has nothing standard between vehicle makes or models. The only way to understand what you are seeing is to either have a scan tool that defines all of the data for you, or to print out the Mode 6 information from service information then compare the printed data to the scan tool data.
- Request emission-related diagnostic trouble codes detected during current or last completed driving cycle- The purpose of Mode 7 data is to allow the scan tool to have access to codes that have been stored on the first drive cycle after an ECM reset. This is the “pending codes” selection you see on many scan tool menus.
- Request control of on-board system, test or component- The purpose of the Mode 8 is to allow a scan tool to do bidirectional control of an onboard system or test. Typically, it is currently limited to some evaporative emissions systems and allows the user to seal the system for leak testing.
- Request vehicle information- The purpose of Mode 9 is to allow a scan tool access to the vehicle identification number and calibration numbers from all emissions-related electronic modules.
- Request emissions-related diagnostic trouble codes with permanent status after a clear/reset emission-related diagnostic information service- The purpose of Mode 10 is to allow a scan tool to obtain DTCs that are stored as “permanent codes.” These are codes only the module can clear. Even if you’ve made a successful repair and have cleared the codes in Mode 4, these codes will remain in memory until the computer has completed its own system test.
OBD-II has changed over the years and it is still a work in progress, so when you hook up your scan tool and go look for Mode 5 (oxygen sensor monitoring test results) you might not see anything especially if you are hooked to a 1998 (or earlier) model vehicle. The reason is this information was not available for that year on some cars. As OBD-II changes, so does the application.
Real world application
Now that we know what all the different modes of OBD-II are, applying the theory to a problem shouldn’t be too difficult. I think that most all techs have been using several of the modes of OBD-II for several years and having great success, but do we know how to get the most out of the tools that we use?
As an example of using Global OBD-II data for a diagnostic problem, let’s take a look at a 2002 Subaru Outback with a ”my check engine light is on” complaint. The vehicle is equipped with an automatic transmission, a 2.5-liter engine and has 168,000 miles on the odometer. There are no drivability complaints other than the MIL is glowing. With a scan tool hooked up, a code P0420 was stored in memory.
Because the P0420 is the only code that is stored, this rules out a lot of things that I normally would want to test. In this case, I want to do a visual of the engine to make sure all of the emission hoses and vacuum hoses are hooked up, check the oxygen sensors for proper operation, check for any air leaks into the exhaust system and if all these check out, then its time for a new catalytic converter.
Wait, since we have several different onboard tests that are accessible using my scan tool’s Global OBD-II mode, maybe it would be a good thing to see what the onboard computer has to say about this problem.
OBD-II is all about the check engine light. The light has been illuminated because the calculated tail pipe emissions are above 1.5 times the federal test procedure (FTP) certification. In this case, the problem being flagged is a catalytic converter with a low oxygen storage capacity. In a case like this my first move is to check the Mode 2 information (freeze frame information). In the freeze frame information, I want to see if the vehicle was in closed loop operation at the time the code was registered, were both long term and short term fuel trims within limits (total fuel trim within 10 percent), was the engine coolant temperature in the normal range, and did other Parameter Identifiers (PIDs) indicate that the engine was in its proper operating ranges. In this case, nothing is out of order.
Mode 1 (current diagnostic data) is the next place to look. With the live data, I want to see if the front and rear oxygen sensors are working properly. Because I did my homework on how the module tests for a P0420 fault, I knew that it relied on the input from these two sensors. In this case, the front sensor is a wide band air fuel ratio sensor. Mode 5 is oxygen sensor monitoring test results. In the case of this vehicle, this function of OBD-II is not functional, so the live oxygen sensor and fuel trim data will be the place to look.
The scan tool was set to record the data, the vehicle was taken on a short test drive and the stored data viewed. No problems were found with fuel control. The next place to look is for any air leaks in the exhaust system or any vacuum leaks. Because both could affect the operation of the sensors and skew the test results, this is an important check when diagnosing this particular code. On inspection nothing was found in these areas.
Mode 6 information will be the next stop on our diagnostic journey. Service information shows that TID 01 and CID 01 (these are test identifiers) is the catalytic converter testing results. The Mode 6 test results show the maximum test value is 180, while the test results are showing 205. These numbers alone don’t mean anything unless you take the time to review the Mode 6 definitions or have a scan tool that does all the translating for you. (For more info on how to use Mode 6, visit the Motor Age website or its community, the AutoPro Workshop.)
The final step in the diagnostic process is to take a look at the Mode 9 information. This is the PCM calibration identification. By going to the Subaru programming website, I found that there is a software update available, but it is not an update related to the P0420 code I found stored.
The diagnostic process is now finished. With no exhaust leaks, the engine in proper fuel control, the front air fuel ratio sensor and the rear oxygen sensor working properly, the only thing left to do is to recommend a new catalytic converter be installed on this car. OBD-II is a great emissions system with a lot of diagnostic power available to the technician and the best thing; this is all available from the comfort of the front seat of the vehicle.
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