When Onboard Diagnostics II was first conceived as a means of monitoring vehicle emissions, it was obvious there would be a lot of information to keep track of. The Society of Automotive Engineers (SAE) developed guidelines for managing all of this information and came up with a list of "diagnostic modes" that scan tools would have to provide. The scan tool modes and other OBD II requirements can be found in a document called SAE J1979.
So what kind of information did SAE think technicians should be able to access with a scan tool? All of the following:
Mode $01 – Data stream (sensor readings and switch status)
Mode $02 – Freeze-frame data (if DTCs are present)
Mode $03 – Diagnostic trouble codes
Mode $04 – Clear codes and freeze-frame data
Mode $05 – Oxygen sensor monitor
Mode $06 – Non-continuous monitors (EVAP, catalyst, EGR, etc.)
Mode $07 – Continuous monitors
Mode $08 – Bidirectional communication (onboard tests) Mode 09 – Vehicle VIN, PCM calibration, etc.
When you plug in a scan tool, you don’t necessarily see all of these things listed as operating modes. They are usually labeled by their function rather than mode number. So if you want to check for any DTCs, you choose "Check DTCs" on the scan tool menu. If you want to clear codes, you choose "Clear Codes" on the scan tool menu. Each scan tool manufacturer labels these functions somewhat differently, but all more or less allow you to access much of this information.
As a rule, basic code reader tools only provide a few limited functions. Code readers can display DTCs and clear codes. Some can also show monitor status. But as a rule, code readers do not display any live sensor data or allow two-way communication with the PCM.
Entry-level and do-it-yourself oriented scan tools provide a little more information. They can read and clear codes, display data stream information and OBD II readiness monitor status. The better tools will also automatically recognize the VIN and vehicle year/ make/model.
Professional-level scan tools, by comparison, do everything an entry level or DIY scan tool will do, plus everything else on the mode list. The trouble is, each scan tool manufacturer handles and lists the information somewhat differently. What’s more, some information may only be available on the "generic" or "global" side of the OBD II system while other information is only available on the "vehicle specific" or "OEM" side of the OBD II system.
Scan Tool Diagnostics
The typical approach when checking a vehicle for codes is to plug in a code reader or scan tool into the diagnostic connector on the vehicle to see if there are any codes present. If the scan tool can also display history or pending codes, the next step might be to check for any old codes or pending codes in the PCM, especially if you didn’t find any current DTCs. From here, you might then use a scan tool to read the freeze-frame data if a DTC is present, to read live system data while the engine is running, to perform various interactive system tests and/or to clear codes.
Most technicians are very familiar with Modes 1 through 4 because these are the scan tool modes that are most often used. You check for codes, you look at system data and freeze-frame data to figure out what’s going on, and then you clear codes to turn off the Malfunction Indicator Lamp (MIL) and to verify a problem has been fixed. If the MIL stays off after you’ve made repairs, you assume everything is fine and send your customer on their merry way. But if the MIL light comes back on, then you’re back to square one and get to repeat the process over again to rediagnose the fault.
One of the main drawbacks of OBD II is that the PCM won’t set a DTC and turn on the MIL light if a fault is still present until after the vehicle has been driven. With most faults, it takes at least two consecutive trips to set a code and turn on the MIL light when a fault is present. The reason why OBD II is set up this way is to reduce false codes and to make sure there really is a problem before it sets a code and turns on the MIL light. The downside of this approach is that you get a lot of comebacks. You think you’ve fixed the problem and have returned the vehicle to the customer, but several days later the MIL lamp comes back on and the customer is back in your face. What happened?
OBD II non-continuous monitors will not run if a fault has already occurred in a sensor or other system monitor that is needed to perform an accurate test. For example, the catalyst monitor won’t run if there are any oxygen sensor codes. The catalyst monitor needs a good signal from both the upstream O2 sensor(s) and downstream O2 sensor(s) to verify the operation of the catalyst converter. If any of the oxygen sensors involved are misbehaving, OBD II will suspend the catalyst monitor until the oxygen sensor codes are gone.
The catalyst monitor compares the switching activity of the upstream and downstream O2 sensors to see if the catalyst is operating at peak efficiency. If the converter is doing its job effectively, the downstream O2 sensor signal should be flatline with little switching activity. If the catalyst is getting old and isn’t working so well, OBD II will detect the increase in switching activity and set a code once it exceeds a certain threshold.
The catalyst monitor requires specific driving conditions before it will run, so it make take several days or even a week or more of driving before all the conditions are just right to run the monitor. As a rule, the engine must be hot (170° to 230° F), the PCM must be operating in closed loop and the vehicle must be cruising at part throttle from 25 to 35 mph. There also must be no DTCs present for the oxygen sensors. Under these conditions, the catalyst monitor will usually run within 15 minutes or less.
The story is similar for the EVAP monitor. The EVAP monitor checks for vapor leaks in the fuel tank and EVAP canister plumbing. The EVAP monitor also has a long list of requirements that must be met before it will run, things like a certain number of hours of "cold soak" with the engine off before it will run a pressure or vacuum test to check for fuel vapor leaks. As with the catalyst monitor, a problem won’t set a DTC until the same fault has occurred twice under nearly similar conditions.
Mode 06 Hex Codes
This brings us to Mode $06. With a couple of exceptions, most professional-level scan tools will display Mode 06 test information. The trick is finding the Mode 06 information in the scan tool menu, and then figuring out what it means. The problem is, most scan tools display Mode 06 data in hexadecimal code.
Hexadecimal code is a 16-digit numbering system that goes as follows: 0, 1, 2, 3, 4, 5, 6. 7, 8, 9, A, B, C, D, E, F. The value is typically proceeded by a "$" to denote the value displayed is in hex code. Hex code is computer programming language that PCMs understand, but makes no sense to us unless we have a reference list and conversion table to translate the hex code.
Hex code numbers can be converted into regular decimal values using a scientific calculator or the calculator that is built into Windows 98 and XP software. Open the calculator, click on "View" and select "Scientific." Click "Hex," enter the value in hex code, then click "Dec" to automatically convert the value to a regular decimal number you can read and understand.
For example, if you convert the hex code number "3C" into a regular decimal number, the value is 60 using this method.
Mode 06 data is typically displayed in three columns. The first column is the "TID" (Test identification) that indicates what system monitor is involved. The second column is the "CID" (Component identification), which corresponds to the component being tested and its test value. The third column is PASS/FAIL that tells you if the test value is in or out of predetermined limits.
When you find the Mode $06 information menu on your scan tool, you’ll see a laundry list of TIDs, CIDs and PASS/FAIL listings on the screen. The first line might read: TID$07 CID$90 PASS.
What does this mean?
Absolutely nothing if you can’t translate it. Unless your scan tool or diagnostic software converts the hexadecimal code into English and decimal units of measure, Mode 06 information is of little use, which is why many technicians have totally ignored it. In fact, many technicians don’t even know this information is available in their scan tool or where to find it.
But now that plug-in OBD II emissions testing has become commonplace in many states, technicians need a better method of diagnosing OBD II problems and making sure the MIL light stays off. Fortunately, some of the newest scan tools and diagnostic software now convert Mode 06 hex code and display it as readable and useful system component test information. The Mode 06 data can be used to verify repairs and to make sure there aren’t any other "hidden" faults waiting to turn on the MIL light after a customer leaves your shop.
In the hex code example we just mentioned, TID$07 is the hex code for the oxygen sensor monitor, CID$90 is the maximum heater current of the bank 1 oxygen sensor (in amps), and because the heater current amperage is within specifications established by the vehicle manufacturer, the oxygen heater circuit passes the test.
Mode $06 Lists
Information on translating and converting Mode $06 hex code can be found on the international Automotive Technicians Network website (www.iatn.com). Go to the "Technical Resources" menu, then look in the Ford and Toyota sections. The Mode $06 information is in a downloadable PowerPoint presentation by Paul Baltusis of Ford Motor Company, called "An Introduction to Vehicle Networks, Scan Tools and Multiplexing."
Unfortunately, GM, Daimler-Chrylser and other vehicle manufacturers have not yet posted any of their Mode 06 information on the iATN website. Mode 06 information on other vehicle manufacturers usually can be found by searching their service websites. These sites, which include all their OBD II-related service information, can be accessed via the Internet for fees ranging from free to about $25 per day. For a complete list of OEM websites, go to www.nastf.org.
Note: We’ve heard that some of the OEM Mode $06 lists are not completely accurate. Some of the TID/CID hex codes listed on certain OEM websites are incorrectly identified. iATN has attempted to post corrections where errors have been discovered, so it’s a good idea to see if there have been any recent postings if you’re an iATN member.
It would be nice if there were a single master list of Mode $06 data and what it means, but each vehicle manufacturer assigns their own unique hex code to their system monitors and components. As a rule, the hex codes are similar from one model year to the next within a vehicle manufacturer’s product line, but some codes may change from one year or model to the next depending on the vehicle application.
If your scan tool only displays Mode $06 information in hex code and you have been unable to find a reference chart to translate and convert the hex code, your only alternative is to create a reference list of your own using a known-good vehicle. One automotive trainer we know did that by hooking up his scan tool to a variety of vehicles and painstakingly compiling his own reference manual of hex code translations.
To identify each TID/CID, he would disconnect a component or create a false failure and then note which TID/CID on the Mode $06 screen changed. He would then write down the item, the corresponding Mode $06 hex code, and convert the test value into regular decimal units that made sense.
One thing to keep in mind here is that if all the Mode $06 entries read PASS, it means all the monitored components are working within specifications. There are no problems lurking in the system waiting to trigger the MIL light and set a DTC. Consequently, you don’t really have to know what each TID/CID means as long as all the lines read PASS.
On the other hand, if one or more lines read FAIL, then you know there’s a problem and further diagnosis is needed to isolate the fault. For that, you do need to know what component is identified by the failed TID/CID hex code.
Using Mode $06 Test Data
Mode 06 is the actual system test data that OBD II looks at when it decides to set a pending code or a current fault code. If the test data is within the limits established by the vehicle manufacturer, the item gets a PASS and no codes are set. But if a value is out of range, OBD II flags it with a FAIL and keeps an eye on the component until the system monitor has run at least twice. Then, if the problem is still there, a DTC is set and the MIL light comes on.
The older scan tools typically display the Mode $06 data in hex code, while some of the newer scan tools translate the data and display it by component and test value.
You can also buy software that will allow a PC, laptop or notebook computer, Palm Pilot or Pocket PC to function as a scan tool. Many of these programs will also display Mode 06 data. One of the best software packages we’ve seen is called "OBD II Pro Plus," which displays Mode $06 TIDs and CIDs in easy-to-read plain English with their values converted into amps, volts, ohms, temperature, pressure, etc. The OBD II Pro Plus software is available from www.aatecusa.com.
Some scan tools also can give you the range of "good/bad" limits for each TID/CID in Mode $06.
In addition to test information on the catalyst, EVAP and EGR monitors, you also can find data on oxygen sensor heater circuits, and on Ford applications, misfire counts for each cylinder. Most other vehicle manufacturers put their cylinder misfire data into the Mode 07 area because misfire detection is a continuous monitor.
Finally, if you make it a habit to scan Mode 06 data for possible failures after other faults have been repaired, you’ll reduce the risk of comebacks caused by other problems that have not yet set a code. For example, if a vehicle has a bad O2 sensor heater circuit and a bad converter, you’ll probably only find a DTC for the O2 sensor.
There won’t be one yet for the converter because the cold O2 sensor will prevent the catalyst monitor from running. A bad TPS sensor, coolant sensor or vehicle speed sensor can also have the same effect. So make the most of Mode 06 test data and use it to prevent comebacks.