Global OBD II Diagnostics: Using All 10 Modules of Your Scan Tool

A friend of mine and automotive instructor by the name of John Forro once stated that 80% of the emission codes that light the malfunction indicator light (MIL) can be easily diagnosed from the generic or global side of the scan tool, and I would have to agree with him.

There are 10 separate modes on the global side of the scan tool. We will discuss each mode individually.

One of the modes from the global side of the scan tool is known as mode 6. Mode 6 contains an enormous amount of test results from what is known as the once-per-trip monitors that pretty much covers much of the source of 80% of the emission diagnostic trouble codes (DTCs).

Ford Motor Co. was the first car maker that gave us the mode 6 test results from the once- per-trip monitors. John Forro must have spent months dissecting this data and came out with a mode 6 manual that created a lot of interest from technicians.

In addition to the Ford systems, he also covered the domestic systems and Asian systems. Back in the mid-1990s the mode 6 test results from Ford required some complex calculations to arrive at actual test results. The federal Environmental Protection Agency (EPA) noticed this data and now requires all manufacturers to make this data available on all controller area network (CAN)-compliant vehicles. More importantly, there is no longer a need to calculate the test results’ values. It is just a matter of looking at a pass or fail test result and to compare the actual test results to the minimum/maximum values. It is now, so to speak, in the king’s language.

I once attended a seminar in Ft. Myers, Fla., where the subject revolved around the 10-minute diagnostics approach concerning the top 10 DTCs. Both instructors were heavily into the global side of the scan tool. Keep in mind that we are not undermining all of the good data and functions available on the enhanced side of the scanner. First of all, let’s look at the global OBD II menu from a Snap-on scanner (see Figure 1). Before model year (MY) 2010 there were nine separate modes. Beginning in MY 2010, mode 10 was added to retain trouble codes even after they were cleared. It takes three complete good drive cycles to erase trouble codes in mode 10.

Mode 1

Let’s begin with mode 1, which is engine data including emission data. A critical point here is that this data is uniform on all makes and models. The second point here is that the data is raw data, meaning that the powertrain control module (PCM) will show you a failed sensor value instead of a substituted sensor value. Most all critical engine data and emission data is readily available in mode 1. From a diagnostic viewpoint, I hope you see the pluses of mode 1. In addition, the CAN-compliant systems have added some new parameters which turn out to be very valuable. For example, one parameter indicates the miles driven, since the MIL was requested. Say, for example, a vehicle comes in with a MIL indicating a misfire code.

We recently had this vehicle come in with a misfire code and the mode 1 data indicated that the vehicle had been driven 387 miles since the MIL was requested. We all know that when a misfire code is set the PCM shuts down the catalyst monitor. After the misfire is corrected, the PCM will run the catalyst monitor to determine if the converter survived the misfire.

You guessed it, the converter was damaged.

After the repair we did not clear the MIL. A short test drive later we retrieved the converter mode 6 test results which indicated a converter failure. Mode 6 test results will show a monitor failure long before the PCM requests the MIL.

There are several other failures that will shut down a monitor.

For example, a failure of an upstream or downstream O2 sensor will also disable the catalyst monitor. Take a look at Figure 2A thru Figure 2E from a late model CAN-compliant vehicle indicating mode 1 data.

There are five separate screens of data. If you recall, GM gave us the miles driven since the PCM requested the MIL back in the 1990s. In addition, mode 1 also includes the valuable short-term fuel trim (STFT) and long-term fuel trim (LTFT) values as well as a Lambda value indicating the corrected air fuel ratio the engine is presently operating under. A number 1 indicates the engine is operating at a good 14.7-to-1 air fuel ratio. Numbers below 1 indicate a rich condition while numbers above 1 indicate a lean condition. The good parameters should always be between .9 and 1.10 (see Figure 3).

I have annotated some critical parameters in Figure 2A thru Figure 2E. Mode 2 is freeze frame values captured when the DTC set. Most of us technicians were disappointed in freeze frame data on the earlier OBD II systems. With the advent of CAN-compliant systems, the PCM baud rate is much quicker, meaning that the freeze frame values are much more reliable.

You would certainly want to duplicate these conditions during your test drive. Mode 3 will give us emission-type DTCs. Mode 4 will clear emission-related codes. Before clearing codes, be aware that this will also dump freeze frame data and the valuable mode 6 test results. In addition, all of the once-per-trip monitors will go back to incomplete.

Figure 4 shows us all of the possible once-per-trip monitors. These will vary somewhat based on the year, make or model. When the code is cleared a complex set of conditions must be met to run and complete the once-per-trip monitors in a certain sequence. The conditions needed to run and complete the once-per-trip monitors vary between different makes and models. A six-hour code soak condition will be required to be met to run an evaporative emissions (EVAP) monitor. If codes were cleared and a technician wants to test drive the vehicle he may want to use mode 7 to see a first trip failure may have set a pending code.

Mode 5 is O2 sensor test results including test results from the O2 heater circuit. Some scan tools combine mode 5 into the mode 6 test results.

The meat and potatoes from the global side of the scan tool is found in mode 6. Keep in mind that the data in mode 6 is not live data, but rather test results from the last time the PCM ran the once-per-trip monitors. The test results will indicate a simple pass or fail. In addition, a minimum and maximum value will be displayed.

Simply comparing the test result value to the minimum and maximum values will give you an idea how well a monitor or component is performing.

For example, let’s look at the test results from the B1 catalyst monitor in Figure 5. Notice the good test results compared to the min/max values. Let’s say the actual test results were very close to the maximum value. Does that tell us the converter is losing its efficiency? Mode 6 test results can show a failure long before the PCM requested the MIL. In addition, mode 6 test results can help us determine what component caused or did not cause the PCM to set a code.

For example, let’s say we have a vehicle in with a MIL and a P0440 code. The code definition in itself would lead us to believe it is a purge problem. But in reality when the PCM commands the purge solenoid on it expects to see a shift in voltage from the FTP sensor when vacuum begins to build in the EVAP system. If a large leak exists this cannot happen and the PCM sets the P0440 code.

On this particular vehicle with a P0440 code, let’s look at the purge monitor test results in Figure 6 which indicate purge is working and the code is likely caused by a large leak. In addition, CAN-compliant vehicles will give us individual cylinder misfires from the last 10 drive cycles. This can be very valuable on type C misfires, which are not flagged by the PCM with the MIL (see Figure 7).

Another valuable once-per-trip is the variable valve timing (VVT) monitor found on engines equipped with variable valve timing (see Figure 8). The test results from the once-per-trip monitors are very robust and reliable. Keep in mind that these monitors require specific enable criteria to be met to run — meaning rpm, engine load, vehicle speed and temperature. When these conditions are met the PCM runs the monitor and documents the test results. Once the monitor is complete it will not run again until a 30-second key off cycle is complete and another drive is conducted.

A good example for using mode 6 test results is when one of your good customer is considering buying a used car but he requests that you, the technician, check it out before he purchases the car.

Take 10 minutes and investigate the mode 6 test results. When doing so you find test results from the catalyst monitor and the upstream and downstream O2 sensors very close to the max values. Would this be something a car buyer may want to know?

Mode 7 covers the continuous monitors which run constantly. They include the air/fuel ratio monitor, the misfire monitor and the comprehensive component monitor. The DTCs set from these monitors are mostly one trip failures to request the MIL. A valuable point in mode 7 is where we find the pending codes.

Remember, 80% of the emission codes require two consecutive failures before the PCM requests the MIL. A first trip failure will set a pending code in mode 7. If the codes have been cleared a test drive can be conducted and then investigate mode 7 for a pending code. Mode 8 was originally designed to activate and close the EVAP vent solenoid during key on engine off (KOEO) conditions.

However, I have found this function does not work on all systems. I don’t know who to blame -- the manufacturer or the scan tool people. Most technicians will use the enhanced side of the scan tool and select purge and seal function during engine running conditions while monitoring the federal test procedure (FTP) values to see if vacuum is building in the system.

During KOEO conditions a technician simply selects the vent solenoid and commands it closed when using a smoke machine to locate the source of the leak.

We all know there are various types of EVAP systems out there and some, such as Chryslers, do not use a vent solenoid. In addition, the split EVAP systems found on some Toyota and Honda systems use a bypass solenoid that separate the canister from the gas tank. When the bypass solenoid is turned off the systems only purges from the canister. When the PCM ground side controls the bypass solenoid the systems pulls a vacuum in the gas tank.

The important thing to remember here is that when pressurizing the EVAP system with a smoke machine you will need to command the bypass solenoid on to pressurize the gas tank. The control of the bypass solenoid would be found on the enhanced side of the scan tool.

Mode 9 from the global side of the scan tool will give us the present PCM calibration number (see Figure 9).

We all know there are many cases where an update is needed to correct for different drivability issues and some for false trouble codes. Mode 10 is mandated for all CAN-compliant vehicles and will retain an emission DTC even after it has been cleared.

Again the PCM would need to see three good trips where the enable criteria had been met before this code is cleared from mode 10.

We mentioned earlier that mode 5 is dedicated to the O2 sensor test results including the heater circuit that some scanners include in the mode 6 menu. Zirconium O2 sensors are basically a galvanic battery that are prone to get weak with age.

Take a look at Figure 10 which indicates the O2 switching time from rich to lean. Figure 11 indicates the switching time of the O2 from lean to rich.

These values are actually shown in a millisecond value and do show a pass.

Figure 12 shows the test results from the O2 sensor heater circuit with a pass.

Well, we hope that we created some awareness for our fellow technicians to the diagnostic value from the global side of the scan tool. We now have some uniformity in scan data on all CAN-compliant makes and models.     ■