Should OBD-II be Your First Step in Diagnosing an Engine Management Issue? Yes, and Here’s Why...

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Should OBD-II be Your First Step in Diagnosing an Engine Management Issue? Yes, and Here’s Why...

When a check engine light comes on, having a game plan for understanding the cause and fixing it quickly is critical for your customer’s satisfaction and profitability for you. Having the correct scan tools at your disposal and running through the 10 diagnostic modes to OBD-II are keys to your success.

We can’t possibly have all the tools that we need to repair every vehicle that comes into our shop, even if we concentrate on only a couple of makes or manufacturers. There are simply too many dedicated tools required today to perform all of the required tasks on today’s engine, transmission and drivetrain systems. Even a simple water pump service can now require highly specialized tools, but dealing with a check engine light doesn’t always require the factory scan tool, even if you have one available.

It’s important to remember that the OBD-II system’s responsibility is to turn on the check engine light and alert the driver if there is a failure in the vehicle’s emission control systems, and it’s this fact that made me change my diagnostic approach.

I typically start to diagnose a check engine light (malfunction indicator light/MIL), by grabbing one of my generic scanners first. I check the OBD-II system to see what information is available to properly diagnose the issue, allowing me to base my decision on this data.

Will I need a factory scanner (that I may or may not have access to) or tell the customer that I don’t have the factory scan tool needed and possibly send them back to the dealer?

There are times that you are going to need a factory tool to complete a diagnostic or need some form of bidirectional control, but research has shown that almost 85% of the emissions-related MIL complaints can be dealt with using nothing more than an OBD-II diagnostic capable scanner.

For example, intake air temperature and coolant temperature should have similar readings after a cold soak. The OBD-II system also has to keep track of these test results, the operating conditions when it ran the tests and other information that can potentially be used to diagnose a MIL light issue. There are 10 diagnostic modes to OBD-II that we can use and these modes are typically available on that generic scanner that you already have, but it’s important to understand what information each of these modes provides us and how to interpret that information so we can use it in our diagnostic process.

Mode $01... Request the current powertrain diagnostic data. (The $ sign in the mode number indicates it’s a hexadecimal number used by the computers involved.)

Before any scan tool can communicate it needs to know what language the vehicle speaks. CAN, ISO 9141-2 and J1850 PWM are examples of communication protocols used. Once the scanner understands the vehicle’s protocol, the PCM will allow the scanner access to current live powertrain data values or PIDs (parameter identification). This data must be actual sensor reading, not default, substituted or enhanced data that some manufacturers’ factory data streams may provide.

Many manufacturers’ factory data streams (Fiat/Chrysler and Honda come to mind) may show false information that the PCM is supplying because it is aware of a failed sensor or skewed data and still wants the vehicle to run properly for the customer. It just bases the false information from another input or sensor but it doesn’t tell you that when you are looking at the factory data.

The vehicle’s system readiness test status and the MIL status are also displayed in this mode. The fewer PIDs that are selected the faster the data refresh rate will be, and having genuine sensor data is a real help.

Mode $02... Request the freeze frame information. Mode $02 provides the data stored at the time the fault was detected by the OBD-II and when the OBD-II system turned on the MIL. Mode $02 output only shows the data from when the code set or matured and caused the MIL. This is important to note, because it may not provide information on when the actual problem started, but it provides clues and these clues can still be useful in replicating the failure conditions.

I frequently use the freeze frame data in Mode $02 to diagnose a Toyota Matrix P0171 lean code in the winter. These vehicles typically aren’t running lean when I see them, but the Mode $02 freeze frame data shows the outside temperature when the code set was -20 degrees Fahrenheit and the intake manifold gaskets are as hard as rock and unable to seal properly (that’s why it always runs fine by the time I see it).

Be aware that the freeze frame data can be overwritten by higher priority DTCs, (a misfire code, for example, will overwrite a lean code) and all of the Mode $02 data be cleared when the codes are cleared.

Mode $03... Request the emissions-related diagnostic trouble codes. Mode $03 allows our scanner access to the stored DTCs in the PCM. These emissions related codes will be “P0, P2 or P34-P39” generic codes or “P1 or P30-P33” manufacturer specific OBD-II codes that have turned on the MIL.

Mode $04... Clear/reset the emissions-related diagnostic information. When you want to clear a stored code or codes, you enter into this mode to do it, but be certain of exactly what you want to do before proceeding.

Clearing/resetting the memory will not just turn off the MIL but also erase the freeze frame data and much more.

Clearing/resetting will reset the monitors status, distance travelled while the MIL was on, number of engine warm-ups, engine run time, oxygen sensor data, on-board monitoring test results and pending codes.

I try to record as much of this information as possible for future reference after the repair (I use screen capture or take a picture with my phone). Performing a clear/reset may erase some learned engine values and adaptive memories that may cause some driveability issues unless properly road-tested. Certain vehicles will not allow a clear/reset if the proper conditions aren’t met (Chryslers and Hondas both need the key on but engine off).

Mode $05...Request the oxygen sensor monitoring test results. This mode was intended to allow access to the PCM’s oxygen sensor monitoring values and test results. Different manufactures used many different and unique methods to calculate the O2 effectiveness and operation (one mode may convert that data to the same standard units of measurement).

This mode is only used if the vehicle is not using CAN (controller area network) electrical architecture. If it is a CAN vehicle this data is now stored in Mode $06.

Mode $06...Request the on-board monitoring test results for specific monitored systems. Mode $06 provides access to the OBD-II test results of the specific components and systems that it has tested.

It will show the results of both continuously monitored (misfire monitoring for example) and non-continuously monitored systems (EGR or evaporative systems for example) in this data.

Mode $06 shows actual test results and information but only if the readiness monitors status from Mode $01 shows that the monitor is complete.

This is important to remember because there may be data there before the monitor has run to completion and this may just be garbage data until the monitor has to run to completion.

Sometimes this data may need to be converted to usable values if your scan tool doesn’t do it automatically.

Many manufacturers’ websites provide information on how to do this and what the values mean. For example, Ford’s OBD-II information is free and a valuable resource that explains how systems are tested as well (

I like to look at Mode $06 data to see just how bad a particular component has failed; this is very helpful in P0420/430 catalytic failure codes. Mode $06 is not only useful for finding Ford misfires; it’s also really handy at identifying other issues that may appear after clearing a code.

I like to take a look at Mode $06 and see if any other tests are close to failing even though they are not related to the code or issue that I’m diagnosing or have just repaired.

Many times, a misfire code will shut off the enabling criteria for many other tests, and it’s good to have the information that the evaporative, or EGR system, was close to failing or that the catalytic converter was already borderline to warn a customer of another impending repair.

Mode $07... Request the emission-related diagnostic trouble codes detected during current or last completed driving cycle. This mode shows the codes that are pending or waiting to mature. Mode $07 was designed to aid in diagnostics after a repair by showing a test fail after one drive cycle. If the test fails again a related DTC will be stored. But if the fault is not seen again for 40 to 80 warm-up cycles the code will clear automatically. But if the test fails again during those cycles then the DTC will set and the MIL will be illuminated.

This mode should always be looked at before the codes are cleared as this information is cleared when a Mode $04 reset is performed. Pending codes that are unrelated to the hard code that turned on the MIL should be noted and mentioned to the customer before any repairs are performed.

Mode $08... Request control of on-board system, test or component. Mode $08 is intended to allow the scan tool to perform some bidirectional control of an onboard test or component. Typically it will allow you to seal the evaporative system to check for leak testing. This mode is really hit or miss and doesn’t usually do anything.

Mode $09... Request vehicle information. Mode $09 provides the vehicle’s VIN, calibration IDs, calibration verification numbers, and other useful data that includes in-use performance tracking. In-use performance tracking logs the number of times that specific component monitors (catalysts, primary 02’s, EGR, AIR, and evaporative system) ran to completion, and it also shows how many times the conditions to run that monitor were enabled and the total number of engine starts.

This information is useful in verifying a repair and knowing if something was inhibiting the enabling conditions (failing thermostats are very common at stopping monitors without trouble codes being generated). I like to look at this area to make sure that the monitor counters are increasing. If they seem to be fixed or display a very low number relative to the vehicle age or mileage, I am often suspicious of a car that has some form of tune or non-factory PCM software and this can help in your diagnostics.

Mode $0A (10)... Emission-related DTC with permanent status. We know that the MIL light can be turned off either by passing the prescribed number of tests without a fault reoccurring or having the fault repaired and using Mode $04 to clear the MIL. But just because you or someone else has turned off the MIL, that doesn’t mean that the PCM has forgotten that any codes were set. Starting on some 2009s and all 2010 and up vehicles, Mode $0A remembers that there was a fault serious enough to request the illumination of the MIL and keeps this information in a non-volatile random access memory (NVRAM).

This NVRAM can’t be erased by using Mode $03 or a battery disconnect and it can hold up to four codes for over a year until the diagnostic executive part of the PCM erases it after it performs a test verifying that it’s been repaired (typically passing three consecutive trips without fault).

Finishing up and putting this all in practical terms, and using it in the shop. Here is a scenario that I have encountered more than once. It’s a 2011 Ford Escape with a 3.0 V6. The customer’s issue is that it drives fine but the MIL is on.

The first thing I do is hook up an OBD-II capable scanner and look for codes. At this point I have used Mode $01 and Mode $03. It displays the code P0420, a catalytic efficiency code for bank 1. I then look at the freeze frame data (making sure that freeze frame data is for the P0420 code) and check the operating conditions when the MIL was turned on.

The Data in Mode $02 ( freeze frame) shows me that the vehicle was at operating temperature, in closed loop, on the highway and the fuel trims on both banks were at about +/-5% (which is OK). From this point I will go to Mode $01 again to make sure the catalytic monitor is ready and run to completion, then off to Mode $06 to look at the catalytic test data. In this case I am able to compare bank 1 with bank 2. Mode $06 data tells me that bank 1 failed badly, while bank 2 is well within specs.

Before I leave the Mode $06 area, I look over all the other test data to make sure nothing is close to failing (O2 values) just in case something is just on the borderline. At this stage I have pretty much decided that I am going to be looking at a converter, but I go to Mode $07 first and see if any other codes are pending.

From here I will check and see if there is any Mode $0A codes other than the P0420 that may have been cleared recently. If misfire, thermostat or O2 codes are still stored, they could be clues as to why the P0420 is setting. In an example case, there were no issues and only the P0420 is set.

The last step is to look at Mode $09 and see if the vehicle has had any issues running this test, and it hadn’t, in fact the test had run over 1,500 times. I have made my decision: It needs a new converter. Most techs are already using several OBD-II modes, but in many cases techs are quite unaware of all the information that is available. Using as many of the 10 OBD-II modes as possible can help in diagnostics, verifying a repair successfully and accomplishing that repair without the use of a factory scan tool.    ■

Jeff Taylor boasts a 31-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.

To read more articles by Jeff Taylor, click:

Servicing 4WD and AWD Systems

Multi-Rib V-Belt Systems

Leak Detection: Smoke, Dye and Electrocin Methods

Brake System NVH Woes: Addressing and Avoiding Noise, Vibration and Harshness Issues

HFO 1234yf Refrigerant and Coolant Mapping

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