What’s the one thing you always want to avoid when diagnosing and repairing driveability and emissions problems? Comebacks.

Nothing is more frustrating than to “fix” a customer’s vehicle and then have it come back a few days or weeks later with the check engine light on and the same fault code or a new fault code. It makes you look bad, it undermines your customer’s confidence in your ability to fix their vehicle, and in states that track your success rate at repairing emission failures, a low score can steer potential customers away from your shop.

So how can you improve your success rate at accurately diagnosing and repairing driveability and emissions faults, and reducing comebacks? Mode $06 may be the answer.

Mode $06 is one of the nine diagnostic modes that is part of the OBD II on-board diagnostics system on all 1996 and newer vehicles. As originally conceived, Mode $06 was not designed for use by technicians. It was supposed to be “hidden” data used by the OBD II system to detect faults and set fault codes. It was not supposed to be readily accessible scan tool data for analyzing the operation of various components in the engine management and emission control systems. Consequently, many older scan tools lacked the capability to access and display Mode $06 data.

But as time went on, some very tech savvy technicians discovered that Mode $06 contained a wealth of diagnostic information. In fact, it turned out to be the mother load of diagnostic information — provided you can figure out how to read and use it.

The fact that Mode $06 data is in hexadecimal code (a base 16 number system that uses digits 0 to 9 plus letters A through F) makes it difficult to translate and convert into familiar decimal values. That’s what the dollar sign is all about. The “$” symbol means the data is in hex code, not ordinary decimal numbers.

To understand what the hex code data means, you need a reference chart from the vehicle manufacturer that identifies what component or system test each line of code refers to, and what the acceptable range of values are for that particular test. Only then can you determine whether or not a particular test value was good or bad. Unfortunately, much of this information was difficult to locate or had never been published!

It seemed like a lot of extra effort just to figure out why a particular fault code was being set. But for those who figured out how to use Mode $06 data, it became apparent that Mode $06 was the key to solving no-code driveability and emission faults; to identifying sensors and other components that were still operating within acceptable limits but would likely fail soon; and to reducing unnecessary comebacks, especially those involving catalytic converter, EVAP system and EGR system repairs.

Digging In
On OBD II systems up to the introduction of Controller Area Network (CAN) electrical systems, there are two code identification tags for each line of data. The first is the Test Identification (TID) that indicates the monitor test, and the second is the Component Identification (CID) that identifies the sensor or other component that is being tested. After that come the actual test results. On CAN vehicles (those from 2003 to 2008 and newer), the TID is now called MID for Monitor Identification. It’s the same thing with a different name. Better yet, the MID test IDs have been standardized across different vehicle makes and models.

Depending on the capabilities and software in the scan tool or scanner software you are using, the hex code TID and CID values may be translated into plain English (or Spanish) followed by the hex code or decimal test results, the range of acceptable values for that particular test, the units of measure for that test (voltage, pressure, etc.), and a PASS or FAIL indication.

Within each of these monitor tests are additional subtests of various components. For example, within the MID$01 oxygen sensor monitor is a CID$11 O2 sensor switch point test, a CID$80 O2 sensor signal amplitude test, and a CID$81 O2 sensor heater circuit amperage test. Under the MID$31 EGR monitor there is a CID$80 delta pressure upstream test for the differential pressure (DPFE) sensor, a CID$81 delta pressure downstream test, a CID$84 delta pressure EGR leak check test and a CID$85 delta pressure EGR flow test.

Each of these tests has an upper and lower limit that is programmed into the PCM by the calibration values for that particular year, make and model of vehicle. The acceptable range of test results for each CID is determined by the vehicle manufacturer. Cut points are then set to assure emissions compliance.

Pass or Fail?
As long as all of the components that are being monitored and tested by the OBD II system pass, no codes are set and the check engine light remains off. If a component fails a test, it won’t necessarily set a code and turn on the check engine light. It depends on the component that failed the test, how many times it failed the test (multiple failures may be required from some tests before OBD II will set a code), and whether or not the test has actually run because another fault has prevented the test from running.

An oxygen sensor fault, for example, will prevent any of the catalyst monitor tests from running. Why? Because the OBD II system requires reliable inputs from all of the upstream and downstream O2 sensors to check catalyst efficiency. If the vehicle has a bad upstream or downstream O2 sensor, it can’t compare before and after readings to calculate converter efficiency. Consequently, an O2 sensor component test failure will prevent the catalyst monitor from running its tests. It’s important to keep this in mind when reviewing Mode $06 data because a failure in one test may prevent other tests from running.

Here’s another mystery of Mode $06: As long as a sensor is functioning within the established limits for a particular CID test, you can’t always assume the sensor is functioning normally. If it is near the upper or lower limit of the acceptable test range, it may be enough to cause a noticeable driveability or emissions problem.

For example, say one of the oxygen sensors is lazy or is biased rich or lean. It may not be bad enough to fail its CID tests, but it might be sluggish or biased enough to throw off the air/fuel mixture, resulting in a loss of fuel economy, poor throttle response or an engine misfire. That’s why you need to look closely at the CID test results for that sensor in Mode $06 so you can see how close the sensor is operating to its upper and lower limits. If the test results are just within the limits, it will tell you the sensor is probably causing a problem and will soon fail. Time to replace it with a new one.

The following is a partial list of the Mode $06 monitor tests that can be accessed on a late-model Ford vehicle with a CAN electrical system:
MID$01 – Oxygen sensor
MID$02 – Oxygen sensor
MID$03 – Oxygen sensor
MID$05 – Oxygen sensor
MID$06 – Oxygen sensor
MID$07 – Oxygen sensor
MID$21 – Catalyst monitor
MID$22 – Catalyst monitor
MID$31 – EGR monitor
MID$32 – EGR monitor
MID$33 – EGR stepper motor
MID$3A – EVAP monitor
MID$3B – EVAP monitor
MID$3C – EVAP monitor
MID$71 – Air injection monitor
MID$A1 – Misfire monitor
MID$A2 through MID$AB – Misfire monitors for each cylinder

Finding the Real Cause
In some cases, OBD II may set a fault code for a problem that is being caused by something else. This includes lean fuel mixture codes, misfire codes and catalyst efficiency codes. A vehicle cannot pass an OBD II plug-in emissions test if the check engine light is on. You have to read and clear the code, diagnose the fault, and make whatever repairs are required so all the OBD II monitors will run and pass.

Let’s say a vehicle failed an OBD II plug-in emissions test because of a P0420 catalyst efficiency code. Should you replace the converter, the oxygen sensors or what?

Here’s how Mode $06 can help. By looking at the CID test results data, you can see which catalyst test failed. Then, you can look at the various CID tests for the upstream and downstream oxygen sensors to see if perhaps one or more sensors are lazy or biased, and are affecting the test results. In some cases, you may find the O2 sensors are all functioning well within their normal limits, but catalyst efficiency is being affected by a crack or pinhole in an exhaust manifold, Y-pipe or flex pipe ahead of the converter that’s leaking air into the exhaust.

EVAP codes are always a pain because EVAP problems can be difficult and time-consuming to diagnose. You can spend hours with a smoke machine trying to find pinhole leaks when the real fault may be in the canister purge control solenoid, a vent solenoid or the fuel pressure sensor in the fuel tank. Again, you can use Mode $06 to look at the various EVAP component tests to see why a particular code might have set. You can also look at the various EVAP test results to make sure all of the components are well within range and not operating near their minimum or maximum threshold limits.

Though OBD II performs a long list of component self-tests, sometimes a test limit may be set too high or too low, causing codes to set unnecessarily. For example, a vehicle may be setting a misfire code repeatedly because the maximum limit for misfires was set too low. The vehicle manufacturer may revise the limit and offer a new calibration that can be downloaded and loaded into the vehicle’s PCM. That’s the beauty of reflashing PCMs. It allows changes in calibration to be easily updated (assuming you have a J2534 pass-through tool or a scan tool with flash capabilities).

Bogus Data
Another thing to watch out for when viewing Mode $06 data are TID/CID tests that contain old data or bogus data. On some Ford V6 applications, Mode $06 will report misfire rates for cylinders that don’t exist! The CID test results may show high misfire rates for cylinders 7 and 8. Of course, there are no cylinders 7 and 8 in a V6. It’s a programming glitch that OBD II ignores as it monitors misfires. But if you don’t know that, you may assume the engine has a misfire problem.
The same can apply to other test lines as well. If the vehicle is not equipped with a particular sensor or component, there may be bogus data displayed for that test result.

The bottom line is that Mode $06 is an advanced diagnostic mode that allows you to peer into the inner workings of the OBD II system. It requires a scan tool that can access and display the TID and CID test results, and an understanding of the data you are looking at and how to use it. Mode $06 is not something you are going to pick up overnight. It takes some hands-on experience to appreciate its subtleties and capabilities. But once you master it, it can provide invaluable diagnostic insight into many OBD II failures.

The best way to get familiar with the mysteries of Mode $06 is to take some online training courses, or attend a hands-on clinic at a community college, vo-tech school or other location. Bring your own scan tool if you don’t know how to access Mode $06 and have the instructor show you how to find and display it. Then, play around with the Mode $06 data on some vehicles to see how the various test results are displayed. This way you can learn to distinguish good readings from borderline readings to detect sensors and other components that may be causing driveability or emission faults without setting a code.