Mode $06 Explained in Detail

Mode $06 Explained in Detail

Mode $06 is one of the nine diagnostic modes that is part of the OBD2 onboard diagnostics system on all 1996 and newer vehicles. As originally created, Mode $06 was not designed for use by technicians. It was supposed to be “hidden” data used by the OBD2 system to find faults and configure fault codes.

It was not supposed to be a readily accessible scantool data for evaluating 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 plethora of useful diagnostic information. It turned out to be the ultimate of diagnostic information — provided you can figure out how to read and use it.

Before we go any further, we should say that few counter professionals will ever use Mode $06 to assist a customer analyze a Check Engine light. Most of the scanners that parts stores employees typically use to perform plug-in diagnostic checks on customers’ cars are basic code fault readers or entry-level scanners.

So many of these diagnostic tools can’t even access Mode $06. But even if you have a scanner that has Mode $06 ability, it’s doubtful you would ever have the need to delve that far into the on-board diagnostics. That level of troubleshooting is best left to a professional mechanic with a lot of experience and know-how in scantool diagnostics arena.

With that said, it’s helpful to be aware of Mode $06 and how a mechanic can use it to troubleshoot a Check Engine light that comes back on after repairs have been made. In fact, if a technician knows how to use Mode $06 to look for specific types of faults, they can usually keep the MIL (Malfunctioning Indicator Lamp) out and prevent unnecessary comebacks.

Mode $06 is often the key to resolving such issues as:

  • no-code driveability and emission faults
  • identifying sensors and other components that are still operating within acceptable limits but are on the verge of failure
  • reducing comebacks involving the catalytic converter
  • EVAP system and EGR system repairs
  • engine misfire


Mode $06 data is information the OBD2 system tracks and compiles on “noncontinuous” monitors. It is tabulated in hexadecimal code (a base 16 number system that uses digits 0 to 9 plus letters A through F), which is the computer language that the PCM uses to manage things. The “$” symbol means the data is in hex code, not ordinary decimal numbers.

Fact that Mode $06 data is in hex code means it has to be converted into familiar decimal values for us to understand it. This requires some math and conversion factors, or scantool software that can do the translation for you.

You also 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 Mode $06 test value is good, bad or somewhere in the middle.

On OBD2 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 compatible vehicles, 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 the capabilities and software in the reader 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.

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

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.


As long as all of the components that are being monitored and tested by the OBD2 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 OBD2 will set a code), and based on 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.


Because the OBD2 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.

As a result, an O2 sensor component test failure will prevent the catalyst monitor from running its tests. It’s crucial 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 fact of Mode $06: Even if 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 would tell you the sensor is probably causing a problem and will soon fail. Time to replace it with a new one.


In some cases, OBD2 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 OBD2 plug-in emissions test if the Check Engine light is on. You have to read and clear the code, diagnose the fault and make the required repairs so all the OBD2 monitors will run and pass.

Let’s suppose the car failed an OBD2 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 instances, 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. One 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, one can use Mode $06 to look at the various EVAP component tests to see why a particular code might have set. One 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 OBD2 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).


Another thing to be on alert 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 OBD2 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 technicians to peer deep into the inner workings of the OBD2 system.

It requires a OBD2 scanner that can access and display the TID and CID test results, and an understanding of the data that is displayed.

Mode $06 Explained in Detail
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