What Is OBD2 Mode 06? On-Board Test Results Explained

Most drivers meet OBD2 through a trouble code or the check engine light. Mode 06 sits one layer deeper: it shows the numbers behind the vehicle’s own pass-or-fail decisions.

What Mode 06 Actually Shows

Mode 06, also written Mode $06 or Service $06, returns on-board monitoring test results for specific monitored systems. The engine computer runs self-tests on emissions-related components, compares each measured result with a limit, and stores that outcome for a scan tool to read.

A typical result has four parts: the monitor and test, the measured test value, the limit or limits for that test, and whether it passed. Some tools translate those fields into plain descriptions; others show only labels, codes, or raw values.

The key point is timing. Mode 06 generally reports the last completed test, not live data. If the monitor has not run since codes were cleared, there may be nothing useful to read yet.

How Mode 06 Differs From Other OBD2 Modes

Several modes deal with codes, monitors, or test data, so the distinction is worth making clear.

A code scan tells you what the computer has decided. Mode 06 helps explain the evidence behind that decision, and sometimes shows deterioration that has not yet crossed the threshold for a code. For the wider set of services, see the overview of OBD2 service modes.

How to Read a Mode 06 Result

Start with the label, not the number. A good scan tool names the monitor, such as catalyst, oxygen sensor, oxygen sensor heater, evaporative emissions (EVAP), exhaust gas recirculation (EGR), or misfire. Cheaper tools may show only identifiers, and those identifiers can be manufacturer-specific, so service information may be needed to translate a raw entry.

On CAN vehicles, results are commonly organized around a monitor identifier, a test identifier, and a component identifier. On older non-CAN vehicles, oxygen sensor results were handled under Mode 05; on CAN vehicles, much of that information moved into Mode 06.

Next, check the direction of the limit. Some tests have only a maximum limit, some only a minimum, and some both. A value can therefore be too high in one test and too low in another, so do not assume that a larger number is always worse. Units matter too: a result may be in volts, milliseconds, counts, or pressure, and an unscaled raw number should not be judged until its unit and scaling are confirmed.

Then compare the test value with the limit:

• Comfortably inside the limits: the test passed with margin. It does not prove every part is new, only that this test passed under its conditions.
• Passing but close to a limit: the early-warning case. Note the value and monitor, then recheck after driving rather than replacing parts on this evidence alone.
• Outside a limit: the test failed. That points to the monitored system, but not to one part: a sensor, leak, wiring fault, fuel issue, or the component itself can affect the same result.

Which Systems Can Appear in Mode 06?

The list depends on the vehicle and the monitors it supports. Common entries include catalyst efficiency, oxygen sensor response and heater operation, EVAP leak and purge tests, EGR flow, variable valve timing, thermostat operation, secondary air, and misfire-related tests. A vehicle will not show every possible monitor, and the same monitor may be organized differently on another model year.

Mode 06 mainly covers non-continuous monitors, which need particular conditions before they run. If a test has not run, its result is not meaningful, even if the menu lists it.

A catalyst result is a good example, because efficiency can decline before the familiar catalyst code sets. The separate guide to whether OBD2 can detect a catalytic converter problem explains what that reading can and cannot prove.

How to Find Mode 06 on a Scanner

Menu names vary more than the data does. Look for On-Board Monitoring, Mode $06, Monitor Test Results, or On-Board Monitor Results, sometimes under monitors or advanced OBD2 functions rather than trouble codes.

1. Connect the scanner with the ignition on, and record stored and pending codes first. Do not clear codes before checking Mode 06, because clearing can also clear the test results you want to see.
2. Open the on-board monitoring menu and let the tool poll the vehicle. A long list is normal: it is a set of individual tests, not one overall score.
3. Find the monitor related to the symptom or code, then note its test value, limits, units, and pass-or-fail result.
4. If a monitor has not completed, drive under normal conditions and recheck. A monitor that has not run cannot confirm a repair.
5. After a repair, compare the new result with the earlier one. The useful question is whether the value moved away from its limit, not only whether the light stayed off for a day.

What a Near-Limit Pass or a Fail Really Means

A near-limit pass is a reason to watch, not a diagnosis. It can reflect normal aging, a borderline sensor, or a small leak, but it can also be the first sign of a part that will fail on the next drive cycle. Repeat the test and check related data rather than acting on one number.

A failed test is stronger evidence, but it still needs context. Check for a related code, a pending code, live data, or freeze frame. Mode 03 stored codes show confirmed faults, while pending codes show a fault detected during the current or most recent drive cycle but not yet confirmed. Mode 06 shows the test measurement itself.

For repairs, Mode 06 is most useful as a before-and-after check. Record the original results, drive until the monitor runs again, and compare. If the value has moved well inside its limits, the repair looks supported; if it is unchanged or still outside a limit, the original cause may not have been fixed.

Readiness Is Not the Same as a Good Mode 06 Result

Readiness tells you whether a monitor has completed since codes were cleared. Mode 06 tells you what that monitor measured. A monitor can be complete and still show a value close to its limit, and an incomplete monitor gives no completed result to trust yet.

This matters before an emissions inspection or after clearing codes: complete readiness means the tests have run, not that every system is healthy. The guide to how OBD2 readiness monitors work covers completion, while Mode 06 covers measurement. See Tomorrow’s Technician’s guide to using readiness monitors to verify emissions repairs.

Why Mode 06 Can Look Blank, Cryptic, or Confusing

Several normal reasons explain a difficult screen. The monitor may not have completed because of short trips, cold starts, low fuel level for some EVAP tests, or a recent code clear. A monitor supported only through manufacturer-enhanced data may also show only a partial list in a generic tool.

Tools also differ in decoding. A modern tool may translate a test into “Catalyst Bank 1” with a clear pass mark, while an older tool shows only hexadecimal identifiers and unscaled numbers. Start with the monitor tied to the symptom, code, or repair, rather than chasing every near-limit value at once.

When Mode 06 Is Worth Checking

Mode 06 is worth the extra effort when there is a symptom but no stored code, when a code returns after a repair, when a used car needs a deeper check than a clean code scan, or when a repair needs evidence beyond “the light stayed off.” It is less useful as a routine check on a car with no symptom, because most results will simply pass.

For a pre-purchase check, the value is in the margins: a clean code scan can hide a catalyst or oxygen sensor test that passed only narrowly. That is a reason to ask whether the value is stable after a proper drive. See Walker’s guide to accessing Mode $06 diagnostic data.

Frequently Asked Questions

Is Mode 06 the same as live data?

No. Live data shows current sensor readings while the engine runs. Mode 06 shows the result of a test the vehicle already completed, including its measured value and limits.

Can Mode 06 tell me exactly which part to replace?

No, not by itself. A failed result identifies the monitored system or test, but several parts, sensors, leaks, or wiring faults can affect the same measurement. Narrow the system first, then check related codes and live data.

Why is my Mode 06 list empty after clearing codes?

Clearing diagnostic information can also clear stored test results, and monitors then need to run again before new results appear. Drive normally, let the relevant monitor complete, and recheck.

Does a passing Mode 06 result mean the car will pass an emissions test?

Not necessarily. A passing result is a good sign for that monitor, but emissions testing also depends on readiness completion, stored and permanent codes, and local test rules.

Why do two scanners show Mode 06 differently?

Scanners differ in how they label monitors, translate identifiers, scale values, and display units. The underlying vehicle data is the same; the translation layer differs, and manufacturer-specific details can require service information.

Should I worry about every value close to its limit?

No. A single near-limit pass is a reason to record the value and recheck it, not an automatic repair. Concern grows if the value moves closer to the limit over time, a related pending code appears, or the same system shows symptoms.

Conclusion

Mode 06 is the measured evidence behind OBD2 monitor decisions: a completed test value compared with its limits. It will not run a test on demand or name the failed part, but it can reveal a system losing margin before a code appears and confirm whether a repair truly improved the measurement. Record the result before clearing codes, check the monitor tied to the symptom, and compare before-and-after values after the monitor runs again. That habit turns a long list of numbers into a practical diagnostic check.