OBD2 Readiness Monitors: Can You Tell If Codes Were Cleared?

A seller who wants a check engine light gone for a weekend does not need a mechanic. The erase command on any code reader does it in seconds, and so, on most cars, does leaving the negative battery cable off for a while. The lamp goes dark, the stored codes vanish, and the car looks clean to anyone who only glances at the dash.

What the erase command cannot do is make the car forget that it was erased. The rules that govern the on-board diagnostic system require it to keep track of exactly that, in three different places, and all three are visible to a cheap generic scan tool plugged into the port under the steering column. This page walks through them with the regulation text behind each one, because the regulation is what tells you which signals a manufacturer was forced to build in and which ones are optional.

Some limits up front. I read regulations and agency pages; I am not a technician and I have not scanned the car you are looking at. Every one of these signals has an innocent explanation, and none of them tells you what the fault was. What they give you is a well-founded question to put to the seller, and a reason to ask the inspector you hire to look harder at one system.

What a code clear resets, according to the rule that requires it

The design requirements for OBD II come from California. The Air Resources Board's regulation at 13 CCR 1968.2 sets them, and the federal side borrows it: 40 CFR 86.1806-17 requires 2017 and later light-duty vehicles to meet the version of 1968.2 that California approved on 31 July 2013, and lets a manufacturer certify to a later version instead. The section numbers below come from the current California text, so a federal-only car may follow the 2013 wording on the newest details. That is why a Honda in Ohio and a Ford in Georgia answer the same generic questions in the same format. I read the California text on Cornell's Legal Information Institute on 9 October 2026; the history note there lists amendments through November 2022.

Section (g)(4.1.1)(A) of that regulation requires the system to report, for each monitored emissions system, whether its self-test is "complete" or "not complete" "since the fault memory was last cleared." Section (g)(4.1.1)(F) then says what clearing does:

the readiness status for each of the monitored components or systems shall indicate "not complete" whenever fault memory has been cleared or erased by a means other than that allowed in section (d)(2). Normal vehicle shut down (i.e., key off, engine off) may not cause the readiness status to indicate "not complete".

Two consequences follow. Turning the car off at night does not reset anything, so a string of incomplete monitors cannot be blamed on the car having sat. And clearing codes resets all of them at once, not just the monitor that found the fault.

There is an exception that matters for reading the screen. Section (g)(4.1.1)(B) says three readiness bits "shall always indicate 'complete'": gasoline misfire, diesel misfire on engines that use a single misfire monitor for both kinds of misfire the rule defines, and the comprehensive component monitor. A code clear will not make those show incomplete. Scan tools usually lump the fuel system monitor in with them as "continuous," but fuel system is not on the forced list; on 2019 and later cars its readiness bit is tied to the cylinder imbalance monitor under section (g)(4.1.1)(D), so it can legitimately read incomplete after a clear. So a misfire that was cleared an hour ago leaves no trace in the readiness list itself, which is the strongest reason to read the counters in the next section too.

Reading the readiness screen

Most generic tools show the readiness list as a column of system names with a status beside each. The names come from the regulation's list of monitored systems, and on a gasoline car you will usually see some subset of these:

Two other statuses look alike and are not. Ohio EPA's readiness fact sheet (revision dated 16 September 2024, read 9 October 2026) describes a vehicle performing "up to 11 diagnostic checks," and its key to the inspection report separates the two. "N/A" means the test "is not equipped on your vehicle by the manufacturer," which is normal, since not every engine has secondary air or EGR. "Unsupported" means the test "is equipped on your vehicle by the manufacturer but is no longer present," and Ohio says that "will need to be corrected" before the car can pass. A monitor the factory built that has since gone missing points toward altered engine software or a removed component, and it deserves a direct question. Many cheap readers print the same "not supported" label for both cases, so the factory specification for that engine is the tiebreaker.

The pattern is more informative than any single line. One incomplete EVAP monitor on a car that has been parked on a dealer lot for two weeks with a full tank is common and is the case the inspection programs were written to tolerate. Catalyst, oxygen sensor, oxygen sensor heater and EGR all incomplete together, on a car the seller says has been driven daily, is the shape a fresh clear leaves. The Ohio sheet says plainly why: after codes are cleared or the battery is disconnected, the owner should "drive the vehicle for at least two or three days (older vehicles may take more time) including both highway and city miles" before the monitors report ready.

That two or three days is also the weakness. Readiness only tells you something during the window before the monitors finish, and a seller who cleared a code a week ago and drove the car to work in between may hand you a screen of solid "complete." The next two signals do not have that window.

The counters that keep counting after the monitors finish

For cars that talk to the scan tool over the CAN protocol, section (g)(4.2.2) of the California regulation adds signals that must be available on demand. Among them, quoted from the section:

distance traveled since fault memory last cleared, and number of warm-up cycles since fault memory last cleared.

That paragraph applies to "all 2005 and subsequent model year vehicles using the ISO 15765-4 protocol," and section (g)(3.4) says ISO 15765-4, the CAN standard, is "required on all 2008 and subsequent model year vehicles." Put together: on any 2008 or newer car, and many 2005 to 2007 ones, the computer reports how far it has gone and how many times it has warmed up since someone last wiped its memory.

These do not reset when the monitors complete. They reset only when memory is cleared again. California's Bureau of Automotive Repair relies on them by number in its OBD test reference, read 9 October 2026, which describes using "Mode $01 PID $30 and $31" to determine distance and warm-ups "since its OBD information was last cleared." Generic tools label them in plain English, usually something like "warm-ups since codes cleared" and "distance since codes cleared," and often display the distance in kilometers because that is how the standard reports it.

How to read them:

• Compare the distance to the odometer. A car with 96,000 miles on the cluster whose computer reports 30 miles since the last clear had its memory wiped within the last day or two of driving.
• Compare the warm-ups to the seller's story. A car described as a daily commuter that shows three warm-ups since the last clear has been started from cold three times since the wipe.
• Treat a high number as weakly reassuring. A distance in the thousands means no one cleared memory recently. It does not mean no one ever did.

BAR uses the same counters for its own threshold, which gives a sense of scale for when a clear stops being "recent." For a car with a stored permanent code, its reference says the code "will be ignored if the vehicle has completed at least 15 warm-up cycles and been driven at least 200 miles since its OBD information was last cleared." Fifteen warm-ups and 200 miles can be read as the regulator's working line for "the car has had a fair chance to check itself again." A reading well below both is worth a question.

Permanent codes: the one a scan tool cannot erase

The third signal was built specifically to defeat clearing before an inspection. Section (g)(4.4.6)(C) of the California regulation:

Permanent fault codes shall be stored in NVRAM and may not be erasable by any scan tool command (generic or enhanced) or by disconnecting power to the on-board computer.

Section (g)(4.4.6)(B) says a confirmed code that is turning the check engine light on must be stored as a permanent code "no later than the end of the ignition cycle." Section (d)(2.5) then gives the way out: the system erases a permanent code itself once its own monitor has rerun and decided the fault is gone. The only other route is reprogramming, covered further down. A code reader cannot do it. Neither can the battery cable.

The phase-in matters when you are shopping older cars. Section (d)(2.2.5) applied the requirement to "50 percent of all 2010, 75 percent of all 2011, and 100 percent of all 2012 and subsequent model year vehicles." BAR's reference repeats the caution in its own words: "Some 2010 vehicles do not support PDTC." On a 2009 car, the screen will simply have nothing to show, and that means nothing.

On a 2012 or later car, a permanent code with the check engine light off and the ordinary code list empty is close to a description of a recent clear. The fault happened, the light came on, somebody erased the confirmed code, and the car has not yet been driven enough to retest that system. It is also what a completed repair looks like for the first few drives when the shop cleared the codes afterward, before the monitor reruns. The difference is a repair invoice.

Not every inexpensive reader can show permanent codes. Many list them under a separate heading labeled "permanent," apart from "stored" or "confirmed" and "pending." If yours has no such screen, assume it cannot read them rather than assuming the car has none.

What an inspection lane would do with the same screen

A car that cannot pass emissions inspection in your state becomes your problem the day you register it. So it is worth knowing how the lane reads readiness.

The federal procedure for state OBD inspections is 40 CFR 85.2222. I pulled it from the eCFR versioner API on 9 October 2026, requesting the text as it stood on 1 October 2026. Section (c) has the test system send "a Mode $01, PID $01 request," which is the readiness query, and paragraph (c)(1) says a car with incomplete monitors is sent away to be driven and failed if it comes back still incomplete. Paragraph (c)(2) is the tolerance most states copied:

An exception to paragraph (c)(1) of this section is allowed for MY 1996 to MY 2000 vehicles, inclusive, with two or fewer unset readiness monitors, and for MY 2001 and newer vehicles with no more than one unset readiness monitor.

The same paragraph adds that a car with an unset monitor and a stored code lighting the check engine light "must be failed." Paragraph (d)(4) has a check worth copying in a parking lot: if the light "does not illuminate at all when the vehicle is in the key-on/engine-off condition, the vehicle shall fail the OBD inspection, even if no DTCs are present." A bulb that never lights at key-on is a failed inspection on its own, and it is the crudest way to hide a fault.

New York's DMV, in its page What Do You Mean My Car's Not Ready? (read 9 October 2026), applies the federal tolerance as it describes it: EPA guidelines "allow only one monitor to be 'not ready' for 2001 or newer model year vehicles to pass inspection." The page lists the causes of a not-ready reading, and the first one is "Recent vehicle repairs in which diagnostic trouble codes have been cleared with a OBD II scan tool." If readiness was the only failure and the old sticker has expired, New York issues a 10-day extension to finish the drive cycle.

California runs tighter than the federal floor. BAR's reference table, read 9 October 2026:

A footnote says the misfire, fuel and comprehensive component "continuous monitors are ignored" during the test on gas vehicles, so the lane never fails a car on those three flags. And BAR's Fall 2025 newsletter, New OBD readiness monitor regulations explained (read 9 October 2026), says BAR adopted rules effective 1 October 2025 requiring "that all readiness monitors must be set for a vehicle to pass," while keeping the current pass/fail chart in force until data show particular vehicles "can reasonably be expected to complete." BAR's stated reason is the one a buyer cares about: old exemptions let some cars pass with a malfunctioning emissions system, which "can result in costly, unexpected repairs for future owners." The test reference adds that California put a permanent code check into the inspection on 1 July 2019 for 2010 and newer vehicles, failing any car with one stored "regardless of whether the MIL is commanded ON or OFF." The same reference flags a newer gap: its vehicles-of-interest table lists all 2026 and 2027 models as using OBDonUDS, a newer diagnostic protocol its current lane equipment cannot talk to, so the OBD portion is skipped on those cars for now. An older generic reader may hit the same wall on a 2026 or later car.

The practical reading: a 2001 or later car showing two or more incomplete monitors would fail most states' lanes today, and in California even one non-EVAP monitor fails a 2000 or newer gas car. If the seller says it just passed, ask to see the inspection report and compare its date to the counters.

Running the check in the parking lot

The whole sequence takes a few minutes with a generic reader. Do it before the test drive if you can, because the drive itself adds distance and a warm-up to the counters.

1. Key on, engine off, and watch the check engine light. It should light. Note whether it does. A lamp that never lights fails 40 CFR 85.2222(d)(4) and may have been disabled.
2. Start the engine. The light should go out within a few seconds. If it stays on, there is an active fault, and the rest of the scan is about which one.
3. Plug in and read stored and pending codes. Pending codes are faults the system has seen once but not yet confirmed, and they show up before the lamp does. A clear wipes them too, and the system drops one on its own after a later drive under similar conditions finds no fault, so an empty pending list on a freshly cleared car proves little.
4. Read permanent codes, if the tool supports them. On a 2012 or later car, any entry here with the lamp off and no stored codes deserves a question.
5. Read the readiness list. Write down each monitor and its status. Misfire and comprehensive component will always say complete, so they tell you nothing.
6. Read distance and warm-ups since codes cleared. Write both down next to the odometer reading. Photograph the screens; the next owner of this information is the inspector you hire.
7. Ask one question with the numbers in hand. "The computer says it was reset 40 miles ago. What was the code, and do you have the invoice for the repair?"

If the seller produces a dated invoice for a catalytic converter, an oxygen sensor or a battery from last week, the signals are explained and you have learned the service history. If the answer is that nothing has been done to the car, the numbers say otherwise, and that is information about the seller as much as the car.

The innocent reasons, and what readiness cannot see

Each signal has ordinary causes, and a fair reading accounts for them before drawing a conclusion.

A new battery or a dead one. New York's list of not-ready causes includes a battery that "has been recently disconnected or replaced." A dealer that jump-started a car sitting on the lot for a month may have wiped its memory without meaning to. The battery's date label and the dealer's reconditioning record will usually settle it.

A recent repair done properly. Shops clear codes after a repair as a matter of routine, which is exactly the situation the counters and readiness flags are designed to expose. Ohio EPA actually asks owners to tell their technician not to clear codes when possible, so the lamp turns itself off once the repair is confirmed. A fresh clear with a matching invoice is a recent repair, not a cover-up.

A software update. Section (g)(4.4.6)(D) of the California regulation allows permanent codes to be erased during reprogramming only if readiness is reset to "not complete" at the same time. A recall or service campaign that reflashed the engine computer leaves the same signature as a clear. Run the VIN through the NHTSA recall lookup and ask whether any campaign was done recently.

EVAP on its own. The evaporative monitor needs a fuel level and temperature window that many cars hit rarely. Every inspection program cited on this page currently tolerates it alone for that reason, although California's 2025 rules leave room to change that for specific vehicles.

The limits run the other way too. A screen of solid "complete" is not a clean bill of health. Section (g)(4.1.1)(D) says that on 2019 and later models a monitor goes to "complete" when it has run and found nothing wrong, or when it "has determined that the component or system is malfunctioning"; section (g)(4.1.1)(C) sets the same rule for 2004 to 2018 models. A completed monitor that found a fault should have produced a confirmed code and a lit lamp, which you would see in steps 2 and 3, but the readiness flag alone does not distinguish the two.

And all of this covers emissions-related systems only. Readiness has nothing to say about worn transmission clutches, a leaking head gasket that has not yet affected a sensor, a bent subframe or a repaired flood. The scan is one line in the scope of an independent pre-purchase inspection, and the cheapest one. Its job is to tell you which system to ask the technician to look at harder, and to give you a dated, photographed record of what the computer said before you paid.

One last use for the numbers. If the cleared code turns out to have been a catalytic converter or an engine computer fault on a car still inside 8 years and 80,000 miles, the repair may fall under the federal emissions warranty, which follows the car to the next owner. A seller who cleared that code rather than claiming it may have left a free repair on the table. That is worth knowing before you agree a price, and more useful than the bare fact that someone pressed erase.