How to Reset the Check Engine Light in 2026: Three Ways to Turn It Off, and What Each One Erases

The question people type is "how do I reset it". The question that saves money is "what happens after I do". This page answers both from the rules that decide it: California's OBD II regulation (which, CARB says, virtually every car sold in the US is designed to meet), the federal inspection rules and EPA's guidance, five state emissions programs' own pages, and the owner's manuals of three carmakers. What the light usually means is in check engine light on; where to get the code read for free is in the AutoZone free scan guide. Neither is repeated here.

The answer in sixty seconds

If you remember one line: the light is a messenger, and every reset method except the first one silences the messenger without touching the message.

Before you reset anything: is the light steady or flashing?

This decides whether the rest of the page applies today. A flashing check engine light is a different situation from a steady one, and every source here that describes the flashing light treats it as the more urgent of the two.

EPA's 1997 consumer fact sheet on OBD (EPA 420-F-97-003) explains the steady light first: it is "intended to inform the driver of the need for service, NOT of the need to stop the vehicle. However, service should be sought as soon as possible." On the flashing light it is blunter: "This indicates a rather severe level of engine misfire. When this occurs, the driver should reduce speed and seek service as soon as possible." The regulation behind it is specific. California's OBD II regulation requires that once a catalyst-damaging misfire is detected, "the MIL shall blink once per second at all times while misfire is occurring during the driving cycle" (MIL is the regulation's name for the check engine light), with an exemption for cars that cut fuel during such a misfire, where "the MIL need not blink" and may stay steady instead.

The makers' manuals turn that into instructions, and they differ in degree. The speeds below are the makers' limits for getting the car to service, not for carrying on driving:

Our reading: none of these sources suggests clearing a flashing light, and doing so would not stop the misfire that makes it flash. If the light is flashing, the job today is getting the car safely off the road and into a shop; the flashing light and shaking guide walks through that. If the light is steady and the car drives normally, it is not an emergency, but the sources say to get it checked promptly: EPA says "as soon as possible", and Honda's manual says "Avoid high speeds and immediately get your vehicle inspected at a dealer." The rest of the page is for you.

Two more checks before any reset. Read and write down every code first, including pending codes and the freeze frame, because two of the three methods delete them; a free parts-store read or an inexpensive reader does it. And look at the fuel cap (section below).

How we checked

Everything on this page was read on 30 September 2026 and saved as text; quotations come only from the saved text.

The rules. California's OBD II regulation, 13 CCR 1968.2, says in numbered sections when the light comes on, when it goes off and what a clear erases. We read the current text as published by Cornell's Legal Information Institute, and CARB's own 2021 amendment order. It is a California rule, but not only a California one: EPA's rule for 2017 and newer vehicles, 40 CFR 86.1806-17, says "Vehicles must comply with the 2013 OBD requirements adopted for California", with listed exceptions (the 2013 version, which we did not read; the wording we quote for 2019 and newer models is from the current text), and CARB's OBD II fact sheet says "In practice, virtually all vehicles sold in the U.S. are designed and certified to meet California's OBD II requirements, regardless of where in the U.S. they are sold." For inspections we used the federal procedure 40 CFR 85.2222 and 40 CFR 51.357, EPA's 2001 guidance to inspection programs (EPA420-R-01-015, whose cover says it "does not necessarily represent final EPA decisions or positions"), and a 2013 EPA memo on readiness and permanent codes.

The states. We read the states' own pages: California's Bureau of Automotive Repair (OBD test reference, Smog Check page), Missouri's Gateway Vehicle Inspection Program, the Nevada DMV, Oregon DEQ and the Vermont Department of Environmental Conservation. Six other states' pages refused or did not respond (listed at the end); we did not work around any refusal.

The makers. Owner's manuals as published by the makers: Honda's online owner's manual pages for the 2024 CR-V and 2023 Civic Sedan, GM's 2018 Chevrolet Tahoe and Suburban owner's manual, and Nissan's 2026 Frontier owner's manual. Toyota's and Ford's manual PDFs refused our requests. For the scan-tool side, OBDLink's support articles on diagnostics and monitors, and a scan-tool manual on Innova's support server. A manual describes that maker's cars or that tool, and we do not stretch one make's procedure to another. PulsCar's app works with the OBDLink MX+, so we have a commercial interest in it; nothing on this page depends on which reader you use.

Way one: fix the cause and let the car turn the light off

This is the method the regulation is built around, and the only one that leaves you with more information rather than less.

California's regulation says that once the light has been illuminated, "the MIL shall be extinguished after three subsequent sequential driving cycles during which the monitoring system responsible for illuminating the MIL functions and the previously detected malfunction is no longer present", provided no other fault would keep it on (that is the wording for 2019 and newer model years; for 2004 through 2018 it reads "at least three"). EPA's 1997 fact sheet says the same in plain words: "If the OBD system evaluates a component or system three consecutive times and no longer detects the initial problem, the dashboard light will turn off automatically." A "driving cycle", in the regulation's definition, "consists of engine start and engine shutoff".

Three details matter in practice.

It is three drives in which that particular test runs, not three drives. In our reading, a test that needs special conditions keeps the light on until those conditions come around. The regulation has separate wording for a few fault types; for misfire and fuel system faults it speaks of three driving cycles "in which similar conditions have been encountered".

The code does not vanish with the light. The regulation lets the car keep the confirmed code on record after the light goes out, for about 40 warm-up cycles (a warm-up is a drive long enough to take the engine from cold to fully warm). So a light that went out on its own last week can still leave a code a scan tool reads. That is useful history, not a new problem.

Vermont's program prefers this method. Vermont's inspection program puts it directly in its readiness sheet: "If possible, when repairing a vehicle, do not clear the Diagnostic Trouble Codes so that the check engine light remains on after the repair. This way, if it was the correct repair, the vehicle's computer confirms the correct repair by turning off the check engine light while driving the vehicle through its drive cycle and signals to the driver that the vehicle is ready to be re-tested." It adds the corollary: "If the check engine light does not turn off, then additional repair may be required." Oregon's DEQ tells owners whose light went off before the test to do "Usually nothing.", and that the on-off sequence "does not indicate a defective OBD system."

EPA says the same for one class of faults: for cars that failed only on codes from the continuously running monitors, "repair technicians should be instructed to not clear the DTCs electronically after performing the necessary repairs". For other faults the same guidance treats clearing after a repair as normal, as the next section shows.

The gas cap: the one fix you can do in the driveway

A loose, damaged or missing fuel cap is the cause EPA's fact sheet, Nissan's manual and Oregon's DEQ each single out: the evaporative system tests itself for leaks, and an unsealed filler neck is one.

EPA's 1997 fact sheet uses it as its example: "if the gas cap is not properly tightened after refueling, the OBD system can detect the vapor leak that exists from the cap not being completely tightened. If the gas cap is subsequently tightened, the dashboard light should be extinguished within a few days. This is not an indication of a faulty OBD system." Nissan's 2026 Frontier manual gives the owner's version: if the cap is loose or missing, "tighten or install the cap and continue" driving; the light "should turn off after a few driving trips." If it does not, the manual says to have the vehicle inspected. Oregon's DEQ says the car "will continue to run its diagnostics even with an illuminated check engine light" and can extinguish it "if the fault condition no longer exists after several “trips.”"

GM's manual adds two cases on its vehicles: a capless-filler funnel adapter left in place ("A few driving trips with the adapter removed may turn off the light.") and poor fuel ("It may require at least one full tank of the proper fuel to turn the light off.").

Some cars have their own cap warning; Nissan's 2026 Frontier manual tells owners to check the cap "if the Loose Fuel Cap warning appears in the vehicle information display". Where the check engine light does come on for a cap, the regulation lets the car switch it off itself once it "has verified that the fuel cap has been securely fastened" and no other fault lit it.

What this does not cover: a cap that was already tight. Evaporative leak codes have many causes beyond the cap, and the regulation lets the smallest-leak test run less often than most. If the light survives a week of driving with a properly fitted cap, see the P0455 guide, the P0456 guide and, for P0457 or P0449, our P0449 and P0457 guide before buying anything.

Way two: clear the codes with a scan tool

Both scan-tool makers whose documents we read describe this function. OBDLink's app page says it can "read diagnostic trouble codes (DTCs), and clear check engine lights"; the Innova manual has an ERASE button. The command itself is standardized: EPA's 2001 guidance, discussing clearing codes electronically, points to SAE J1979, the standard for the diagnostic test modes a scan tool uses, and its "Mode $04".

How to do it, per the tool makers. OBDLink's support article gives the one safety rule that matters: "You should only clear codes with the ignition ON and the engine OFF. You should never try to clear codes with the engine running." That matches the regulation, which requires the car to accept the command at a minimum "while in the key on, engine off position". Read and save everything first, then clear. On a car that is fixed, the light stays off; on one that is not, it comes back.

What the tool actually erases is not your choice. California's regulation lists what a clear must wipe on 2019-2021 and newer models: at least the readiness status, data-stream values such as the "number of warm-up cycles since fault memory last cleared", freeze frame information, pending and confirmed fault codes, and test results, from every control unit that reports readiness for a monitor other than comprehensive components; the car "may not erase only one of three stored fault codes". EPA's 2001 guidance, written for older cars, puts it the same way: the clearing feature "is an all-or-nothing proposition".

The tool makers repeat the warning. Innova's manual: "When the Scan Tool’s ERASE function is used to erase DTCs from the vehicle's on-board computer, "Freeze Frame" data and manufacturer-specific enhanced data are also erased. "Permanent" DTCs ARE NOT erased by the ERASE function." And, with the key words in capitals: "If you plan to take the vehicle to a Service Center for repair, DO NOT erase the codes from the vehicle's computer." And: "When DTCs are erased, the I/M Readiness Monitor Status program resets status of all Monitors to a not run condition." OBDLink's diagnostics article: "The freeze frame data is deleted when you clear codes."

When clearing is the right call. After a real repair, clearing is normal practice. Nevada's DMV describes it plainly: "Once a technician fixes the issue, they will use a diagnostic tool to clear the code and turn off the light." EPA's 2001 guidance describes a car returning to the inspection lane after repair as one that "will likely have just had the fault code memory cleared by the repair technician (the proper step following a repair)". The consequence: "all readiness codes will also be cleared and set to “not ready.”" So the same document says motorists "should plan to allow a week’s worth of ordinary driving between receiving repairs and getting a vehicle retested". Vermont's program, quoted above, would rather the shop left the light on so the car confirms the repair itself. Both positions are reasonable, and they agree on the one thing that costs money: clearing right before a test.

How do I reset the check engine light with a scan tool, and what does it erase? Plug a reader into the diagnostic port, turn the ignition on without starting the engine, save every stored and pending code and the freeze frame, then use the clear or erase function. In PulsCar's reading of California's OBD II regulation (its wording for 2019-2021 and newer models), that command wipes stored and pending codes, freeze frame data, test results, the warm-up and distance counters since the last clear, and the readiness status of every monitor, all at once. It cannot erase a permanent code on a 2010-2012 or newer car. So clear only after the fault is fixed, never to hide it, and never right before an emissions test: EPA's own guidance says to plan about a week of ordinary driving between a repair and a retest, because the monitors start again from not ready.

Way three: disconnect the battery

This is the method people reach for when they have no reader, and it is the least reliable of the three.

It is not guaranteed to clear anything. The regulation's wording is deliberate: the diagnostic information "shall be erased as a result of a command by any scan tool (generic or enhanced) and may be erased if the power to the on-board computer is disconnected". "Shall" for the tool, "may" for the battery. (That wording applies to 2019-2021 and newer models; the regulation's definition of keep-alive memory, worded the same in its 2006 text and today, likewise says that memory "may be erased if power to the on-board control unit is interrupted".) Where it does clear, the makers' manuals we read say readiness goes with it. Honda's owner's manual: "The readiness codes are erased when the battery is disconnected, and set again only after several days of driving under a variety of conditions." Nissan's: "If a powertrain system component is repaired or the battery is disconnected, the vehicle may be reset to a “not ready” condition."

It cannot clear a permanent code. The regulation says permanent codes "may not be erasable by any scan tool command (generic or enhanced) or by disconnecting power to the on-board computer". California's Bureau of Automotive Repair says the same in its fall 2019 newsletter: permanent codes "cannot be reset by disconnecting the vehicle’s battery or cleared using an On-Board Diagnostic (OBD) scan tool."

It resets things that are not the engine. Owner's manuals list what a battery disconnect touches, and the lists are specific to each car. In GM's 2018 Tahoe and Suburban manual: "Programming may be necessary if the vehicle battery has been disconnected or discharged" for the express-close power windows, and the electric liftgate latch will not open with the battery disconnected until it is "reconnected and charged". In Nissan's 2026 Frontier manual, after the battery is disconnected or discharged "the predicted course lines may be displayed incorrectly" on the parking camera, and the manual's fix is to turn the steering wheel lock to lock with the engine running and drive straight for more than 5 minutes; and the service precautions say: "Never connect or disconnect the battery or any transistorized component while the ignition switch is in the ON position." Check your own manual's battery and electrical sections before you pull a terminal; we did not find a general list that applies to every car.

The manuals attach warnings to battery work. GM's manual, in its section on servicing the airbag-equipped vehicle: "For up to 10 seconds after the vehicle is turned off and the battery is disconnected, an airbag can still inflate during improper service." Nissan's precaution about the ignition applies too. Both warnings are about doing the work wrongly, which is a reason to follow the manual or leave it to a shop.

Side by side, wherever it works the battery method has every downside of the scan tool (freeze frame gone, readiness reset, fault still there), may not even turn the light off, and adds resets and precautions of its own. To see whether a repair worked, way one is free; to clear after a repair, a reader does it cleanly.

Letting the shop or the parts store do it

A shop that fixes the fault often clears the codes when it is done; EPA calls that the proper step, except for the one class of faults noted above. Ask when you collect the car: was the code cleared, and have the monitors run since? If cleared, wait before any inspection. At a parts-store counter the free scan is a reading, not a repair; the AutoZone free scan guide found that AutoZone's annual report and official video descriptions, as we read them, do not say whether staff will clear the light. If anyone offers to clear it before the cause is known, say no: the freeze frame goes, the monitors reset, and the fault stays.

What a reset erases, and what it cannot touch

Here is the whole picture in one table, from California's regulation in its wording for 2019-2021 and newer models (which says a clear erases "at least" these items), the EPA memo on permanent codes, and the Innova and OBDLink manuals.

The counters are why a reset is visible afterwards: anyone with a reader that shows these counters can see that the codes were cleared recently, which matters when you sell (below).

Will the light come back? How the car decides

If the fault is still there, yes. The regulation spells out how quickly.

Step one, a pending code. The regulation: "Upon detection of a malfunction, the OBD II system shall store a pending fault code within ten seconds indicating the likely area of the malfunction." The light is not on yet.

Step two, the light. Then: "if the identified malfunction is again detected before the end of the next driving cycle in which monitoring occurs, the MIL shall illuminate continuously and a confirmed fault code shall be stored within 10 seconds." If the fault is not seen on that next cycle, the pending code is erased. Under this default rule a fault must be seen on two separate drives in which that test runs before the light comes on, which is why a cleared light can stay dark for a while and then return.

The exception is the dangerous one. A misfire severe enough to damage the catalytic converter does not wait: the pending code is stored and the light blinks while the misfire is happening, except on cars that cut fuel during such a misfire, where it may stay steady, as quoted above.

The slow tests. Some monitors run only when their conditions are met, so a light can stay off long enough to be mistaken for a fix; the smallest evaporative leak test is allowed to run less often than most (see the P0456 guide). EPA's 2001 guidance also names the opposite effect, pending codes that "frequently self clear without requiring repair of the vehicle"; it adds that such codes "usually reflect an intermittent condition which may or may not be a problem at the time of testing".

So the honest reading of a light that stayed off after a clear depends on the readiness monitors. If they have all completed and the light is still off, the car has run its self-tests since the clear and not turned the light back on. That is good news but not proof: a fault seen once shows first as a pending code, so read pending codes too. (Vermont's confirmation is a different case: a light left on after a repair that goes out by itself.) If they are still not ready, the car has not looked yet, and the absence of the light means nothing.

Readiness monitors: the part of the reset you cannot see

Readiness is the car's record of which self-tests have finished since the memory was last cleared. EPA's 2001 guidance describes up to 11 monitored subsystems, three running continuously (misfire, fuel trim, comprehensive components) and the rest running only when their conditions are met; each reports "ready", "“not ready” (meaning the monitor has not yet been evaluated)", or not applicable. Vermont's program explains the link to a reset in one line: readiness "identifies whether the vehicle's computer has completed the required tests since the last time the vehicle was serviced with a scan tool or had its battery disconnected."

A not-ready monitor is not a fault: EPA says "an unset readiness code is not itself an indication of an emission problem." But it is not a clean bill of health either. Oregon's DEQ: "A component failure may exist, but has not yet been identified because the system testing has not been completed."

You may be able to check readiness without a scan tool. Two of the three makers whose manuals we read publish a key-on check. Honda's 2024 CR-V and 2023 Civic manuals: "To check if they are set, set the power mode to ON, without starting the engine. The malfunction indicator lamp will come on for several seconds. If it then goes off, the readiness codes are set. If it blinks five times, the readiness codes are not set." Nissan's 2026 Frontier manual: "If the Malfunction Indicator Light (MIL) comes on steady for 20 seconds and then blinks for 10 seconds, the I/M test condition is “not ready”. If the MIL does not blink after 20 seconds, the I/M test condition is “ready”." Nissan adds that ordinary driving usually sets the ready condition, and recommends a Nissan dealer to set it or prepare the vehicle for testing. The regulation allows makers to use the light this way, at their option, so check your own manual; the pattern differs between Honda and Nissan and may not exist on your car. With a reader, the monitors screen shows the same thing: Innova's manual calls it the "I/M Readiness Monitor Status" (I/M: inspection and maintenance, the emissions-test program), and OBDLink's app lets you pick "the readiness standard for your state and your vehicle's model year".

The emissions test after a reset: what the federal rules say

Much of the trouble a reset causes shows up here. The federal inspection procedure, 40 CFR 85.2222, sends a car with any incomplete self-test away to be driven and fails it if the monitors are still incomplete on its return; states may pass 1996-2000 cars with up to two unset monitors and 2001 and newer cars with one. A lit light fails, and so does a bulb that does not light at key-on.

A freshly cleared car can easily show more unset monitors than that allowance, which is the whole problem in one line. The AutoZone scan guide quotes the rule in full, with 40 CFR 51.357, and explains why a code is not a diagnosis; what a state inspection costs is in car inspection cost.

Will clearing the check engine light help me pass an emissions test? Usually not, in PulsCar's reading of the rules, in a state that tests through the diagnostic port. Clearing codes resets the readiness monitors to not ready, a battery disconnect can do the same (Honda's manual says it does; the regulation says only that it may), and the federal procedure sends a car with incomplete monitors away to be driven, then fails it if they are still incomplete. The federal allowance, where a state uses it, is small: two unset monitors for 1996 to 2000 models, one for 2001 and newer. Nevada's DMV warns that erasing codes right before a test brings an automatic rejection. On a 2010 or newer car, California's Smog Check also fails a stored permanent code unless the car has done 15 warm-up cycles and 200 miles since the last clear. The way through is repair first, then about a week or two of normal driving, then the test.

What five state programs tell owners, in their own words

States set their own readiness limits, and the five pages below differ from each other and from the federal allowance, so the practical answer depends on where the car is registered. These five pages were readable on 30 September 2026 (Vermont's sheet is dated October 2017, California's test reference July 2020).

Beyond the numbers, the five pages agree on the cause and differ usefully on the remedy.

On the cause, four of the five name a reset. Missouri lists "Battery replaced or disconnected." and "Memory cleared by a repair shop (removal of Diagnostic Trouble Codes)." Vermont lists "Recent clearing of diagnostic trouble codes with an OBD II scan tool". Nevada says a rejection usually happens "because the battery was recently disconnected or codes were cleared." California's BAR says readiness monitors "must be rerun after a repair activity like disconnecting a vehicle’s battery". Oregon does not name causes.

On clearing before a test, two are explicit. Nevada: "Using a scanner to erase trouble codes right before a test will result in an automatic rejection. Clearing codes doesn’t fix the problem; it simply erases the memory the inspector needs to see." Vermont: "Do not use a scan tool or disconnect the battery to turn off the check engine light in an attempt to pass the OBD inspection. This practice is counterproductive, because it will lead to a readiness failure."

On the remedy, details differ. Nevada: "You don’t need repairs. Simply drive the vehicle for up to one week under varying conditions, city and highway, to reset the system", and after a rejected test your local emission lab "can check whether your vehicle is ready to be tested at no cost." Missouri gives a grace period: owners failing only on readiness "can legally drive their vehicle for up to 30 days beyond expiration of vehicle’s registration to reset the monitors", but its FAQ says "there are no waivers for readiness as per EPA guidelines." (a note on the same page describes one narrow catalyst case that may be waived). California's BAR: "If you recently disconnected or replaced your battery, drive your vehicle for a week or two." If weeks pass with no change, Missouri says "You may need to repair your vehicle".

Missouri adds a rule for catalyst codes: a gasoline car that reported a catalytic converter code (P0420 to P0439) must show the catalyst monitor completed on a later inspection before readiness can pass; Missouri says the rule exists to make sure a repair has "truly resolved the cause of that code." What P0420 usually means is in the P0420 guide.

Drive cycles: getting the monitors ready again

A "drive cycle" in the everyday sense is a pattern of driving that lets every monitor run.

How long. California's air board: owners should "drive their vehicle as they normally do for about a week or so". EPA's 2013 diesel best-practices memo (which EPA says is not official guidance): "A gasoline OBD system may typically run all monitors within two weeks, while a diesel OBD system can take eight weeks or more to run all monitors." For diesel testing, the same paper suggests drivers be told to drive "for two or three weeks after codes are cleared" as a starting point. Oregon's DEQ: "There is no set amount of mileage needed to complete a drive cycle." Vermont: "In most cases, a few days of normal driving, both city and highway, will reset readiness status to “ready”."

What it involves. Oregon: "The drive cycle varies in length and includes highway, stop and go, and letting the engine cool completely between trips." Vermont's sheet gives the only generic list we found when you do not know your car's own procedure: drive at least three separate times, turning the car on and off; after a cold start, idle about three minutes; do one drive in town and one on the highway; do one after the car has been parked at least eight hours (overnight); keep the tank between a quarter and three quarters full.

A maker's own procedure. Honda's 2024 CR-V and 2023 Civic manuals each publish one for owners who must be tested before the codes are ready. Condensed from the 2024 CR-V manual: fill the tank to about three quarters; leave the engine off for 6 hours or more; make sure it is between 40 and 95 degrees Fahrenheit outside; start without touching the accelerator and idle 20 seconds; in P, hold 2,000 rpm for about 3 minutes, then idle 20 seconds; drive on a lightly traveled highway at 50 to 60 mph for at least 20 minutes without cruise control, including 90 seconds without moving the accelerator; then at least 10 minutes of city or suburban driving with some coasting; then park and leave the engine off for 30 minutes. That is Honda's procedure for those two models only; do not apply it to another make. California's BAR notes that "On occasion, the vehicle owners-manual contains driving procedures to complete the monitors, but usually this drive cycle information is only available to the OEM or found in repair industry service literature."

When driving is not enough. California's air board suggests a technician, who "may be able to operate the vehicle (most likely on a dynamometer in the shop) to set the monitors." And BAR adds: "Monitor completion is also dependent on properly functioning components, so a vehicle may never complete a monitor until a thorough diagnosis and repair of components enabling the monitor to run is performed."

How long should you wait after clearing codes before an emissions test? From the sources PulsCar read, about one to two weeks of ordinary mixed driving for most gasoline cars. California's Air Resources Board says to drive normally for about a week or so; its Bureau of Automotive Repair says a week or two after a battery disconnect or replacement; EPA's 2001 guidance tells programs to advise motorists to allow about one week after repairs; and EPA's 2013 memo says a gasoline system may typically run all monitors within two weeks, a diesel eight weeks or more. The driving should mix city and highway and include an overnight cold start. Before driving to the station, check readiness: with a reader's monitors screen, at an emission lab where one offers a check (Nevada's labs do after a rejected test), or with the key-on light pattern if your owner's manual describes it.

Permanent codes: why a 2010-and-newer car remembers

Permanent codes are the part of the system designed to make a reset harder to hide behind. California's regulation required that a confirmed code currently lighting the lamp also be stored as a permanent code, phased in over "50 percent of all 2010, 75 percent of all 2011, and 100 percent of all 2012 and subsequent model year vehicles". EPA's 2013 memo explains their purpose: they "have been implemented to make it more difficult for a vehicle owner or technician to clear the fault memory, which can potentially allow the vehicle to pass inspection without being repaired."

How one clears. The same memo gives the two ways, both run by the car. If nobody cleared the codes, the permanent code goes when the light goes (its example: the fault absent on three consecutive trips). If somebody did clear them, "the Permanent-DTC will not be erased until the monitor responsible for setting that specific Permanent-DTC has run at least once and confirmed that the fault is no longer present." For some monitors the regulation adds a minimum drive, on a single driving cycle, before it may erase: at least 600 seconds since engine start, at least 300 seconds cumulative at or above 25 mph, and at least 30 seconds of continuous idle (for misfire, fuel system and circuit-type monitors; hybrids count propulsion-system time). A trip around the block is unlikely to meet all three.

Why it matters for a test. California made permanent codes a Smog Check failure for 2010 and newer vehicles on July 1, 2019. In the first month, BAR's newsletter counted "3,124 vehicles" that failed on a permanent code and "all had recent codes cleared"; our used-car checklist walks through what happened to them. BAR also set a limit so motorists trying to comply are not stuck: permanent code issues "are ignored only after the vehicle has completed at least 15 warm-up cycles and been driven at least 200 miles since its OBD information was last cleared."

Those figures are California's; we do not know which other states check permanent codes. The mechanism is in the car, though: on a 2012 or newer car, the permanent code is there for any tool that asks for permanent codes (Mode $0A). BAR's own test reference lists exceptions: "Some 2010 vehicles do not support PDTC" (PDTC: permanent diagnostic trouble code), and its Table 6 names models whose permanent codes do not clear themselves properly, with BAR's instruction for each (for some the code is ignored at Smog Check; for others the owner is referred to the maker's repair program); check that table for your car.

Clearing the codes before you sell: what the rules we found say

We are not lawyers, and this section is not legal advice.

The short answer. We found no federal or state rule that forbids clearing the codes on your own car, but clearing does not make a car sellable. A buyer with a suitable reader can see signs of a recent clear; a California seller normally owes the buyer a valid smog certification, which a freshly cleared car is unlikely to get until its monitors have run; and a dealer may not misrepresent a car's mechanical condition. We did not research private-sale fraud law. The straightforward route: fix it, or disclose it.

What we did not find. In the federal rules, EPA documents and state program pages listed at the bottom, we found no provision that forbids an owner from clearing the codes on their own car. The function itself is required: for 2019-2021 and newer models, California's regulation requires the car to erase its diagnostic information "as a result of a command by any scan tool (generic or enhanced)", and OBDLink's app page and Innova's manual both describe the function. We did not research state fraud, misrepresentation or consumer-protection law for private sales, and we make no claim about it.

What applies to dealers. The FTC's Used Car Rule, 16 CFR 455.1, says: "It is a deceptive act or practice for any used vehicle dealer, when that dealer sells or offers for sale a used vehicle in or affecting commerce as commerce is defined in the Federal Trade Commission Act: (1) To misrepresent the mechanical condition of a used vehicle". The rule defines a dealer as "any person or business which sells or offers for sale a used vehicle after selling or offering for sale five (5) or more used vehicles in the previous twelve months", with exceptions for banks, some employee sales and lessors. The FTC's Dealer's Guide to the Used Car Rule adds that the rule "applies in all states except Maine and Wisconsin", which have similar rules of their own. The rule does not mention trouble codes; whether a particular clear misrepresents a car's condition is not for us to say.

What applies in California. The state's DMV says: "If you are selling your car, you need to give the new owner a valid smog certification when you sell the car." It adds: "If the car you are selling is less than four years old, you do not need to get a smog inspection. Instead, the new owner of the car will need to pay a smog transfer fee." BAR limits that newer-car exemption to gasoline, hybrid and alternative-fuel vehicles, and tells sellers to "give the buyer a copy of the vehicle inspection report." The consequence for a reset follows from the Smog Check rules quoted above (our reading): a car whose monitors were just cleared is likely to be turned away until enough of them have run, except where the allowance covers those still incomplete, and on a 2010 or newer car that supports them a permanent code fails it unless the 15 warm-up and 200-mile limit has been reached. BAR's page lists other vehicles that need no Smog Check (electric vehicles, gasoline cars 1975 and older, diesels 1997 and older, among others), and the DMV says a car given by a family member generally needs none. We read only California's transfer rules.

What EPA calls it in the inspection context. Among the reasons a car can arrive at a test not ready, EPA's 2001 guidance lists "A fraudulent attempt to avoid I/M program requirements by clearing OBD codes just prior to OBD-I/M testing"; that describes clearing to get through a test, not a sale. Oregon's DEQ, in a paragraph about defeat devices and OBD "simulators", says "It is a criminal violation of the Federal Clean Air Act to alter or tamper with OBD systems." The page does not say whether that covers clearing codes, and we make no claim about its scope.

What a buyer can see anyway. A clear leaves fingerprints: monitors not ready, the counters since the last clear near zero, and on 2010-2012 and newer cars any permanent code still stored. Our used car inspection checklist walks a buyer through all three. Those traces show only where the car and the reader support them, and a permanent code exists only if a fault had lit the light, so a buyer should check all three, and a seller should not count on a clear going unnoticed. The straightforward options are to fix the fault and let the monitors complete, or to disclose the code and put the scan printout in the folder.

The traps: three ways a reset costs money

Trap one: clearing the light the day before the test. The situation: the inspection is due, the light is on, and a reader or a battery terminal promises a quick way through. What is real: clearing or a battery disconnect sets the monitors to not ready; 40 CFR 85.2222 sends the car away and then fails it if still not ready; Nevada says the result is an automatic rejection, Vermont calls it counterproductive, and on a 2010 or newer car in California a permanent code fails it on its own. The price: a wasted trip, a retest, and the same fault still waiting. The defense: repair first, then a week or two of mixed driving, then check readiness before the test.

Trap two: clearing the evidence before the shop sees it. The situation: the light comes on, the owner clears it to see whether it comes back, and it does, a week later, on the way to the shop. What is real: the clear deleted the freeze frame and the stored codes (on newer cars, pending codes too); EPA calls those codes and freeze frame data "important for the performance of proper diagnostics prior to repair", and the Innova manual warns that erasing before a repair visit loses "valuable information that might help the technician troubleshoot the problem". The price: a slower diagnosis, possibly a longer paid one. The defense: write down or photograph the codes, pending codes and freeze frame first; if you want to see whether a fault repeats, let the car show you rather than clearing it.

Trap three: reading a dark light as a repair. The situation: after a clear, the light stays off for days, and the problem is filed as solved. What is real: under the regulation most faults need to be detected again on the next driving cycle in which that monitor runs before the light returns, and some monitors run only when their conditions are met; until readiness shows complete, the car has not re-checked. The price: a fault that keeps developing, and a light that returns at a worse moment. The defense: check readiness after a week or two; if the monitors are complete and the light is still off, the car has re-tested itself; read pending codes as well. If any stay not ready for weeks, BAR's warning applies: a component can stop a monitor from ever completing.

Three composite situations, decoded

These are composite scenarios, not individual customers. They are built only from the rules and manuals quoted on this page, written in the conditional on purpose, with no invented names, amounts or dates.

Scenario 1: the gas cap after a fill-up. Suppose a steady light came on after refueling and the cap had not clicked. Tightening it and driving may be the whole fix; EPA's fact sheet says the light should go out within a few days. Clearing it would erase the record and reset the monitors without fixing anything. Lesson: the cheapest reset is the one the car does itself.

Scenario 2: the battery swap before the smog check. Suppose a battery were replaced the week of a California Smog Check. Readiness would likely be erased, and a 2000 or newer gasoline car typically may have only the evaporative monitor incomplete to pass. BAR's advice applies: drive a week or two first. Lesson: a new battery is an innocent reset, but the test does not know that.

Scenario 3: the cleared code that came back. Suppose a cleared light returned days later and was cleared again. Each clear would delete the freeze frame and reset readiness, and on a 2012 or newer car the permanent code would stay stored. Lesson: a code that returns is the car repeating its diagnosis; the next step is a repair.

Where a free sound check fits, and where it does not

This is the part of the page about our own product; the terms below are quoted from our homepage. PulsCar's sound check asks you to film your engine running for 30 seconds and returns a report in about 10 minutes; the homepage terms are "No payment, no card, no subscription." If a clip cannot support a confident verdict, the homepage says the report says "inconclusive" rather than guessing.

It has nothing to do with resetting a light: it reads no codes, cannot clear any, cannot show readiness or permanent codes, and cannot tell you whether a car will pass an emissions test.

Where it can help is a car that sounds wrong. Our FIXD versus PulsCar page counted federal complaints for ten high-volume nameplates, model years 2015 to 2022, and found that about four out of five complaints in which an owner described a sound mention no light, no code and no scan (a count of mentions, not proof that no light was on). A dark dashboard is not an answer to a new noise; sound versus scanner makes that argument in full.

What we looked for and did not find

Four state pages that refused, and two that did not answer. New York's, New Jersey's, Arizona's and Maryland's pages returned HTTP 403 on 30 September 2026; Texas's and Virginia's did not respond. We did not retry through another route. If your state is one of these, its own page is the authority.

Toyota and Ford manuals. Both refused our requests, so nothing here says what their cars do.

A universal list of battery-disconnect side effects. We found car-specific items in manuals (window programming, a liftgate latch, parking-camera guide lines) and no general list, so we list only those.

The SAE J1979 standard itself. We did not read it; we cite the public documents that refer to it.

Any test of our own. We did not clear codes on a car or time a drive cycle; every procedure described here comes from a quoted source, apart from the plain steps in the action plan.

Your action plan

Today, before touching anything:

1. Look at the light. Flashing: slow down, get off the road safely, and follow your owner's manual; do not reset. Steady and the car drives normally: EPA says the light is not a reason to stop, but service should be sought as soon as possible.
2. Read the codes, pending codes and freeze frame, and write them down or photograph the screen. A parts-store read or any basic reader does it.
3. Check the fuel cap: remove it, look for damage, and refit it fully (Nissan's manual says until a single click is heard; check your own manual).

This week: 4. Look up the code in check engine light on or the code index, and fix the cause (or have it fixed). What a diagnosis costs is in our diagnostic cost guide. 5. After the repair, either let the car turn the light off over the next few drives (Vermont's preference) or clear it with a reader, ignition on and engine off. Avoid the battery method. 6. If you cleared, expect every readiness monitor to read not ready.

Before an emissions test: 7. Drive one to two weeks of mixed city and highway driving with at least one overnight cold start; a diesel may need longer. 8. Check readiness: a reader's monitors screen, the key-on light pattern if your owner's manual describes one, or, in Nevada after a rejected test, the free check at a local emission lab. 9. Compare with your state's limit, then test. Do not clear anything in the days before.

Before a sale: 10. Fix and let the monitors complete, or disclose the code and hand over the scan printout. In California a valid smog certification normally goes with the sale (see the selling section for the exemptions).

If the light comes back: 11. Treat it as the car repeating its diagnosis: read the codes again and take them to a shop. Which reader shows pending codes, freeze frame and permanent codes is covered in the 2026 scanner shortlist and which diagnostic tool fits which problem.

Sources and documents

Every quotation above comes from one of these documents, each opened and saved as text on 30 September 2026 (the eCFR sections through eCFR's own text renderer). Pages that refused to open are listed as such and are not quoted.

1. California Code of Regulations, title 13, section 1968.2, OBD II malfunction and diagnostic system requirements, current text as published by Cornell LII (sections quoted: (c) definitions, (d)(2.2)-(2.5), (d)(3.2.1), (e)(3.4), (e)(4.4.2), (g)(4.4.6), (g)(4.10))
2. California Air Resources Board, OBD II regulation final regulation order (2021 amendments), and the regulation's 2006 amendment text — keep-alive memory definition
3. 40 CFR 86.1806-17, onboard diagnostics for model year 2017 and later vehicles — adoption of California's 2013 OBD requirements
4. 40 CFR 85.2222, onboard diagnostic test procedures — readiness evaluation, the one-and-two monitor allowance, MIL and bulb checks
5. 40 CFR 51.357, inspection test procedures and standards — rejection for "not ready" codes
6. U.S. EPA, Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program, EPA420-R-01-015, June 2001
7. U.S. EPA, Best Practices for Addressing OBD Readiness in IM Testing of Diesel Vehicles Under 14,000 Pounds Gross Vehicle Weight Rating, memo dated 7 March 2013 — permanent codes, two weeks gasoline, eight weeks or more diesel; EPA states it is not official guidance
8. U.S. EPA, Frequently Asked Questions About On-Board Diagnostics, EPA 420-F-97-003, May 1997 — steady versus flashing light, turning the light off, gas cap
9. U.S. EPA, Vehicle Emissions Inspection and Maintenance: Information for State and Local I/M Agencies and General Information for Motorists
10. California Air Resources Board, On-Board Diagnostic II (OBD II) Systems Fact Sheet
11. California Bureau of Automotive Repair, On-Board Diagnostic Test Reference (dated 7/27/2020); Smog Check page — battery advice, selling; Fall 2019 newsletter on permanent codes — 3,124 failures, 4.2% to 5.6%, re-inspection outcomes
12. California DMV, Smog Inspections — smog certification when selling, four-year exemption
13. Missouri Department of Natural Resources, Gateway Vehicle Inspection Program: Readiness Monitors
14. Nevada DMV, On-Board Diagnostics System
15. Oregon DEQ, On-board Diagnostics Frequently Asked Questions
16. Vermont Department of Environmental Conservation, Vehicle Readiness FAQ, October 2017
17. Honda owner's manuals: 2024 CR-V, Testing of Readiness Codes, 2023 Civic Sedan, Testing of Readiness Codes, 2024 CR-V, indicators coming on or blinking
18. GM, 2018 Chevrolet Tahoe and Suburban Owner's Manual
19. Nissan, 2026 Frontier Owner's Manual
20. OBDLink support, Get Started with Diagnostics and Get Started with Monitors; OBDLink app page
21. Innova, scan tool owner's manual on Innova's support server
22. Federal Trade Commission, Used Car Rule, 16 CFR Part 455 — dealer definition, misrepresenting mechanical condition; Dealer's Guide to the Used Car Rule — states where the rule applies
23. PulsCar homepage — free sound check terms; FIXD versus PulsCar — the count of sound complaints without a light or code
24. Could not read on 30 September 2026 (not quoted): New York DMV, New Jersey DEP, Arizona ADEQ and Maryland MVA readiness pages (HTTP 403); Texas DPS and Virginia DEQ pages (no response on two addresses each); Toyota owner's manual PDFs and a Ford owner guide PDF (HTTP 403)

How this page was built: every statement about when the check engine light comes on and goes off, what a scan tool or battery disconnect erases, and what an inspection does with the result comes from California's OBD II regulation, the federal rules EPA applies, EPA's published guidance, or a state emissions program's own page, read and saved on 30 September 2026. Procedures for a specific car come from that maker's owner's manual and apply to the models named; procedures for a tool come from that tool maker's own text. We did not clear codes on a car or time a drive cycle ourselves. Nothing here is legal advice; the selling section says exactly what we found and what we did not research. The three scenarios are composite and contain no invented names, amounts or dates.

Prices verified: September 2026 (no prices are quoted on this page) · Technical documents verified: September 2026 · Reviewed quarterly

Did your state's inspection treat a reset differently from what this page describes, or does your owner's manual publish a drive cycle we should quote? Email [email protected] with the state or the model and year. A documented counter-example changes this page faster than anything we could write.