Best OBD2 Scanners for Smog and Emissions Checks
October 30, 2026 · 14 min read
Your smog check is booked, and you want to know the car will pass before you pay. An OBD2 scanner for smog check reads the same readiness monitors the station reads, so a “not ready” shows up at home, not at the counter. Below: what the test reads, what your state allows, and five scanners that show it clearly.
Updated — first published
Quick answer
The best OBD2 scanner for a smog check is one with a one-press I/M readiness screen. Most OBD test programs allow one unready monitor on a 2001 or newer gasoline car. California is stricter and allows only the EVAP monitor to be unready on 2000 and newer gasoline cars, so check readiness before you go.
Innova CarScan Advisor 5210
It shows which monitor is unready and the drive-cycle steps to finish it.
Topdon ArtiLink 500
One button shows readiness, but it gives no drive-cycle steps.
BlueDriver Bluetooth Pro
A small plug-in sensor that puts a Smog Check screen on your phone.
Who this guide is for
It is for owners of 1996 and newer gasoline cars, or 1997 and newer diesels, in a state with a plug-in emissions test.
This is for you if
- Your car was turned away with a “not ready” result
- You just had a repair or a new battery, and the test is due
- You bought a used car and must register it
Look elsewhere if
- Your car is a 1995 or older model, which has no OBD2 port to read
- You need ABS or airbag codes. These five cover engine and emissions
Our picks at a glance
All five show I/M readiness; the middle column is what separates them.
| Product | Readiness display | Extra for emissions | Best for | Link |
|---|---|---|---|---|
| Innova CarScan Advisor 5210Best overall | 8 monitors listed, rechecked about every 30 s | Drive-cycle steps for each monitor | Most owners | Amazon → |
| Topdon ArtiLink 500Best budget | One-click I/M key | O2 sensor test | A quick yes-or-no check | Amazon → |
| BlueDriver Bluetooth ProPhone-based | Smog Check screen in the app | Mode 6 test results | Phone users | Amazon → |
| Foxwell NT301 | One-click I/M key plus green, yellow and red LEDs | On-board monitor test | Reading status at a glance | Amazon → |
| Autel AutoLink AL329 | One-click I/M key with LED indicator | Reads permanent codes | Keeping in the glovebox | Amazon → |
What the table does not show
A scanner only reports what the car says. If a monitor will not complete because a part is failing, only a repair and a drive cycle fix it.
What a smog check actually reads
On a 1996 or newer car, a smog check reads the car’s computer, not the tailpipe. The station plugs into the OBD2 port (also written OBD-II, the 16-pin socket under the dashboard). It checks whether the check engine light is on, which trouble codes are stored, and whether the readiness monitors have run.
New York’s DMV says OBD2 cars run up to 11 self-tests and fail unless the required ones report “ready”. A lit check engine light fails the car outright.
California adds a fourth check. Since July 1, 2019, its Smog Check also reads permanent trouble codes, which a scan tool cannot erase.
Readiness monitors explained
A readiness monitor is a self-test the car’s computer runs on one emissions system, such as the catalytic converter, the oxygen sensors or the EVAP system (evaporative emissions, which traps fuel vapor from the tank). Each reports “ready” once its test has run. “Not ready” does not mean broken. It means untested, and too many untested systems fail the car.
How many unready monitors you are allowed depends on your state, model year and fuel:
| Test program | Vehicles | Unready monitors allowed | Typical drive to get ready |
|---|---|---|---|
| EPA-based OBD test (New York, Wisconsin, Utah County and others) | Gasoline, 1996–2000 | Up to 2 | 2–3 days, mixed driving |
| EPA-based OBD test | Gasoline, 2001 and newer | 1 | 2–3 days, mixed driving |
| Oregon DEQ OBD test | Gasoline, 1996–2000 / 2001 and newer | 2 / 1, and it can never be the catalyst or EVAP monitor | 2–3 days, mixed driving |
| California Smog Check | Gasoline, 1996–1999 | Any 1 | 2–3 days, mixed driving |
| California Smog Check | Gasoline, 2000 and newer | Only EVAP may be unready | 2–3 days, mixed driving |
| California Smog Check | Diesel, 1998–2006 / 2007 and newer | 0 / only the particulate filter and NMHC catalyst monitors | Varies; follow the maker’s procedure |
| Tailpipe or visual test | 1995 and older | Not checked, because these cars have no OBD2 | Not applicable |
California is tightening. Its Bureau of Automotive Repair adopted a rule that all readiness monitors must be set, effective October 1, 2025, but is phasing it in; the limits above apply meanwhile. Oregon’s limits come from Oregon Administrative Rule 340-256-0355. Drive times are typical figures from Ohio EPA guidance, and older cars can take longer.
Why clearing codes fails you the test
Clearing codes fails the test because it resets every readiness monitor to “not ready” at once. The light goes off, but the station sees a car with nothing tested. Innova’s 5210 manual notes that its erase button resets monitor status too.
Disconnecting the battery does the same. Texas DPS calls it the most common reason monitors read “not ready”.
Reading readiness changes nothing, so check as often as you like. If the port itself worries you, see can an OBD2 scanner damage your car? The rule: read before the test, and clear only after a repair, with days to drive afterward.
The drive cycle
A drive cycle is the set of driving conditions your car needs before each monitor runs its test. It usually mixes a cold start, idling, city driving and steady highway speed. The exact steps differ by make, model and monitor.
Ohio EPA’s readiness fact sheet advises two or three days of mixed highway and city driving after codes are cleared, longer for older cars. Book the test after that window.
A scanner ends the guessing: plug in each evening and see what is still “not ready”. If you check while driving, let a passenger watch the screen.
Innova CarScan Advisor 5210 — Best overall
The Innova CarScan Advisor 5210 wins because it shows which monitor is unready and the steps to drive it to ready.
Innova CarScan Advisor 5210
Best for owners chasing an unready monitorA corded handheld that lists eight monitors and blinks incomplete ones. Auto Re-Link rechecks them about every 30 seconds and reports when a drive cycle finishes.
Price and availability change often — check the current listing.
Check price on Amazon → Paid link.Strengths
- Drive-cycle steps for the unready monitor
- Live recheck shows when a monitor turns ready
- Code Priority flags the main code
Limitations
- No airbag (SRS) or other module codes
- Innova says it works in the US and Canada only
Editorial opinion
Get it if “not ready” is what stands between you and a pass. For airbag codes, see Innova scanners compared (2026).
If you were turned away for readiness, this is the one.
See it on Amazon →Topdon ArtiLink 500 — Best budget
The Topdon ArtiLink 500 does the one job a smog check needs: one key shows readiness. It gives up guidance on finishing an unready monitor.
Topdon ArtiLink 500
Best for a quick yes-or-no before the stationA corded code reader with a one-click I/M readiness key and a 2.8-inch screen. Three LEDs show trouble-code status.
Price and availability change often — check the current listing.
Check price on Amazon → Paid link.Strengths
- Readiness in one key press
- 56.5-inch cable
- Code definitions on screen
Limitations
- No drive-cycle steps
- Its LEDs track codes, not each monitor
Editorial opinion
Buy it to confirm a healthy car is ready. If a monitor stays stuck, the Innova helps more. See Topdon scanners compared (2026) for the range.
If your car usually passes and you just want to check first, this is the one.
See it on Amazon →BlueDriver Bluetooth Pro — Best phone-based
The BlueDriver Bluetooth Pro suits you if you would rather use your phone: a small sensor plugs in, and the app’s Smog Check screen shows readiness.
BlueDriver Bluetooth Pro OBDII Scan Tool
Best for phone users who want reportsA Bluetooth sensor (model LSB2) from Lemur Vehicle Monitors, run by an iOS or Android app. It also shows Mode 6 results: how close each part is to failing its self-test.
Price and availability change often — check the current listing.
Check price on Amazon → Paid link.Strengths
- No cable in the footwell
- Mode 6 shows parts close to failing
- Reports you can save and share
Limitations
- Useless without a charged phone
- No on-device screen to glance at
Editorial opinion
Pick it if your phone is always with you. For a shared family tool, a handheld is simpler.
If you would rather check readiness on your phone, this is the one.
See it on Amazon →Foxwell NT301 — Best LED readiness display
The Foxwell NT301 pairs a one-click I/M key with green, yellow and red lights that read faster than a menu.
Foxwell NT301
Best for checking status at a glanceA corded code reader with a 2.8-inch color screen. Its I/M key checks readiness and verifies drive cycles; the LEDs mean normal, possible problem or problem found.
Price and availability change often — check the current listing.
Check price on Amazon → Paid link.Strengths
- Lights readable from the seat
- Help key explains codes on screen
- On-board monitor test
Limitations
- Engine and emissions only
- Updates need a Windows computer
Editorial opinion
Good for a clear signal. With no drive-cycle steps, it suits checking more than fixing.
If you want a color-coded answer in a few seconds, this is the one.
See it on Amazon →Autel AutoLink AL329 — Also worth a look
The Autel AutoLink AL329 is the smallest unit here and still has a one-click I/M key.
Autel AutoLink AL329
Best for keeping in the gloveboxA pocket-size reader, Autel’s update of the AutoLink AL319. It reads current, pending and permanent codes and shows I/M readiness.
Price and availability change often — check the current listing.
Check price on Amazon → Paid link.Strengths
- Small enough for the glovebox
- Reads permanent codes
- Runs on 8–18 V car power
Limitations
- Small screen
- No O2 sensor or on-board monitor test
Editorial opinion
Best as a second scanner that lives in the car.
If you want a reader that never leaves the car, this is the one.
See it on Amazon →How we chose
Cedar Buy chose these five on how clearly each answers “will this car pass the readiness check today?”
Our selection method
- Cedar Buy compared manufacturer pages and manuals in October 2026. Readiness rules came from California BAR, Oregon DEQ, New York DMV and Ohio EPA.
- The readiness display weighed most, then help with unready monitors.
- Discontinued models, such as the Autel AutoLink AL519, were excluded.
- Specifications come from manufacturer documentation. Cedar Buy does not physically test products — see our editorial policy.
What to look for in an OBD2 scanner for smog check
The feature that matters most is a clear I/M readiness screen. After that, it is about help when a monitor is stuck.
A readiness screen that names each monitor
A good screen lists each monitor as ready, not ready or not supported. A single light hides which system is holding you up.
Help finishing an unready monitor
Drive-cycle steps or a live recheck save days of guessing. Only the Innova 5210 here gives both.
Pending and permanent codes
Pending codes warn before the light comes on. Permanent codes matter in California, where the test reads them.
Ignore the vehicle-count claims
Any OBD2 scanner reads the generic emissions data a smog check uses. Big coverage numbers matter for ABS and airbag work, not here.
Key terms, explained
These three terms appear on scanner screens and test reports alike.
Plain-language definitions
- I/M readiness
- I/M stands for inspection and maintenance. I/M readiness is the list of emissions self-tests and whether each one has run since codes were last cleared.
- Permanent code
- A trouble code that a scan tool cannot erase. It clears only after the car’s own self-test passes. California’s Smog Check reads them.
- Mode 6
- The detailed results of the monitors the car runs less often. They show how close each part sits to its pass limit.
Mistakes that cost people money
The costliest mistake is clearing codes the morning of the test, which guarantees a second trip.
- Clearing codes right before the testEvery monitor resets. Clear only after a repair.
- Swapping the battery the week beforeIt resets monitors too. Swap it after the test if you can.
- Trusting a dark check engine lightPending codes and unready monitors do not light it. Read readiness anyway.
- Assuming another state’s rule appliesCalifornia is stricter than most. Check your limit in the table above.
Frequently asked questions
Will my car pass a smog check with one monitor not ready?
Often, yes. Programs following EPA guidance allow one unready monitor on 2001 and newer gasoline cars, and two on 1996 to 2000 models. Oregon will not accept catalyst or EVAP as the unready one. California allows only EVAP unready on 2000 and newer gasoline cars.
How long after clearing codes can I take a smog test?
Plan on two to three days of mixed city and highway driving, as Ohio EPA advises after codes are cleared or the battery is disconnected. Older cars can take longer. Check readiness each evening and book once enough monitors read ready.
Can the smog station tell if I just cleared my codes?
Yes. Clearing codes sets every readiness monitor to not ready, and the station reads that status straight from the car. California also reads permanent codes, which a scan tool cannot erase. Clearing codes to hide a problem gets the car turned away.
Do I need a scanner that reads ABS or airbag codes for a smog check?
No. A smog check reads only the engine and emissions side: the check engine light, trouble codes and readiness monitors. Any scanner with an I/M readiness screen covers that. ABS and airbag coverage helps with other warning lights, not an emissions test.
So which one should you buy?
Cedar Buy’s pick is the Innova CarScan Advisor 5210: it shows which monitor is unready and how to fix that.
| If you need | Best pick | Key spec | Buy |
|---|---|---|---|
| Best overall | Innova CarScan Advisor 5210Best overall | Drive-cycle steps for each unready monitor | Amazon → |
| Best budget | Topdon ArtiLink 500Best budget | One-click I/M key | Amazon → |
| Your phone instead of a handheld | BlueDriver Bluetooth ProPhone-based | Smog Check screen plus Mode 6 | Amazon → |
| Status at a glance | Foxwell NT301 | Green, yellow and red LEDs | Amazon → |
| If you are still unsure | Innova CarScan Advisor 5210 | Rechecks monitors about every 30 s | Amazon → |
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