Best OBD2 scanners for a smog check

The three tiers

What does a smog check inspector actually look for?

Most jurisdictions using OBD2 emissions testing check three things: whether the check engine light is on, whether there are stored or pending diagnostic trouble codes, and whether enough readiness monitors report "ready" rather than "not ready" or "incomplete." Any of these three can cause a failure independent of the others.

A scanner that shows readiness monitor status clearly lets you catch a likely failure before driving to the test station, rather than finding out only after paying the test fee.

How many readiness monitors need to be ready to pass?

This varies by jurisdiction and by vehicle model year, but many programs allow one or two monitors to remain "not ready" (often called the allowed exceptions) while still passing, provided the rest are complete and no codes are present. Check your specific state or local testing program requirements, since this number is not universal.

• Model year and jurisdiction both affect how many incomplete monitors are allowed
• Newer vehicles generally allow fewer incomplete monitors than older ones in most programs
• A recently disconnected battery or cleared code resets all monitors, not just one

What should I do if my scanner shows a pending catalytic converter code?

A pending P0420 or P0430 code (catalytic converter efficiency below threshold) close to a smog test is worth taking seriously, since catalytic converter faults are a common cause of emissions test failures. Confirm the code with a scan, consider a catalytic converter cleaner treatment if the code is borderline, and recheck with the same scanner before assuming it is resolved.

What if my car fails smog even with monitors ready and no codes?

This usually points to an actual tailpipe emissions or visual/functional inspection failure rather than an OBD2 monitoring issue, since a scanner only checks the computer's self-reported readiness and codes, not actual tailpipe emissions output on programs that still measure it directly. Some jurisdictions combine an OBD2 check with a separate tailpipe or visual inspection, and a scanner cannot predict the result of that separate test.

If this happens, the test station report typically specifies which portion of the test failed, which tells you whether the issue is mechanical, such as a genuinely high-emissions engine condition, rather than anything a scanner would have flagged in advance.

How we chose

This roundup is researched from published manufacturer specifications and listing capability claims across our catalog, not from hands-on testing of every unit. Readiness monitor and I/M status claims here come directly from what each product listing states.

Smog and emissions testing requirements vary by state and jurisdiction, and the specific readiness monitors checked can differ depending on your vehicle's model year and engine type. This roundup covers the tools that display readiness status; it does not replace checking your specific jurisdiction's testing requirements.

Clearing codes before a smog test resets all readiness monitors to "not ready," which typically causes an automatic test failure until the vehicle completes enough driving to reset them. This is one of the most common and avoidable mistakes people make right before a scheduled emissions test.

Buying tips

Should I clear my codes right before a smog test?

No, in almost every case. Clearing codes resets the readiness monitors to "not ready," and most testing programs require a minimum number of monitors to be ready before they will even run the test. Clearing codes shortly before a scheduled test very often causes an automatic failure for incomplete monitors, not a pass. See our guide on clearing codes safely for the timing that actually works.

What does "readiness monitor" actually mean?

A readiness monitor is a self-test the vehicle computer runs on a specific emissions-related system, such as the catalytic converter or evaporative system, during normal driving. A monitor shows as "not ready" until the computer has completed that self-test, which can take anywhere from one short trip to several days of varied driving depending on the monitor and the vehicle.

How long does it take to complete readiness monitors after clearing codes?

This varies by vehicle and by which monitors were reset, but it generally requires a mix of highway and city driving over one to several days, not a single short trip. If a smog test is scheduled soon, avoid clearing any codes beforehand and instead scan for readiness status without clearing anything.

What if a pending code shows up during readiness prep?

A pending code that has not yet fully failed can sometimes resolve on its own if it was a one-time event, or it can indicate a real developing problem, such as an early catalytic converter or oxygen sensor issue. If the code is emissions-related, addressing it before the test is usually faster than risking a failure and a retest fee.