Any OBD2 scanner, including an inexpensive one, can tell you in about ten minutes whether the OBD2 portion of an emissions test is likely to reject your car today. It reads the three things the test reads: whether the warning light is commanded on, whether emissions faults are stored, and whether the car's self-test monitors have completed. It cannot tell you the car will pass, because a test usually involves more than the OBD2 check.
What a scanner can and cannot pre-check
It can show you the same OBD2 data the inspection equipment queries. That is not an approximation; it is the same interface. If your monitors are incomplete, you will see that before the station does.
It cannot replicate a tailpipe measurement, a visual inspection of emissions equipment, or anything a technician does with their eyes. Which of those apply to you depends entirely on your vehicle's age and your jurisdiction's programme, and some programmes for newer vehicles are OBD2-only while others are not.
We are describing what the checks are, not advising you on your obligations. Your own testing authority publishes the current rules for your area and your vehicle, and those are the rules that apply.
The three things the OBD2 portion of the test looks at
Malfunction indicator lamp status. Not whether the bulb is lit on your dashboard, but whether the car is commanding it on. A car that reports the lamp commanded on is a rejection. Related to this, a lamp that does not illuminate at all during the bulb check is generally also a problem, because a burnt-out or removed lamp defeats the point of the system.
Stored emissions-related fault codes. Confirmed codes relating to emissions systems. This includes permanent codes, which cannot be cleared by any tool and are specifically designed to survive an attempt to hide them; that mechanism is explained in why your scanner refuses to erase permanent codes.
Readiness monitor status. Whether the car has finished testing itself. This is the one that catches people out most often, because it is the one that clearing codes resets. Full detail is in readiness monitors and drive cycles.
The ten-minute pre-check, in order
- Plug in and connect with the ignition on. If you are new to the tool, the first plug-in walkthrough covers the sequence.
- Read stored codes. Note anything present, and note specifically whether the tool flags any of them as permanent.
- Read pending codes too. These are faults the car has seen once but not yet confirmed. They are not usually a rejection in themselves, but they are a warning that a confirmed code may be coming.
- Read the I/M readiness or monitor status screen. Count how many non-continuous monitors report not ready. Monitors reported as not supported do not count against you; those systems are simply not fitted.
- Check the MIL status field if your tool shows it, which tells you whether the lamp is commanded on independently of what the dashboard is doing.
- Write it all down or photograph the screens before you clear anything. Which brings us to the most important instruction on this page: do not clear anything. Clearing resets monitors and guarantees a readiness rejection. The full case is in should you clear OBD2 codes.
Reading the result: go, wait, or fix
No stored emissions codes, lamp not commanded on, monitors within your jurisdiction's allowance: go. The OBD2 portion should not be the thing that stops you.
No codes, but too many monitors incomplete: wait and drive. This is a timing problem, not a fault. Recovery is a matter of driving under the right conditions, usually across several days with separate cold starts rather than one long trip. The tolerance depends on where you are and how old the car is: a common United States pattern, described by programmes including the Texas Department of Public Safety and the Arizona Department of Environmental Quality, allows two incomplete non-continuous monitors for 1996 to 2000 model years and one for 2001 and later, while California's Bureau of Automotive Repair adopted a requirement effective 1 October 2025 that all monitors be set. Check your own authority's current rules.
Stored emissions codes or a permanent code: fix first. A test at this point is money spent to be told what you already know. What the code means for your particular car is a repair-diagnosis question and belongs with a qualified mechanic; within our own network, fault interpretation is autoartificer.com's lane rather than ours, which we say in words rather than sending you off-site.
Codes cleared recently: wait. You have reset the monitors, and driving is the only thing that brings them back.
What this check does not cover
Worth being explicit, because a clean OBD2 pre-check reads more reassuringly than it should.
It does not measure anything coming out of the tailpipe, so on programmes that include a tailpipe measurement you have checked one half of the test. It does not inspect emissions hardware visually, so a missing or obviously modified component is not something your scanner will mention. It does not know your jurisdiction's rules, so a result that would pass in one place may be rejected in another. And it does not check anything unrelated to emissions, so a safety inspection running alongside is entirely outside its view.
Used for what it is, though, it is one of the best returns on a cheap scanner there is: ten minutes at home against a wasted trip and a fee.
FAQ
Can an OBD2 scanner tell me if I will pass a smog check?
It can tell you whether the OBD2 portion is likely to reject you, because it reads the same data the inspection equipment does. It cannot confirm a pass, since many programmes also include tailpipe or visual checks that no scanner performs.
Will clearing codes before a smog check help me pass?
No, and it usually causes a rejection. Clearing resets readiness monitors, and a vehicle with too many incomplete monitors is generally not accepted for testing regardless of its emissions.
What is I/M readiness on my scanner?
It is the screen showing whether each of the car's emissions self-tests has completed. Inspection programmes read the same data, which is why checking it yourself first is worth doing.
Do pending codes cause a smog failure?
Generally not on their own, since a pending code is a fault seen once and not yet confirmed. It is worth treating as a warning that a confirmed code may follow.
How soon before a test should I do this check?
Far enough ahead that you can act on the answer. If monitors are incomplete, recovering them takes days of driving rather than hours, so a check the morning of the appointment leaves you no options.
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