Lauren HollandBy Lauren Holland

Updated November 2026

An OBD2 scanner for smog check lets you look at your car’s emissions computer before the inspection station does. In a few minutes you can read any stored trouble codes, confirm that the emissions readiness monitors have completed, and check whether the check engine light is working as it should. Done in the right order, this gives you a realistic idea of whether your vehicle will pass, and it gives you time to fix small problems before you pay for a test.

What a Smog Check Looks at Electronically

Most states that require smog testing use the onboard diagnostic system as the primary check on cars built from about 1996 onward. The inspection station connects to the OBD2 port under the dashboard and asks the car’s computer three things: is the malfunction indicator lamp (the check engine light) commanded on, are there any stored or pending emissions-related trouble codes, and have the readiness monitors completed their self-tests. A car can fail on any one of those three items even if it runs perfectly.

Shopping for one? See our guide: 3 Best OBD2 Scanners for Nissan Rogue Compared (2026).

A scanner that reads live data and freeze-frame information is useful here, but the core job is simple: report codes, report monitor status, and clear codes when you have fixed something. Keep in mind that the inspection itself is not a repair. It is a snapshot of the emissions system’s health, and your scanner shows you the same snapshot the station will see.

Step 1: Confirm Your Car Is Covered and Ready to Test

Before you plug anything in, check the basics. Vehicles model year 1996 and newer generally use the OBD2 test. Older cars may get a tailpipe test or a different procedure, and rules vary by state and by county, so check your local program’s requirements first. Diesel vehicles, certain heavy trucks and some classic cars follow separate rules.

Your scanner can only do its job if the engine is in normal operating condition. Make sure:

  • The check engine light has been diagnosed and any real fault has been repaired, not just hidden.
  • The battery is fully charged and the battery terminals are clean and tight. A weak battery can cause voltage dips that set false codes.
  • No one has disconnected the battery or cleared codes in the last several days, because that resets the readiness monitors.
  • The fuel tank is between roughly one-quarter and three-quarters full. Very low fuel can prevent the evaporative system monitor from running.

Step 2: Read Stored and Pending Codes

Plug the scanner into the 16-pin port, usually located under the driver’s side of the dashboard, and turn the ignition to the on position without starting the engine. Pair the scanner with the vehicle, then open the diagnostic trouble code menu. You will typically see two kinds of codes:

  • Stored (confirmed) codes: faults that have triggered the check engine light or persisted over multiple drive cycles. Any stored emissions code is an automatic fail.
  • Pending codes: faults the computer has seen once but has not yet confirmed. These also matter, because they often become stored codes during the inspection drive.

Write down every code exactly as shown, including the first letter (P for powertrain, B for body, C for chassis, U for network). Look up each one in a reliable code database before deciding what to do. A code like P0420 (catalyst efficiency below threshold) and a code like P0171 (system too lean) point to very different repairs. If you are working with a make-specific guide, the codes listed in articles such as Toyota Corolla OBD2 Scanner: How to Read and Clear Codes at Home can help you narrow the cause.

Step 3: Check the Readiness Monitors

Readiness monitors are self-tests the computer runs when conditions are right. They include the catalyst, oxygen sensors, EGR system, evaporative system, and others, depending on the vehicle. Your scanner’s readiness or I/M status screen shows each one as complete or incomplete.

Most stations allow a limited number of incomplete monitors depending on the model year. A common rule is that newer vehicles may have one incomplete monitor, while older ones may be allowed two. Those allowances change, so confirm them for your area. Do not assume you can pass with monitors open just because the check engine light is off.

How to Complete Incomplete Monitors

  1. Make sure the codes are clear and the check engine light is off.
  2. Fill the tank to the appropriate level and keep it there through the drive.
  3. Drive for several days with a mix of city and highway conditions. A typical drive cycle includes a cold start, a steady cruise at around 55 to 60 mph for several minutes, deceleration without braking, and idling for a short period.
  4. Check the readiness screen again after a few days. Each monitor may need a specific sequence, so your vehicle’s service information or an owner forum for your model is worth reading.

Step 4: Clear Codes Only When the Repair Is Done

Clearing codes is the step most people get wrong. Clearing a code does not fix the fault. It erases the stored data and resets the readiness monitors to incomplete, so if the underlying problem is still present, the code will return and the monitor will fail again. If you clear codes to hide a problem before the test, the station will usually detect the open monitors and send you home.

Only clear codes after you have repaired the cause and verified the repair. Then give the vehicle the full drive cycle needed to complete the monitors before testing.

Checklist: What to Have Before You Scan

  • An OBD2 scanner that reads codes, shows readiness monitor status, and displays live data
  • A reliable code reference or service manual for your make and model
  • A notepad or phone camera to record codes and freeze-frame data
  • A multimeter for checking battery voltage (about 12.4 to 12.7 volts with the engine off is healthy)
  • Basic tools for tightening battery terminals and checking the gas cap seal

Common Mistakes

  • Clearing codes right before the test. This almost guarantees an incomplete-monitor failure.
  • Ignoring pending codes. They often become stored codes during the inspection drive.
  • Replacing parts based on a code alone. An oxygen sensor code can be caused by a vacuum leak, a wiring fault or a bad sensor. Test before you buy.
  • Driving only short trips. Short trips rarely complete the evaporative or catalyst monitors.
  • Forgetting the gas cap. A cracked or loose cap can trigger an evaporative leak code. Check the seal and tighten it until it clicks.

Tips for a First-Time Pass

  • Scan the car a few days before the appointment, not the morning of.
  • Take the car on a highway stretch if it is safe to do so, since sustained cruising helps several monitors finish.
  • Keep the battery in good condition. If the car has been sitting, a slow charge can make a difference.
  • Note the oil change interval and any recent repairs, since some stations ask about them.

A scanner that shows readiness monitor status is the single most useful tool for this job. Look for a model that reads both generic and manufacturer-enhanced codes if your car is newer, since some faults only appear in enhanced data. Owners often report that scanners with live graphing make it easier to spot intermittent sensor faults.

  • Autel MaxiCOM MK808: a full-featured handheld scanner with readiness monitor display and live data, suited to someone who checks several vehicles. View on Amazon
  • BlueDriver Bluetooth Professional OBDII Scan Tool: pairs with a smartphone app and provides a detailed emissions readiness report. View on Amazon
  • Innova 3100j: a simple code reader with readiness status, a practical choice if you mainly need to read and clear codes. View on Amazon

FAQ

Can I pass a smog check with the check engine light on?

No. A commanded-on check engine light is an automatic failure in most OBD2 testing programs. You need to diagnose and repair the fault that turned it on.

How long after clearing codes should I wait before testing?

Most vehicles need several days of mixed driving to complete all monitors, and some need more. Check the readiness screen before you go, and do not schedule the test until the required monitors show complete.

Will disconnecting the battery work instead of a scanner?

Disconnecting the battery also clears codes and resets monitors, which causes the same problem. It also erases useful freeze-frame data, so a scanner is the better tool.

Does a pending code mean I will fail?

It can. Pending codes indicate a fault the computer has detected once. Many become stored codes during the inspection drive, so treat them seriously.

Can any OBD2 scanner be used for smog prep?

Most generic OBD2 scanners can read codes and readiness status. For best results, choose one that displays readiness monitors clearly and reads both stored and pending codes.

Why did my car fail for an incomplete monitor even though the light is off?

The check engine light only reports faults. Readiness monitors are separate self-tests, and the station counts how many are still open. Too many open monitors can cause a failure.

Should I see a mechanic if a code keeps returning?

Yes, if you cannot find the cause or the repair does not hold. Persistent emissions codes can involve catalytic converters or fuel system faults that need professional diagnosis.

Conclusion

An OBD2 scanner for smog check turns a stressful inspection into a checklist you control. Read the codes, repair the real faults, let the readiness monitors complete through normal driving, and only then schedule the test. Doing it in that order gives you the best chance of passing on the first attempt.

Ready to choose one?

Lauren Holland

About Lauren Holland

Lauren Holland is part of our editorial team and reviews and edits our car care & garage supplies guides, checking product details against manufacturer specifications and owner feedback.