What an OBD2 scanner does during a smog check
An OBD2 scanner is a handheld device that reads diagnostic trouble codes from your vehicle's onboard computer. During a smog check, the testing station plugs a scanner into your car's OBD2 port — a standardized connector usually located under the steering wheel or dashboard — and retrieves data about your emissions system. The scanner tells you whether your vehicle has any stored or pending fault codes that would cause it to fail the test.
The smog check itself uses a similar scanner, but the station's equipment also measures actual tailpipe emissions. Knowing what codes your car has before you arrive at the testing station gives you time to address problems or decide whether repairs are worth the cost. Many vehicles fail smog checks because of emissions-related fault codes, not because the tailpipe emissions are actually too high.
A personal OBD2 scanner cannot tell you whether your car will pass — only the official test can do that. What it can tell you is whether your vehicle's computer has flagged any emissions problems, which is the first thing a smog technician will check.
Key Takeaways
- An OBD2 scanner reads fault codes from your vehicle's computer and shows whether emissions-related problems are stored in the system.
- Most smog test failures are caused by fault codes rather than high tailpipe emissions, so scanning before your test can reveal fixable problems.
- You can buy a basic OBD2 scanner for $25 to $100, or borrow one from an auto parts store, a mechanic, or a friend.
- The OBD2 port is a standardized connector, usually under the steering wheel, and works on all vehicles sold in the United States since 1996.
- A personal scanner is informational only — the official smog test uses different equipment and is the only result that matters for registration.
Where to find the OBD2 port on your vehicle
The OBD2 port is a small rectangular connector with 16 pins, and it is located in a standard position on every vehicle sold in the United States since 1996. Start by looking under the steering wheel on the driver's side, about 6 to 12 inches below the dashboard. This is the most common location. If you do not see it there, check under the steering wheel on the passenger side, or look along the dashboard edge near the center console.
Some vehicles have the port mounted higher up on the dashboard itself, or tucked into a panel that you may need to remove. If you cannot locate it by looking, check your vehicle's owner manual — it will show the exact location. Once you find the port, you can straightforward plug in your scanner. There is no setup required, and you cannot damage anything by plugging in a scanner.
What fault codes mean for your smog test
When your OBD2 scanner reads codes, it will display them as alphanumeric strings like "P0420" or "P0171." These codes correspond to specific emissions system problems — a catalytic converter issue, an oxygen sensor fault, a fuel system leak, or similar. Not all fault codes will cause a smog test failure. Some are informational, and some relate to systems that do not affect emissions.
The codes that matter for smog testing are those related to the catalytic converter, oxygen sensors, fuel system, and evaporative emissions system. If your scanner shows codes in these categories, your vehicle will likely fail the official smog test. Codes related to other systems — transmission, suspension, or engine knock — typically do not affect smog results.
A mechanic can tell you which codes are emissions-related and whether they are worth fixing before your test. Some repairs are inexpensive; others cost several hundred dollars. Knowing this before you pay the smog testing fee helps you make an informed decision.
Buying versus borrowing an OBD2 scanner
A basic OBD2 scanner costs between $25 and $100 and is available at most auto parts stores, online retailers, and big-box stores. You do not need an expensive model — a straightforward scanner that reads and clears codes is sufficient for checking before a smog test. More advanced scanners with wireless connectivity or detailed diagnostics cost more but offer features you may not need for this purpose.
If you do not want to buy one, many auto parts stores will lend or rent a scanner for free or a small fee. Some mechanics will scan your vehicle for free or for $10 to $20. You can also ask friends or family members who own vehicles — most people who have owned a car for several years have a scanner in their garage. Borrowing is a practical option if you only need to check your vehicle once before a smog test.
What to do if your scanner shows fault codes
If your scanner reveals emissions-related fault codes, you have several options. The first is to take your vehicle to a mechanic and have them diagnose the specific problem. Some faults are straightforward — a loose gas cap, for example, can trigger a code and is free to fix. Others require parts replacement or system repairs that cost more.
You can also try clearing the codes yourself using your scanner, which will erase them from the vehicle's memory. However, if the underlying problem still exists, the codes will return within a few days of driving. This is why clearing codes without fixing the problem is not a reliable strategy for passing a smog test — the codes will reappear during the official test.
Some vehicles have a "monitor not ready" status, which means the emissions system has not completed its self-check cycle. Driving the vehicle normally for several days — typically 50 to 100 miles of mixed city and highway driving — can allow the monitors to complete. If your scanner shows monitors as "not ready," you may want to wait and drive before scheduling your smog test.
How scanner results differ from the official smog test
Your personal OBD2 scanner reads only the fault codes stored in your vehicle's computer. The official smog test does this too, but it also measures actual tailpipe emissions using specialized equipment. A vehicle can have no fault codes but still fail because its actual emissions are too high. Conversely, a vehicle with fault codes might pass if the emissions themselves are within limits — though this is less common.
The smog testing station will plug in their own scanner as part of the test, so they will see the same codes you see. If you have codes, they will know about them. The advantage of scanning beforehand is that you can address obvious problems before paying the testing fee, rather than discovering them at the station and having to reschedule.
The official test result is what matters for your vehicle registration. A personal scanner is a planning tool, not a substitute for the actual test.
Frequently Asked Questions
Can I clear fault codes myself to pass a smog check?
You can clear codes using your scanner, but if the underlying problem still exists, the codes will return within days of driving. The smog test will then show the same codes again. Clearing codes without fixing the problem is not an effective strategy for passing the official test.
What if my scanner shows no codes but I still fail the smog test?
This means your vehicle's computer is not reporting any faults, but your actual tailpipe emissions are too high. This usually indicates a problem with the catalytic converter, fuel injectors, or engine tuning that has not yet triggered a fault code. A mechanic can run a more detailed diagnostic to identify the issue.
Do I need a scanner if I am just renewing my registration?
Only if your state or county requires a smog test for your vehicle. If a test is required, scanning beforehand can help you avoid a failed test and the cost of retesting. If no test is required, you do not need a scanner for registration purposes.
Will using a scanner void my vehicle's warranty?
No. Plugging in a scanner is a read-only action that does not modify your vehicle's computer or systems. It cannot void a warranty. Clearing codes also does not void a warranty, though it may prevent your dealer from diagnosing a problem if you clear codes before bringing the vehicle in for service.
Can I use a smartphone app instead of a physical scanner?
Some smartphone apps work with a small Bluetooth adapter that plugs into the OBD2 port. These are less expensive than standalone scanners but require your phone to be compatible and the app to be reliable. A basic physical scanner is more straightforward and does not depend on your phone's battery or connectivity.