Your car is due for a California SMOG check, and you are wondering whether it will pass. The single best thing you can do before driving to the station is plug in an OBD-II scanner and read your vehicle's diagnostic trouble codes. It takes about five minutes, costs nothing if you borrow a scanner, and can save you the time and expense of a failed test. Here is everything you need to know about reading OBD-II codes before your SMOG inspection.
What Is OBD-II and Why Does It Matter for SMOG?
OBD-II stands for On-Board Diagnostics, second generation. Every gasoline-powered car sold in the United States since 1996 is required to have an OBD-II system. The system continuously monitors your engine, transmission, and emissions equipment. When it finds a problem, it stores a diagnostic trouble code and usually turns on the check engine light.
California SMOG technicians plug directly into your OBD-II port during an inspection. If your vehicle has any stored fault codes or if your check engine light is on, your car will fail automatically. Reading those codes yourself first means you know exactly what the inspector will see before you ever pull into the lane.
How to Read Your OBD-II Codes at Home
You do not need special training to check your own codes. Here is what to do:
- Get a scanner. Basic OBD-II scan tools cost as little as $20 at an auto parts store. Many stores, including AutoZone, O'Reilly, and Advance Auto Parts, will also lend you one for free or read the codes for you in the parking lot.
- Locate the OBD-II port. It is almost always under the dashboard on the driver's side, within a foot of the steering column.
- Plug in the scanner with the ignition off. Then turn the key to the "on" position without starting the engine.
- Read the codes. The scanner will display any stored diagnostic trouble codes, such as P0420 or P0171. Write them down.
- Check readiness monitors. A good scanner also shows whether your emissions monitors have completed their self-tests. More on that in a moment.
Understanding the Codes You Find
OBD-II codes follow a standard format. The first letter tells you which system is affected: P for powertrain (engine and transmission), B for body, C for chassis, and U for network. The number that follows indicates whether it is a generic code (0) or a manufacturer-specific code (1).
For SMOG purposes, the codes that matter most are powertrain codes in the P0xxx and P1xxx ranges, especially those related to the catalytic converter, oxygen sensors, evaporative emissions system, and mass airflow sensor. A code like P0420 (catalyst system efficiency below threshold) almost guarantees a SMOG failure and usually means your catalytic converter is worn out. A code like P0455 points to a large evaporative emissions leak, which is another common failure point. You can look up any code on the California BAR's smog resources or on a trusted repair database to understand what it means before your appointment.
If you find active codes, do not simply clear them and drive in for a test. Clearing codes also resets your readiness monitors, which brings us to the next important point.
Readiness Monitors: The Hidden SMOG Hurdle
Even with no active fault codes, your car can still fail a California SMOG inspection if too many readiness monitors show "not ready" or "incomplete." Readiness monitors are self-tests the OBD-II system runs on specific emissions components, such as the catalytic converter, oxygen sensors, EGR system, and EVAP system. They only complete after you drive the car through specific conditions, sometimes called a drive cycle.
California allows a maximum of one incomplete monitor on 1996 to 2000 model year vehicles, and zero incomplete monitors on 2001 and newer vehicles. If your battery was recently disconnected or your codes were recently cleared, your monitors may not be ready. You need to drive the vehicle through a complete drive cycle first. Our guide on preparing your car after a battery disconnect walks through the full process.
When to Repair Before Testing
If your scan reveals active codes, repair the underlying problem before you test. Paying a shop to diagnose and fix a P0420 costs money upfront, but it is far less frustrating than paying for a failed SMOG test and then paying for repairs anyway. Low-income drivers may also qualify for help through the Consumer Assistance Program. Visit our CAP repair assistance page for details on income-based repair vouchers.
Once the repair is done, clear the codes, complete the appropriate drive cycle, confirm all monitors are ready with your scanner, and then head in for your test.
Get a Professional Read Before You Commit
If the codes are confusing or you are not sure whether your car is ready to test, a STAR-certified station can give you a pre-inspection check. Drivers in Concord can use the $49.75 coupon at Super Station Concord -- see $49.75 coupon or book your smog check in Concord.
For a broader overview of what to expect during your appointment, the California SMOG check guide covers the full inspection process from start to finish.
Official sources
For the most current rules on OBD-II testing and California SMOG inspection requirements, visit the Bureau of Automotive Repair Smog Check inspections page and the California DMV SMOG certification page.