Smog Plus is a California emissions test add-on that checks your vehicle's onboard diagnostic system
Smog Plus is an optional service offered during California's standard smog check. Instead of just the basic tailpipe emissions test, Smog Plus runs a more thorough scan of your vehicle's onboard diagnostic (OBD) system — the computer that monitors your engine's emissions equipment. The test costs extra, typically $15 to $30 more than a standard smog check, depending on your test station.
You do not need Smog Plus for every vehicle or every smog check. It becomes relevant when your vehicle has a check engine light, when you are buying or selling a used car, or when you want a complete picture of your emissions system before a repair decision. The test reads fault codes stored in your vehicle's computer and can identify problems that a basic tailpipe test would miss.
California's Bureau of Automotive Repair (BAR) does not require Smog Plus for standard biennial smog checks. It is a voluntary service that test stations offer as an add-on. Whether you need it depends on your vehicle's condition and what you are trying to accomplish.
Key Takeaways
- Smog Plus scans your vehicle's onboard diagnostic system and costs $15 to $30 more than a standard smog check.
- A check engine light is the most common reason to get Smog Plus, because it tells you what code triggered the light.
- Smog Plus can identify emissions-related problems that a basic tailpipe test cannot detect.
- You do not need Smog Plus to pass a routine biennial smog check unless your vehicle has a check engine light or fails the basic test.
- Test results include fault codes that you can take to a mechanic to understand what repair is needed.
When a check engine light makes Smog Plus worth the cost
If your check engine light is on, Smog Plus tells you which emissions system component is causing the problem. The test reads the diagnostic trouble code (DTC) stored in your vehicle's computer. That code points to a specific part — your oxygen sensor, catalytic converter, evaporative emissions system, or another component — rather than leaving you guessing.
Without Smog Plus, you would need to pay a mechanic to scan your vehicle separately. Many independent shops charge $50 to $100 for a diagnostic scan. Getting the scan at your smog test station for $15 to $30 more saves you money and time, especially if you are already there for your regular smog check.
The fault code alone does not tell you whether the part has failed or whether a loose gas cap triggered the light. But it narrows the problem enough that a mechanic can make a faster, more accurate repair decision. You can also search the code online to understand what system is involved before you call a shop.
Smog Plus during a used car purchase or sale
If you are buying a used vehicle, Smog Plus gives you a complete emissions picture before you commit to the purchase. A standard smog check passes or fails based on tailpipe emissions and visual inspection. Smog Plus reveals whether the onboard diagnostic system has stored fault codes that might indicate a developing problem.
A vehicle can pass a standard smog check and still have fault codes in its computer. Those codes may point to an emissions component that is failing but has not yet affected the tailpipe reading. Knowing about them before you buy lets you negotiate the price or walk away.
If you are selling a vehicle, offering a Smog Plus report to a buyer builds confidence that the vehicle's emissions system is sound. It shows you have nothing to hide and can justify your asking price.
How Smog Plus results differ from a standard smog check
A standard California smog check measures tailpipe emissions (carbon monoxide, nitrogen oxides, and hydrocarbons) and performs a visual inspection of emissions equipment. For vehicles 1996 and newer, it also includes a basic OBD system check that confirms the system is ready for testing.
Smog Plus goes deeper. It reads all stored and pending fault codes in your vehicle's computer, not just whether the system is ready. It can identify problems that have not yet affected your tailpipe emissions enough to fail the test. A vehicle might pass the standard test but show multiple fault codes on Smog Plus.
The results come as a printout listing each fault code, its description, and whether the code is currently active or has been stored from a previous occurrence. You can take this report to any mechanic for interpretation and repair planning.
What happens if Smog Plus finds a fault code
Finding a fault code does not automatically mean your vehicle will fail its smog check. The standard test measures actual emissions output. A fault code indicates a problem in the emissions system, but that problem may not yet be severe enough to increase tailpipe emissions above the legal limit.
However, if the fault code is related to a component that directly affects emissions — such as your oxygen sensor or catalytic converter — and that component is malfunctioning, your vehicle may fail the standard test. Smog Plus straightforward identifies the problem earlier, before it becomes a failure.
If you get Smog Plus and it shows a fault code, you have options. You can repair the component before your next smog check, monitor the code to see if it clears on its own, or take the report to a mechanic for a second opinion on whether repair is urgent. The code itself does not force you to do anything — it is information to help you make a repair decision.
Smog Plus for vehicles that fail the standard test
If your vehicle fails a standard smog check, Smog Plus can help identify why. The failure might be due to high tailpipe emissions, a visual inspection failure (such as a missing or damaged emissions component), or an OBD system problem. Smog Plus focuses on the OBD system piece.
If your vehicle failed because the OBD system was not ready for testing, Smog Plus will not help — you need to drive the vehicle for a set number of miles to allow the system to complete its self-test cycle. But if the failure was due to high emissions or a component issue, Smog Plus can point to the specific fault code causing the problem.
California allows you to retest after repairs. Taking your Smog Plus report to a mechanic gives them a clear starting point for diagnosis and repair, which can save time and money compared to a general "your emissions are too high" failure notice.
Cost comparison: Smog Plus versus separate diagnostic scanning
A standard California smog check costs between $30 and $70, depending on your vehicle type and location. Smog Plus adds $15 to $30 to that cost. A separate diagnostic scan at an independent mechanic typically costs $50 to $100, and a dealership scan can run $100 to $150.
If you need a diagnostic scan anyway — because you have a check engine light or you want a complete emissions picture before buying a used vehicle — getting Smog Plus at your test station is usually the cheaper option. You are already there for the smog check, so you are not paying for a separate visit.
If your vehicle passes the standard smog check and has no check engine light, Smog Plus is optional and adds cost with no when ready benefit. You would only choose it for peace of mind or as part of a used car purchase inspection.
Frequently Asked Questions
Will Smog Plus make my vehicle fail if it passes the standard test?
No. Smog Plus is informational only and does not affect your pass or fail result. Your vehicle passes or fails based on the standard smog check — tailpipe emissions and visual inspection. Smog Plus straightforward shows you what fault codes are stored in your vehicle's computer. You can choose to repair them or not.
Can I get Smog Plus at any smog test station?
Most California smog test stations offer Smog Plus as an add-on service, but not all. Call ahead or ask when you arrive for your smog check. If your test station does not offer it, you can get a separate diagnostic scan at an independent mechanic or dealership, though it will cost more.
What do I do with the fault codes Smog Plus finds?
Take the report to a mechanic and ask them to explain what each code means and whether repair is urgent. Some codes indicate minor issues that can wait; others point to components that will likely cause a smog check failure soon. A mechanic can help you prioritize repairs based on cost and impact.
Does Smog Plus tell me if my catalytic converter is failing?
Smog Plus can show a fault code related to your catalytic converter, but it cannot directly measure whether the converter is physically damaged or clogged. The code indicates the converter is not performing as expected. A mechanic can then perform additional tests to confirm whether replacement is needed.
Is Smog Plus required for a vehicle with a check engine light?
No, but it is practical. California does not require Smog Plus, but a vehicle with a check engine light will likely fail the standard smog check. Getting Smog Plus at your test station tells you what is causing the light for $15 to $30 more, rather than paying a mechanic $50 to $100 for the same scan separately.