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What Happens During a Smog Check?

Posted by DEKRA North America on Sep 16, 2015 3:57:53 PM

California requires a smog check on most vehicles older than six model years once every two years, or once a year if your vehicle is tagged as a high-polluting vehicle. If you do not abide by this law, you cannot legally drive your car in the state. So, what exactly happens during smog checks? Here, we’ll break down the process. smog checks entail

Measure / Inspect

A licensed technician will attach measuring equipment to your vehicle and perform the following inspections: 

  • Emissions Inspection - Emissions measured include carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC) and oxygen (O2), and oxides of nitrogen (NOx).
  • Visual Inspection and Functional Inspection - Ensure your emissions equipment is in place, functioning, and doing its job of decreasing pollutants from your car's exhaust.

Current testing methods include:

  • Two-Speed Idle (TSI) This test analyzes exhaust emissions from the tailpipe as the engine idles, first at high and then at low speed, and is usually for older cars registered before 1995.
  • Acceleration Simulation Mode (ASM) - Most often reserved for older vehicles, the ASM test measures emissions under simulated driving conditions using a drive-on dynamometer, which is kind of like a treadmill for your car.
  • On-Board Diagnostics (OBDI) - During this test, information is downloaded from the vehicle’s on-board diagnostics computer.


The measurements taken are recorded and become immediately available to your state’s agency responsible for monitoring smog and vehicle emissions. In California, it is transmitted to the Bureau of Automotive Repair (BAR). 

Get Results 

Following completion of the smog check, whether pass or fail, the technician will give you a copy of the test results, called the Vehicle Inspection Report (VIR), along with a receipt for payment. 

Pass or Fail 

If your vehicle passes inspection, you can legally drive in California! (Until it’s time for your next smog check.)

If your vehicle fails its smog test, you will receive instructions on what needs to be repaired. You can repair it yourself or being it to a licensed repair shop, then stop back at any DEKRA station for your free retest. Your receipt will note the amount of time allotted for your repair and retest, and it is important to be prompt as owners will be ticketed if they are caught operating a vehicle past its due date. 

This step-by-step explanation should give you a clear picture of what actually happens during a smog check. Check your vehicle on time and drive safer!

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Topics: Vehicle inspections in California

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