What Is Done During A Smog Check?

Some state laws require vehicles to undergo periodic or annual emissions testing in an attempt to improve air quality. These services are designed to recognize and fix cars that emit excessive pollution in compliance with local regulations. Depending on where you live, pollution regulations differ dramatically. Some states require a yearly pollution test for all cars, while others only require trucks and larger vehicles to pass a smog check. 

A licensed technician can mount measuring equipment to your vehicle during a smog inspection. The smog check test consists of pollution inspection, visual inspection, and practical inspection—all of which ensure that your emission equipment is in place, working, and doing its job of removing contaminants from the exhaust of your vehicle. Carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC) and oxygen are among the emissions measured (O2). The emissions test also measures levels of nitrogen oxides in some of the nation’s most polluted urban areas.

The measurements taken are registered during the smog check and instantly accessible to the agency responsible for tracking smog and vehicle pollution in your state. Cities, states and counties that require DMV car registration smog and pollution tests decide which type of testing or combination of methods to use.

Testing Methods

• Two-speed idle (TSI) – This test analyzes exhaust emissions from the tailpipe as the engine idles first at high speeds, then at low speeds, and is usually for older cars registered before 1995.
  
• Acceleration simulation mode (ASM)– Again, the ASM test tests emissions under simulated driving conditions using a dynamometer, most commonly reserved for older vehicles.
• On-board diagnostics (OBD)– Information is downloaded from the on-board diagnostics computer of the vehicle during this test.

Upon completion of the smog check, whether passed or not, the technician will provide you with a copy of the test results, called the Vehicle Inspection Report (VIR), along with a receipt for payment of the smog check. If your vehicle fails to perform a smog check, you can choose to have it repaired at a licensed test and repair facility and then have it retested to pass a smog check. Vehicles that have not passed the inspection are not allowed on public roads, and owners will be charged if they are caught operating those vehicles.

Emissions issues are generally cheap and quick to repair. If you believe that your vehicle is not going to pass a smog check, it may be best to go to a facility that can perform a check and make any necessary repairs and then re-test the vehicle to pass a smog inspection.