How to Interpret Smog Check Information

Smog tests were begun in the 1960s to help raise the air quality in major urban areas.

Although most identified with California, the state with the most stringent emissions requirements, smog checks are required in 30 states plus the District of Columbia. Also known as a smog inspection or an emissions test, the process examines atmospheric gases released while the car is running. Frequency of the test varies by state, but most require either annual or biennial testing as well as during a transfer of ownership. Similarly, testing standards and procedures vary by state, but as California is the most stringent, it is often the standard for interpretation.

Instructions

1. Have the vehicle tested. States that require smog tests have stations, both state-run and privately run, where owners bring their vehicles. Using California, the most populous state and the one with the most rigorous testing, as an example, vehicles are tested at two speeds: 15 and 25 miles per hour. A special device (a dynamometer) is attached to the tailpipe to measure the output of hydrocarbon, carbon monoxide and nitrous oxides, as well as carbon dioxide and oxygen.

   There is also a visual test in which the tester will inspect the muffler and engine for any missing or bad parts.

2. Receive the smog check results. Testers and technicians are required to interpret the results, make recommendations for any repairs and answer any questions, but they may not always have the time to go through the results in great detail.

3. Read the smog check results. Again, using California as an example, the result will look something like this:

   HC

   Test RPM MAX AVG MEAS

   15mph 1950 57 8 17

   25mph 2202 41 6 13

4. Read the gas tested. The first item of each test refers to the acronym of the chemical name: "CO2" for carbon dioxide, "O2" for oxygen, "HC" for hydrocarbon, "CO" for carbon monoxide and "NO" for nitrous oxide. Although at times carbon dioxide and oxygen are listed together, the three major pollutants are listed separately.

5. Read the results for each emissions test. "Test" refers to the speed at which the measurement was taken (15 or 25 miles per hour), while rpm refers to the revolutions per minute of the engine at that speed. "Max" is the highest amount of that particular gas released during the test as measured in "ppm" (parts per million), "Avg" refers to the average amount of that gas emitted throughout the test and "Meas" is the total amount emitted. If any of these values are above the acceptable quantity, the vehicle will fail inspection.

6. Make any necessary changes to the vehicle. While major repairs may be necessary, a failing test can also be the result of a dirty air filter, extra release of unburned fuel from a recent fill-up or simply not allowing the car to sufficiently warm up before the check.

References

• Long Beach City College: Reference Guide Final
• California Department of Motor Vehicles: Smog Check Information
• DMV.org: California Smog Checks
• Smog Check Tips: How to Pass the Smog Inspection