How to Read Smog Test Results

by Kenrick Callwood

More than half of the states in the U.S. now require your vehicle to pass a smog test periodically. This specialized test analyzes the amount of pollutant emissions your car produces. You must understand the testing procedure to read smog test results properly. California is famous for having some of the strictest policies on smog control. Because of this, its smog tests are often used as the base for nationwide interpretation of test results.

Step 1

Review the testing procedure. Using California as an example, vehicles are run through each test at two different speeds. In general, the vehicle sits on a treadmill device and is tested at 15 miles per hour and 25 miles per hour. A dynamometer is attached to the exhaust pipe. This special machine measures the emission levels of hydrocarbon (HC), carbon monoxide (CO), carbon dioxide (CO2), nitrous oxide (NOx) and oxygen (O). You are issued a report that details the pass or fail status, a list of any suggested maintenance issues and a list of the emissions test readings.

Step 2

Read the first line of the emissions test results. Each gas has a separate section on most smog test results. The first line in the section is the two letter abbreviation for the emissions gas detected as described in the previous step.

Step 3

Read the results in each emissions category. Each section shows the testing velocity, the engine revolutions per minute (RPM) at that speed, the maximum amount of the specific gas released at that speed in parts per million (PPM), the average amount of gas emitted during the test and the total amount of gas detected during the test. If any of the measurements exceed the maximum allowable level in your particular state, the car fails the smog inspection.

Step 4

Read and follow any of the maintenance notes included in the report. Most states require technicians to include data about faulty or deficient components that may be responsible for a car failing the test and components that may lead to future smog-related issues. Take the car to a mechanic or address the concerns raised in the report on your own.

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