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Vehicle Inspection and Maintenance Program

Air Pollution and Motor Vehicles

Under certain meteorological conditions, high levels of air pollution can accumulate in the Salt Lake Valley. Concentrations of particulate matter, ozone or carbon monoxide, often referred to as "inversion," can reach moderate or unhealthy (for sensitive groups) levels in our air.

Motor vehicles are responsible for about 70% of the pollution that affects public health. All motorized vehicles emit pollution, however when a vehicle is operation properly emissions levels are very low.

Malfunctioning vehicles can emit one hundred times the amount of pollution they would if they were operating as designed.

Properly tuned and well maintained vehicles provide better performance, better fuel economy and reduced levels of air pollution.

The Salt Lake County Vehicle Inspection/Maintenance (I/M) Program tests motor vehicles and requires that malfunctioning vehicles be repaired. The program keeps tons of pollutants out of the Salt Lake Valley each day:

• 82 tons of carbon monoxide (CO)
• 4 tons of hydrocarbons (HC)
• 4 tons of nitrogen oxide (NOx)

Vehicle Emissions Inspections

To minimize the pollution from motor vehicles, 1968 and newer vehicles are inspected and repaired if needed. Inspection items include:

• Emissions control devices
• Gas caps
• Tail pipe emissions of HC, CO and NOx
• Proper functioning of the emissions On-Board Diagnostics (OBD) system

Emissions Control Devices

Vehicles are inspected to determine that the emissions control devices originally installed on the vehicle are still present and appear to be operating. For 1984 and newer vehicles, these devices must be installed and operating before the test can continue.

Gas Caps

Gas Caps are tested to make sure they are sealing properly.

Tailpipe Emissions

Most 1995 and older vehicles are emissions tested using a tailpipe test with the vehicle running on a chassis dynamometer (a treadmill for a car). Emissions of HC, CO and NOx are measured.

If emissions exceed predetermined standards, based on the age and weight of the vehicle, the vehicle will fail the test. Some vehicles (such as those with full-time four wheel drive) cannot be run on the dynamometer. These vehicles are tested at idle and 2500 RPM for HC and CO only.

On-Board Diagnostics (OBD) System

Most 1996 and newer vehicles are tested using the OBD test. These vehicles have a malfunction indicator lamp (MIL) in the form of a “check engine” or “service engine soon” light that comes on when the vehicle’s computer system indicates that one or more of the emissions control systems is about to fail or has already failed.

During the OBD test, the I/M technician connects an analyzer to the OBD system to make sure that it is operation properly. Each emissions control system has a monitor that checks vehicle performance as it is driven.

Should any of the emissions systems malfunction, the monitor alerts the on-board computer and captures the data from the failure. A qualified repair technician is able to access this information to aid them in properly diagnosing and repairing the vehicle.

Not Ready Code

Note: If the vehicle’s battery has recently been disconnected or replaced, the OBD system may not be “ready” for testing. This does NOT mean that the vehicle has failed the inspection. Normal driving usually allows the OBD system to become “ready” for testing.