Carbon monoxide (CO)—

A colorless, odorless, tasteless, and toxic gas resulting from the incomplete combustion of fuels containing carbon.


Molecule formed when CO reacts with hemoglobin, an intracellular protein that transports oxygen in the blood. The presence of carboxyhemoglobin increases the hemoglobin’s affinity for oxygen, thereby reducing the transport of oxygen from the blood to the body’s tissues.

Clean Air Act (CAA)—

The original Clean Air Act was passed in 1963, but our national air pollution control program is actually based on the 1970 version of the law. The 1990 Clean Air Act Amendments (CAAA90) are the most recent and far-reaching revisions of the 1970 law.

Closed-loop fuel control—

A fuel-metering system that uses feedback for more effective emissions control. The air-to-fuel ratio of a contemporary vehicle is “closed-loop,” using a sensor in the exhaust to evaluate the mixture exiting the engine and adjusting the air-to-fuel ratio through the use of an onboard computer to optimize emissions performance.

Cold-start emissions—

Tailpipe emissions that occur before a vehicle is fully warmed up. Vehicle emissions are higher during the first few minutes of operation because the engine and the catalytic converter must come to operating temperature before they can become effective.

Criteria air pollutants—

A group of six common air pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide) regulated by the federal government since the passage of the Clean Air Act in 1970 on the basis of information on the health and/or environmental effects of each pollutant.

Cut point—

For each pollutant, the emissions level above which a car is considered to have failed the emissions test for that pollutant.

Data link connector—

The connector where the scan tool interfaces with a vehicle’s onboard diagnostic system. Also know as the diagnostic link connector.

Diagnostic trouble codes—

Codes stored in the engine’s computer that identify emissions-control systems and components that are malfunctioning; they can be retrieved using a scan tool.


A treadmill-like machine that allows cars to be tested under the loads typical of onroad driving.

Emissions budget—

Allowable emissions levels identified as part of a state implementation plan (SIP) for pollutants emitted from mobile, industrial, stationary, and area sources. These emissions levels are used for meeting emission reduction milestones, attainment, or maintenance demonstrations.

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