http://www.epa.gov/OMS/imports/quiktext.htm
Extracts:
A. What is a U.S. Version Vehicle?
A U.S. version vehicle is manufactured in conformity with Federal emission requirements. The manufacturer attaches a label in the engine compartment that states, in the English language, that the vehicle conforms to all U.S. EPA regulations.
A vehicle lacking the U.S. EPA emission compliance label is considered non-complying. Alternative proof is a written statement from the manufacturer's U.S. representative stating the vehicle is a U.S. version.
EPA does not accept passing a state inspection test as proof. State inspection tests do not determine conformity with Federal emission standards.
EPA also does not accept having a catalytic converter as proof. The presence of a catalytic converter does not mean the vehicle is a U.S. version, or that it meets Federal emission standards.
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C. EPA Requirements for Importing a U.S. Version Vehicle
The importer must be able to demonstrate the vehicle is a U.S. version. A U.S. version vehicle is manufactured with a label in the engine compartment that states, in the English language, the vehicle conforms to all U.S. EPA regulations.
Note that Customs may seize a non-U.S. version vehicle falsely declared as a U.S. version.
Military members should import their U.S. version vehicle through the military P.O.V. program, if possible.