Here we are, information directly from the government.

A converted vehicle is one that was originally designed to operate on gasoline but has been altered to run on an alternative fuel such as compressed natural gas (CNG) or liquefied petroleum gas (LPG, or propane), the two most common types of alternative fuel vehicle conversions.

Converted vehicles may be "dedicated," which means that they operate only on an alternative fuel, "dual-fuel," or "flex fuel," which means that they can operate on either an alternative fuel or gasoline, with separate tanks and fuel systems for each fuel. "Bi-fuel" vehicles are designed to run on combinations of an alternative fuel with a conventional fuel such as gasoline. Unlike dual-fuel systems, which allow the use of only one fuel at a time, bi-fuel systems supply both fuels into the combustion chamber at the same time (please note that according to industry, these definitions of "bi-fuel" and "dual-fuel" may be reversed).

All vehicle conversions must be certified according to Mobile Source Enforcement Memorandum 1A (Memo 1A), the Addendum to Memo 1A, and the Revision to the Addendum to Memo 1A, which were issued by EPA. There are three options specified under the Addendum to Memo 1A through which a converted vehicle may be certified. Since Option 3 of the Addendum to Memo 1A expired on August 29, 2002, only companies who obtain either a EPA Certificate of Conformity according to Option 1 or a retrofit system certification from the California Air Resources Board (CARB) according to Option 2 of the Addendum to Memo 1A may perform alternative fuel vehicle conversions.

For information about whether a Certificate of Conformity has been issued for your vehicle and whether your vehicle can be converted, please see the information on the Conversion Company Industry Contacts Web page. If a Certificate of Conformity has not already been issued for your vehicle, it is possible that a certificate holder would consider applying for a Certificate of Conformity for that vehicle so that it could be converted. The certificate holder can also advise you regarding the cost of having your vehicle converted. Additionally, federal and state incentives can help offset the cost of a conversion.

The following is a description of the EPA certification procedure.

EPA now certifies converted vehicles, rather than conversion systems or "kits." Typically, EPA refers to a fuels converter (the certificate holder), as a "small volume manufacturer." An individual or entity that wishes to have a vehicle converted to operate on an alternative fuel must do so through a company or organization associated with a certificate holder. Examples of types of companies or organizations that hold Certificates of Conformity issued by EPA include the designer of the conversion equipment, the producer or manufacturer of the equipment, and the person or entity that plans to perform installations. It is the responsibility of the certificate holder to ensure that the equipment is properly installed and that the system is safe, durable, and results in the vehicle meeting the emission standards of the original model year of the vehicle.

Certificates of Conformity for "aftermarket" conversions (conversions on vehicles that are owned by an individual, company, or organization rather than the OEM) are signed by EPA and certify that the appropriate sections of the Code of Federal Regulations (40 CFR Parts 85 through 88) have been met. Certificates indicate the following:

• The original test group of vehicles as determined and
  provided by the OEM. Engine families (since the 2001 model year, light-duty vehicle engine families are known as "test groups") are a subset of vehicles that the OEM certifies and have certain common operating characteristics in terms of emissions control systems. There are many different test groups; for example, the Ford F-150 could have 10 or more test groups, depending on the engine size, gross vehicle weight, and drive train of the vehicle.
• The evaporative emissions family.
• The state(s) in which the test group is certified (e.g. California vs. 50 state sales areas).
• The "car line." For example, "F-150, 2 wheel drive, extended cab, 5.4 Liter engine."
• The model year of the vehicles included in the test group.
• The emissions standards that are met.

An aftermarket conversion may only be performed on a vehicle if a Certificate of Conformity or a CARB certification has been issued for that vehicle’s particular model year and exhaust and evaporative emissions test groups.

For aftermarket AFV conversions, potential certificate holders must complete an application and submit emissions test data to EPA. Each year, certificate holders must file a new application to renew their certificates for a test group of a specific model year, but they do not need to submit new test data in order to renew. For example, for the conversion of a model year 2003 vehicle, EPA can issue a 2004 model year certificate (enabling the certificate holder to convert that model year 2003 vehicle test group through the end of 2004 calendar year) or a 2005 model year certificate (enabling the certificate holder to convert that model year 2003 vehicle through the end of 2005 calendar year). The certificate holder could later apply for a 2006 model year certificate, once EPA begins issuing those certificates (enabling the certificate holder to convert that model year 2003 vehicle test group through end of 2006 calendar year). The certificates are valid through December 31 of each certification year.

For information about the CARB procedures, please visit the
Aftermarket, Performance, and Add-On Parts Regulations Web page.

For additional information on small volume manufacturer conversion procedures, please see the EPA’s Certification Guidance for Alternative Fuel Converters.