Alternate Fuel Retrofit Systems (Aftermarket)

This page last reviewed December 18, 2014

Certification of Alternative Fuel Retrofit Systems

California regulations prohibit the conversion of emission-controlled vehicles with retrofit systems to operate on an alternative fuel, such as natural gas, propane, or ethanol, in lieu of the original gasoline or diesel fuel unless the retrofit systems have been evaluated and certified by the Air Resources Board (ARB). The certification of an alternative fuel retrofit system must be obtained by its manufacturer. Certification is issued by ARB once the manufacturer demonstrates compliance with the emission, warranty, and durability requirements. A manufacturer is defined as a person who manufactures or assembles an alternative fuel retrofit system for sale in California. An individual who wishes to convert a vehicle for personal use does not fall under this definition and cannot certify a retrofit system in California.

The United States Environmental Protection Agency (U.S. EPA) streamlined its rules for alternative fuel converters using a 3-tier approach based on vehicle age/useful life (published April 8, 2011).  This has prompted numerous inquiries regarding their applicability in California.  As clarified in U.S. EPA guidance letter CISD-10-24 (Q&A # 9), California’s requirements for alternative fuel converters are different than U.S. EPA’s requirements.  Revisions to California’s Alternative Fuel Conversion regulation and test procedures were approved by the Board on September 26, 2013.  2003 model year and earlier vehicles were not affected by these changes.  The revisions provided for streamlining of the conversion approval process for 2004 Model year and later Light Duty (LD) vehicles/Heavy Duty (HD) engines.  Small volume manufacturers were given some additional relief which sunsets after the 2017 model year.  

What's New: New Alternate Fuel Retrofit System Certification Procedures

Approved Alternate Fuel Retrofit Systems

Individuals interested in converting their vehicles to operate on an alternative fuel must ensure that the alternative fuel retrofit systems used for their vehicles have been certified by ARB.  Lists of certified retrofit systems for various vehicle makes and models are available online, as follows:

  • Conversion Systems for 1994 and newer model year vehicles (PDF - 142K)
  • Conversion Systems for 1993 and older model year vehicles (PDF - 1.95MB).

Procedures for Alternate Fuel Retrofit System Certification

The following are the procedures for certifying retrofit systems designed to convert on-road vehicles and engines to operate on alternative fuels:

  • 2004 and Subsequent Model Year Conventional Vehicles Converted to Alternate Fuel Vehicles
  • 1994-2003 Model Year Conventional Vehicles Converted to Alternate Fuel Vehicles
  • Test Procedures for 1994-2003 Model Year Light Duty Vehicles and Heavy Duty Vehicles/Engines (PDF - 91K) 
  • 2000 and Subsequent Model Year Hybrid Vehicles Converted to Plug In Vehicles
  • Test Procedures for 2000 and Subsequent Hybrid Vehicle Conversions to Off-Vehicle Charge Capable Systems (PDF - 56K)

To obtain additional information on certification requirements of Vehicles Converted to Alternate Fuel Vehicles, please contact Jackie Lourenco, Chief New Vehicles and Engine Programs Branch at (626) 450-6152 or

For information on Hybrid Vehicles Converted to Plug In Vehicles, please contact Antonio Martino, Manager Aftermarket Parts Section at (626) 575-6848 or

Additional Resources

For hydrogen, biodiesel or non-hybrid to hybrid questions, please contact Antonio Martino, Manager Aftermarket Parts Section, at (626) 575-6848 or