Retrofit systems are after-market conversion kits that allow a gasoline or diesel-fueled engine to run on an alternative fuel, usually natural gas or propane. Most require a qualified automotive mechanic to install.

According to the California Air Resources Board (CARB):

"Under current California regulations, conversion of emission-controlled vehicles with retrofit systems to operate on alternative fuels, such as propane and natural gas, in lieu of the original gasoline or diesel fuel is only allowed if the retrofit systems have been evaluated and certified by the Air Resources Board. The certification must be obtained by the manufacturer of the retrofit system and is issued by the Air Resources Board once the manufacturer demonstrates compliance with the emission, warranty, and durability requirements."

To view CARB's list of certified retrofit systems for 1994 and newer model-year vehicles (latest information is from October 2002).

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