Aftermarket Conversion Systems: EPA and CARB Regulations


EPA streamlines rules for aftermarket conversions
U.S. Environmental Protection Agency (EPA) regulations require manufacturers of alternative fuel aftermarket conversions to prove that their systems continue to meet emission requirements and do not increase emissions beyond the levels originally prescribed for the vehicle undergoing modification. Since 2003, EPA regulations required that all manufacturers of natural gas conversion systems submit to extremely rigorous emissions testing requirements and receive a certificate of conformity for their systems before they could be legally sold. The significant burden and cost of these regulations led to a decline in conversion systems offered in the U.S. In response to this situation, NGVAmerica and other organizations worked with EPA to develop new streamlined regulations for aftermarket conversion systems.

In March 2011, EPA issued new rules for aftermarket conversion systems. The new rules expand compliance options to include less burdensome demonstration requirements for vehicles that are two years or older and for vehicles or engines that have exceeded their useful life. There is now greater flexibility concerning testing requirements, and multiple vehicles can be combined into single test groups. The revised regulations establish three new vehicle tiers: new and relatively new, intermediate age, and outside useful life, which is different depending on the size and weight of the vehicle. See below for more details. The goal of the new rules is to provide flexibility but to also maintain EPA’s oversight and longstanding commitment to the environmental integrity of clean alternative fuel conversions.

Overview of Program Elements1
Vehicle applicability
Certificate issued?
Produced in current calendar year or previous model year
Exhaust, Evap, and OBD testing
Certification application
Intermediate age
Two or more model years old and does not exceed useful life
Exhaust and Evap testing and OBD scan tool test and attestation
Compliance submission
Outside useful life
Engine exceeds useful life
Technical justification and OBD scan tool test and attestation
Compliance submission

1This table is modified from that provided by EPA.

The new rules help streamline exemption requirements by allowing manufacturers to apply a single set of test data to multiple candidate vehicles. Another advantage of the new rules is that manufacturers of systems for vehicles that are more than 2 years old no longer have to re-certify those vehicles. That means they can continue to sell systems without re-applying to EPA or paying any additional fees. Also, the new rules for intermediate and outside useful life vehicles do not impose any fees for submission or approval.

Outside useful life vehicles 
EPA’s new rules provide different requirements for vehicles and engines depending on whether the vehicle or engine is still within its useful life or has exceeded it. All vehicles and engines, however, unless they predate EPA regulations and certification (1960’s) must go through EPA’s approval process. The useful life periods presented here for light-duty vehicles are Tier 2 light-duty vehicles, which have generally been in production since about 2004. Consult EPA rules for older vehicles.

Outside Useful Life Vehicles
Vehicle class
Gross vehicle mass (GVM)
Vehicle mileage/age
Light-duty gasoline engines/vehicles
Less than 8,500 lbs.
120,000 miles/10 years or optional 150,000 miles/15 years
2005–present complete heavy-duty spark ignition vehicles
Less than 14,000 lbs.
110,000 miles/11 years
2004–present light heavy-duty diesel engines: LHDDE
Between 8,500 and 19,500 lbs.
110,000 miles/10 years
Medium heavy-duty diesel engines: MHDDE
Between 19,500 and 33,000 lbs.
185,000 miles/10 years
2004–2007/present heavy heavy-duty diesel engines (including urban buses): HHDDE
Greater than 33,000 lbs.
435,000 miles/10 years/22,000 hours
2005–present heavy-duty spark-ignition engines
Less than 14,000 lbs.
110,000 miles/10 years

Anti-tampering provisions and policy guidance 
EPA policy has always required manufacturers of conversion equipment to ensure that their systems do not degrade the emissions performance of motor vehicles. This policy directive was first set out in EPA’s Mobile Source Enforcement Memorandum 1A, which is based on the Clean Air Act‘s prohibition against tampering with motor vehicle emissions, section 203(a), 42 U.S.C. 7522(a). Section 203(a) makes it illegal to install any component or part if it defeats or “render[s] inoperative any device or element of design installed” on a certified motor vehicle or engine. EPA’s stated purpose in issuing Memorandum 1A was to “provide criteria by which dealers [and now conversion manufacturers] can determine in advance that certain of their acts do not constitute tampering.” Memorandum 1A requires that manufacturers have a “reasonable basis” for believing that their equipment or actions will not increase emissions. A manufacturer could satisfy the “reasonable basis” requirement if “emissions tests… have been performed according to testing procedures prescribed in 40 C.F.R. section 85” and if the tests show “that the act [e.g., conversion] does not cause similar vehicles or engines to fail to meet the applicable emission standards.”

Initially, Memorandum 1A acted to insulate manufactures from tampering violations as long as they complied with the testing procedures outlined above. However, compliance with Memorandum 1A never relieved manufactures of responsibility for in-use emissions performance, durability, and compliance with useful-life emission standards.

Policy change 
In September 1997, EPA issued an addendum to Memorandum 1A. The addendum altered EPA policy with respect to conversions by requiring manufacturers to start certifying their systems in order to protect themselves from tampering violations. Under the new guidance, the only way manufacturers can satisfy the “reasonable basis” test is full certification (either EPA or CARB). The policy change was prompted by evidence it collected that suggested conversion systems were increasing vehicle emissions. EPA believed that the only way to effectively ensure conversions were not increasing emissions was to subject them to full certification. This policy has since evolved with the latest changes coming in March 2011 in the form of the new streamlined regulations, discussed above.

California’s Air Resources Board (CARB)
California has its own certification rules for alternative fuel conversion systems. See the Final Regulatory Order for certification requirements. These rules also impact the sale and use of conversion systems in some but not all of the states that have adopted CARB’s motor vehicle regulations (e.g., Maryland and New York require CARB certified systems). Pennsylvania and New Jersey have issued guidance indicating that they will accept EPA certified systems in addition to CARB systems. CARB’s certification requirements for aftermarket systems have not been updated since 1995 and are essentially obsolete. As a result, manufacturers seeking CARB certification for conversion systems instead use the certification rules for new vehicles. Complying with CARB’s rules is very costly and time-consuming, especially for bi-fuel vehicles. As a result, manufacturers are currently only certifying dedicated NGV systems; although, a number of manufacturers have indicated that they plan to seek certification for bi-fuel conversions.

Further resources and contact information
EPA regulations and guidance:

EPA contact information:

CARB contact information:

For a list of vehicles and conversion systems approved by CARB for model years 1994 and after, click here.
For NGVAmerica’s comprehensive, regularly updated list of both EPA and CARB certified aftermarket conversion systems, click here.

NGVAmerica supports the EPA’s changes to its regulations that provide greater flexibility:

  • Click here to read NGVAmerica oral comments presented during a public hearing on EPA regulations
  • Click here to read NGVAmerica’s written comments, filed July 23, 2010.