California: Heavy-duty: Emissions


Although California’s emissions standards for heavy-duty vehicles are harmonized with federal US standards, California has imposed its own regulations, including additional testing, stricter emissions limits for public buses, and optional heightened NOx standards for HDVs.

Standard type
Conventional pollutant emission limits
Regulating Body
California Air Resources Board (CARB) within the California Environmental Protection Agency (Cal/EPA)
Vehicles with gross vehicle weight rating (GVWR) above 14,000 lbs


In 2004, federal emission standards for heavy-duty vehicles were harmonized with California standards, with the intent that manufacturers can use a single engine or machine design for both markets. However, California certifications for model years (MYs) 2005-2007 require additional SET testing beyond federal requirements, and NTE limits of 1.25 × FTP standards. California also adopted more stringent standards for MY 2004-2006 engines for public urban bus fleets.

Technical Standards

Model Years 1987-2003

Model year 1987-2003 California emission standards for heavy-duty diesel truck and bus engines are summarized in the following table. Applicable to the 1994 and following year standards, sulfur content in the certification fuel was reduced to 500 ppm wt.

California Emission Standards for Heavy-Duty Diesel Engines, g/bhp·hr
Heavy-Duty Diesel Truck Engines
1987 1.3 15.5 6.0 0.60
1991 1.2 1.3 15.5 5.0 0.25
1994 1.2 1.3 15.5 5.0 0.10
Urban Bus Engines
1991 1.2 1.3 15.5 5.0 0.10
1994 1.2 1.3 15.5 5.0 0.07
1996 1.2 1.3 15.5 4.0 0.05

Useful Life and Warranty Periods – Compliance with emission standards has to be demonstrated over the useful life of the engine, which was adopted as follows (federal & California):

  • LHDDE—8 years/110,000 miles (whichever occurs first)
  • MHDDE—8 years/185,000 miles
  • HHDDE—8 years/290,000 miles

Applicable federal standards – Federal useful life requirements were later increased to 10 years, with no change to the above mileage numbers, for the urban bus PM standard (1994+) and for the NOx standard (1998+). The emission warranty period is 5 years/100,000 miles (5 years/100,000 miles/3,000 hours in California), but no less than the basic mechanical warranty for the engine family.

Consent Decrees – In October 1998, a court settlement was reached between US Environmental Protection Agency (EPA), Department of Justice, CARB, and engine manufacturers (Caterpillar, Cummins, Detroit Diesel, Volvo, Mack Trucks/Renault and Navistar) over the issue of high NOx emissions from heavy-duty diesel engines during certain driving modes. Since the early 1990’s, the manufacturers used engine control software that caused engines to switch to a more fuel efficient (but higher NOx) driving mode during steady highway cruising. EPA considered this engine control strategy an illegal “emission defeat device”.

Provisions of the Consent Decree included the following:

  • Civil penalties for engine manufacturers and requirements to allocate funds for pollution research
  • Upgrading existing engines to lower NOx emissions
  • Supplemental Emission Test (steady-state) with a limit equal to the FTP standard and NTE limits of 1.25 × FTP (with the exception of Navistar)
  • Meeting the 2004 emission standards by October 2002, 15 months ahead of time

Clean Fuel Fleet Program – Voluntary Clean Fuel Fleet (CFF) program was a federal standard that applied to 1998-2003 model year engines, both CI and SI, over 8,500 lbs GVWR. In addition to the CFF standard, vehicles must have met applicable conventional standards for other pollutants. Additional information can be found on the EPA website.

Clean Fuel Fleet Program for Heavy-Duty SI and CI Engines, g/bhp·hr
LEV (Federal Fuel) 3.8
LEV (California Fuel) 3.5
ILEV 14.4 2.5 0.050
ULEV 7.2 2.5 0.05 0.025
ZLEV 0 0 0 0
* LEV – low emission vehicle; ILEV – inherently low emission vehicle; ULEV – ultra low emission vehicle; ZEV – zero emission vehicle

Model Years 2004 and later

On December 21, 2004, EPA signed emission standards for model year 2007 and later heavy-duty highway engines (CARB adopted virtually identical 2007 heavy-duty engine standards in October 2001). The rule includes emission standards and diesel fuel regulations.

For additional information on heavy-duty emissions, see the US Heavy-duty Emissions page.

Optional NOx Standards

California is home to two air basins with the worst air quality in the country. Due in large part to their unique geography, topography, and climate, the San Joaquin Valley air basin in central California and the South Coast basin, which covers all or parts of Los Angeles, Orange, Riverside, and San Bernardino, are both in extreme non-attainment of the national ozone requirements. With new, more stringent federal ozone standards under development, policymakers in California are responding to the significant challenge with a suite of regulatory and incentive-based measures aimed at reducing nitrogen oxides (NOx), an ozone precursor, and other harmful emissions.

In December 2013, the California Air Resources Board adopted an optional low-NOx standard for on-road heavy-duty engines. In this program, manufacturers can voluntary opt to certify their engines at levels below the 0.2 grams per horsepower-hour federal standard, which has been in place since 2010. There are three levels of certification: 0.1 g/hp-hr, 0.05 g/hp-hr, and 0.02 g/hp-hr, which correspond to reductions below the current federal standard of 50%, 75%, and 90% respectively. The figure below shows heavy-duty engine certification data for 2012. As shown, a significant number of engines are certified well below the 0.2 g/hp-hr limit, which gives the ARB confidence that the optional NOx levels are achievable targets in the near-term.

Certified NOx Levels of Heavy-Duty Engines: California ARB Data from 2012


With this optional NOx standard, engines that certify to lower levels could be integrated into incentive efforts such as the Carl Moyer Program, which provides funding for vehicles and equipment that provide emission benefits beyond what is required by regulation. Given the severe air quality concerns facing the state, it is conceivable that at some point in the future, the California Air Resources Board will look to transition from a voluntary to mandatory approach for promoting low-NOx engines.

Rated Power Date CO NOx+HC PM
kW Construction Farming g/kWh
130 ≤ P ≤ 560 January 2015 January 2017 3.5 4.0 0.2
75 ≤ P < 130 January 2015 January 2017 5.0 4.0 0.3
37 ≤ P < 75 January 2015 January 2019 5.0 4.7 0.4
19 ≤ P < 37 January 2017 January 2019 5.5 7.5 0.6

Emissions are measured in accordance with ISO 8178-1.

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