On November 13, 2018, the U.S. Environmental Protection Agency (EPA) launched its Cleaner Trucks Initiative (CTI), which will decrease nitrogen oxide (NOx) emissions by updating the existing NOx standard for heavy-duty trucks. EPA’s announcement comes just as the California Air Resources Board (CARB) updated its Heavy-Duty On-board Diagnostic (HD OBD) requirements and prepares to implement its own Phase 2 GHG regulation for heavy-duty vehicles and trailers.

Background on NOx Emissions from Heavy-Duty Vehicles

The South Coast Air Quality Management District (South Coast), which regulates the notoriously bad air pollution in the Los Angeles metropolitan area, led a coalition of state and local environmental agencies that petitioned the EPA in 2016 to adopt more stringent NOx standards for heavy-duty trucks. South Coast, along with a separate petition filed by the San Joaquin Valley Air Pollution Control District, argued that they will need significant additional reductions of NOx emissions from heavy-duty trucks to meet the National Ambient Air Quality Standards. EPA has not revised NOx standards for on-highway heavy-duty trucks and engines since January 2001.

Although CARB can adopt more stringent vehicle standards than EPA,[1] the petitioners cannot themselves set vehicle standards. Because vehicles purchased out-of-state account for the majority of heavy-duty vehicle miles traveling in the South Coast’s jurisdiction—which includes two of the nation’s largest marine ports that bring in nearly 40% of the nation’s goods—they argued that a federal standard was critical and therefore sought a 90% reduction from the current 0.2 grams per brake horsepower-hour (g/bhp-hr) NOx standard down to 0.02 g/bhp-hr.

In late 2016, EPA responded to the petitioners and, without committing to a specific standard, agreed to prepare a Notice of Proposed Rulemaking for a new on-highway heavy-duty NOx program. The EPA stressed that it would “work closely with CARB” to develop “a new harmonized Federal and California program to reduce NOx emissions.”

The Cleaner Trucks Initiative

The CTI will culminate in a future rulemaking that will update the existing NOx standard. EPA plans on supplementing the new standard with deregulatory reforms focused on the requirements for assuring “real world” compliance of in-use vehicles, including OBD requirements, as well as annual recertification requirements.

EPA intends to publish its proposed rule in early 2020 and intends for the rule to take effect by the 2024 model year. The agency has not begun accepting public comments, nor detailed the specific deregulatory actions it is preparing to take.

The CTI announcement reaffirms the prior administration’s commitment to reduce NOx emissions from on-highway heavy-duty trucks and engines, despite the current administration’s proposal to revise light-duty vehicle GHG standards and abridge California’s authority to regulate GHG emissions more stringently. (Separate battles could also emerge between the federal government and California with respect to GHG standards for heavy-duty vehicles and trailers.[2])

California’s HD OBD Regulation

On November 15, 2018, CARB adopted revisions to California’s HD OBD rules for the first time since 2012. The most noteworthy revision creates a new emissions tracking program—Real Emissions Assessment Logging (REAL)—for OBD systems, which monitor the performance of the engine’s emission-related components for malfunctions. The REAL program requires OBD systems to collect and store NOx emissions data—not just flag malfunctions—on in-use medium- and heavy-duty diesel vehicles starting with the 2022 model year.

The amended OBD regulations also require OBD systems to collect and store fuel consumption data for all heavy-duty vehicles in-use. This data would be used to characterize the vehicles’ CO2 emissions. A similar requirement is already in place for light- and medium-duty vehicles starting in model year 2019.

Alignment of California and Federal Standards

CARB’s movement forward at the same time as EPA has announced its CTI creates both opportunities for alignment and risks of divergence between federal and California standards for heavy-duty vehicles. But, with EPA affirming that “reducing NOx emissions from heavy-duty vehicles is a clean air priority for this administration” and California’s strong interest in a federal standard to reduce truck emissions in areas of the state that have long had some of the worst air quality, the prospects for a 50-state NOx standards for heavy-duty trucks may be relatively favorable.

[1] In 2013, CARB adopted optional low-NOx emission standards for heavy-duty vehicles, intending to allow engine manufacturers to showcase new technologies that could reduce emissions to 90% below the current standard.


[2] With the National Highway Traffic Safety Administration (NHTSA), EPA has separately adopted GHG emission standards for heavy-duty vehicles, with the Phase 1 regulation for heavy-duty engines and vehicles going into effect for model year (MY) 2014 vehicles and Phase 2 regulation establishing the first emission requirements for trailers hauled by heavy-duty tractors starting in MY 2018. The Phase 2 regulation has been stayed to the extent it applies to trailers (see Truck Trailer Manufacturers v. EPA, et al., No. 16-1430 (D.C. Cir. stayed Oct. 27, 2017)), and EPA is reportedly reconsidering the trailer standards; but, on September 27, 2018, CARB adopted its own Phase 2 rule to harmonize with the existing federal GHG standards, with some differences, including requirements for CARB to verify heavy-duty certification information due in part to concerns that EPA might weaken the existing GHG standards.