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Emission Standards

United States: California Emission Programs for In-Use Diesel Engines


Following the identification of diesel particulate matter as a toxic air contaminant (TAC) in 1998, the California Air Resources Board (CARB) developed a strategy to control diesel PM emissions. The “Risk Reduction Plan to Reduce Particulate Matter Emissions from Diesel-Fueled Engines and Vehicles”, approved by CARB in September 2000, set goals to reduce diesel PM emissions in California by 75% by 2010 and 85% by 2020.

This objective was to be achieved by a combination of approaches—including emission regulations for new diesel engines and a low sulfur fuel program. Important to the program were measures for various categories of in-use on- and off-road diesel engines, generally based on three types of controls:

  1. Retrofitting engines with emission control systems, such as diesel particulate filters,
  2. Accelerated replacement of existing engines with new technology diesel engines or alternatively fueled engines, and
  3. Restrictions placed on the operation of existing equipment (e.g., fuel quality).

Once the Diesel Risk Reduction Plan was adopted, CARB started developing PM emission regulations for a number of categories of in-use diesel vehicles and equipment. The regulations required diesel engine operators to apply certain control measures, but in most cases included flexibilities to allow a choice of approach. The general objective of the regulations was to reduce PM emissions from diesel engines to levels of below 0.01 g/bhp-hr, equivalent to the 2007 PM emission standard for new heavy-duty onroad engines.

Ongoing CARB initiatives for new and in-use engines aim to reduce ambient ozone, PM2.5, and toxic air pollution to levels protective of public health by the mid-2030s in all urban areas, and to eventually transform all vehicles on- and off-road to zero emissions [5374].

Control Equipment Verification Program

To support diesel retrofit programs in California, CARB has developed a verification program for Diesel Emission Control Strategies. The Verification Procedure [3584] provides a way to evaluate the PM emission reduction capabilities and durability of the diesel emission control strategies to be used for retrofitting of in-use diesel engines.

Depending on their PM emission reduction capability—as demonstrated by laboratory emission testing—emission control equipment can be verified as a Level 1, 2, or 3 strategy, Table 1. Optionally, strategies can be also verified for their NOx reduction effect.

Table 1
Verification classification for diesel emission control strategies
Verification for PM Emission Reduction
PM< 25%Not verified
≥ 25%Level 1
≥ 50%Level 2
≥ 85%, or
≤ 0.01 g/bhp-hr
Level 3
Optional Verification for NOx Reduction
NOx< 15%Not verified
≥ 15%Verified in 5% increments

In addition to the above emission performance, verified technologies must meet the following requirements:

  • Durability Demonstration: Manufacturers must demonstrate through a field or laboratory test that the required emission reduction can be still achieved after a certain period of operation. The required durability demonstration periods are listed in Table 2.
    Table 2
    Minimum durability demonstration periods
    Engine TypeDurability Demonstration Period
    On-road50,000 miles or 1000 hours
    Off-road (including portable) & stationary1000 hours
    Stationary emergency standby engines500 hours
  • Warranty Requirements: Manufacturers must provide a warranty for the verified emission control device, covering both materials/workmanship and emission performance. The warranty must also cover the engine. Should an engine failure be caused by the retrofit device, the warranty must cover the full repair or replacement cost of the affected engine components. The required warranty periods are listed in Table 3.
    Table 3
    Minimum warranty periods
    Engine TypeEngine SizeWarranty Period
    On-roadLight heavy-duty, 70 - 170 hp, GVWR < 19,500 lbs5 years or 60,000 miles
    Medium heavy-duty 170 - 250 hp, GVWR 19,500 - 33,000 lbs5 years or 100,000 miles
    Heavy heavy-duty, above 250 hp, GVWR > 33,000 lbs5 years or 150,000 miles
    Heavy heavy-duty, above 250 hp, GVWR > 33,000 lbs, and the truck is:
    1. Typically driven over 100,000 miles per year, and
    2. Has less than 300,000 miles on the odometer at the time of installation
    2 years, unlimited miles
    Off-road & stationaryP < 25 hp, and constant speed engines < 50 hp with rated speeds ≥ 3,000 rpm3 years or 1,600 hours
    25 hp ≤ P < 50 hp4 years or 2,600 hours
    P ≥ 50 hp5 years or 4,200 hours

    If a manufacturer sells a CARB verified retrofit technology outside of California, CARB mandates that the retrofit technology be sold with the CARB mandated warranty coverage.

  • Nitrogen Dioxide Emission Limit: The first version of the protocol included a post-control NO2 emission limit—defined as 20% (by mass) of the total baseline NOx emission—which was later suspended. A modified NO2 limit of 30% was introduced effective 2007, and of 20% effective 2009, applicable to both new and existing verifications. The modified limit is defined as the maximum percentage increase incremental over the baseline NO2 emission level. For instance, for an engine with a baseline NO2 fraction of 12%, it corresponds to total NO2 emissions of 42%/32% of the NOx, effective 2007/2009, respectively. Devices must be pre-conditioned before testing to remove any stored PM that could react with NO2.
  • Ammonia Emission Limit: The verified technology must not increase NH3 emissions to a level greater than 25 ppm by volume at the tailpipe on average over any test cycle used to support emission reduction claims.

Adopted Control Regulations

Adopted diesel PM control regulations include rules for:

  1. public transit agency fleets,
  2. solid waste collection vehicles,
  3. idling regulations & APSs,
  4. transport refrigeration units,
  5. cargo handling equipment,
  6. commercial harbor craft,
  7. drayage trucks,
  8. public fleets,
  9. truck and bus fleets (including private fleets),
  10. in-use off-road diesel equipment,
  11. stationary & portable diesel engines,
  12. marine engines.

Some of the early rules, adopted in the early 2000s, have already been fully implemented. Other programs, adopted later, are still ongoing.

Public Transit Agency Fleets

The public transit bus fleet rule adopted in February 2000 [CCR 13, 1956.2] applied to urban buses powered by heavy heavy-duty diesel engines, capable of carrying 15 or more passengers, and intended primarily for inter-city operation. Under the rule, California fleet operators had to choose between a “diesel path”; and an “alternative fuel path”; for their future urban bus procurements. The alternative fuel path required that 85% of buses purchased or leased each year through MY 2015 were fueled by alternative fuels. Transit operators who stayed on the diesel path could purchase diesel fueled buses but were required to follow a more aggressive emission reduction schedule.

The regulation provided numerous detailed provisions and schedules, which can be summarized as follows:

  • A NOx fleet average limit of 4.8 g/bhp-hr was effective from 2002.10 for both diesel and alternative fuel paths.
  • Ultra low sulfur diesel fuel (15 ppm wt.) was required from 2002.07.
  • The total PM emission from the fleet had to be reduced by 85% relative to the emissions in January 2002. This requirement was phased-in by 2007 on the diesel path, and by 2009 on the alternative fuel path. The PM emission reduction could have been achieved by introducing cleaner engines or retrofitting existing engines with verified emission control equipment.
  • New model year 2004-2006 diesel bus engines had to be certified to 0.5 g/bhp-hr NOx, 0.01 g/bhp-hr PM, 0.05 g/bhp-hr NMHC, 5.0 g/bhp-hr CO, and 0.01 g/bhp-hr HCHO emission standards.
  • Zero emission bus purchase quota of 15% applied for fleets of 200 or more buses from 2008 on the diesel path and from 2010 on the alternative fuel path, through MY 2015.

Solid Waste Collection Vehicles

The rule “Diesel Particulate Matter Control Measure for On-Road Heavy-Duty Residential and Commercial Solid Waste Collection Vehicles” was adopted in September 2003 and became effective on 20 July 2004 [CCR 13, 2020-2021]. The regulation covered MY 1960-2006 engine on-road diesel-fueled heavy-duty residential and commercial solid waste collection vehicles with a GVW rating over 14,000 lbs. The implementation schedule started in 2004 and continued through 2011.

The rule required that the “best available control technology” be implemented according to a specified schedule. Three best available control technology options were available to truck owners:

  • To use a diesel engine, alone or in combination with a verified diesel emission control strategy, that was certified to the 0.01 g/bhp-hr PM emission standard,
  • To use an alternative fuel engine, or a heavy-duty pilot ignition (dual fuel) engine, or
  • To apply the highest level diesel emission control strategy verified by CARB for a specific engine.

Idling Regulations & APSs

Several regulations have been adopted to control idling emissions from heavy-duty engines:

  1. A control measure to limit school bus idling became effective in 2003 [CCR 13, 2480]. The rule requires a driver of a school bus, transit bus, or other commercial motor vehicle to manually turn off the bus or vehicle engine whenever they are within 100 feet of a school, and to restart no more than 30 seconds before departing.
  2. Commercial diesel vehicles with a GVWR > 10,000 lbs are subject to idling restrictions effective February 1, 2005 [CCR 13, 2485]. Affected vehicles are required to limit idling to no longer than 5 minutes under most circumstances.
  3. A rule to further limit idling of new and in-use sleeper berth equipped diesel trucks was adopted in 2005. The regulation consists of new engine and in-use truck requirements, as well as emission performance requirements for technologies used as alternatives to the truck’s main engine idling. The new engine requirements require MY 2008 and newer heavy-duty diesel engines to be equipped with a non-programmable engine shutdown system that automatically shuts down the engine after 5 minutes of idling or optionally meet a NOx idling emission standard of 30 g/hr. The in-use truck requirements require operators of both in-state and out-of-state registered sleeper berth equipped trucks to manually shut down their engine when idling more than 5 minutes at any location within California beginning in 2008.
  4. Under the low NOx emission rule for new heavy-duty engines adopted in 2020, the NOx standard at idle—an alternative to the engine shutdown system—declines from 30 to 10 g/hr NOx from 2024, and to 5 g/hr NOx from 2027.

The 2008 regulation also included emission requirements for cab comfort devices, such as internal combustion auxiliary power systems (APS) or units (APU), and fuel-fired heaters. Performance requirements for these systems differ depending on whether the truck’s engine is a 2007 or later model:

  • Beginning in 2008, trucks with MY 2007 and later engines equipped with a particulate filter are required to either route the APS’s exhaust through the particulate filter of the main truck engine or to retrofit the APS separately with a Level 3 PM control device.
  • Trucks equipped with MY 2006 or older engines may use a diesel-fueled APS without adding PM control devices.

Beginning in 2008, all 2007 and subsequent model year trucks equipped with fuel-fired heaters need to comply with the fuel-fired heater emissions requirements specified in the Low Emission Vehicle Program to operate in California.

Transport Refrigeration Units (TRU)

The TRU regulation was adopted in 2003 and became effective from 10 December 2004 [CCR 13, 2477]. It applies to owners and operators of diesel fueled TRUs and TRU generator sets that are installed on trucks, trailers, shipping containers, or railcars that operate in the state of California (including out-of-state vehicles). CARB is evaluating further requirements to reduce TRU emissions by phasing-in zero-emission TRU units on certain truck categories.

TRU units must meet in-use performance standards, shown in Table 4, by either:

  • Using a certified engine meeting the applicable nonroad emissions standards for all regulated pollutants and the in-use PM performance standard, or
  • Equipping the engine with the required Level of Verified Diesel Emission Control Strategy (VDECS).
Table 4
In-use performance standards for TRU engines
Emission CategoryEngine PM CertificationVDECS Retrofit
TRU engines < 25 hp
Low emission TRU (LETRU)0.30 g/bhp-hrLevel 2
Ultra low emission TRU (ULETRU)n/aLevel 3
TRU engines ≥ 25 hp
Low emission TRU (LETRU)0.22 g/bhp-hrLevel 2
Ultra low emission TRU (ULETRU)0.02 g/bhp-hrLevel 3

The engine certification levels in Table 4 correspond to the nonroad Tier 4 PM emission standards. Thus, the TRU regulation enforced Tier 4 PM emission levels on older in-use TRU engines and ensured that in the long term, particulate filters would also be used on older engines below 25 hp (which do not require filters to meet the Tier 4 rule).

The TRU emission requirements apply to in-use engines older than seven years. Owners/operators must meet the TRU engine performance standards on the following schedule:

  • MY 2001 and older engines must meet LETRU standards by 31 December 2008, and ULETRU standards by 31 December 2015.
  • MY 2002 engines must meet LETRU standards by 31 December 2009, and ULETRU standards by 31 December 2016.
  • MY 2003 and later engines must meet ULETRU standards by 31 December of the seventh year past the unit’s model year (i.e., MY 2003 engines must meet ULETRU standards by 31 December 2010, MY 2004 by 31 December 2011, and so on).

As an alternative to meeting the in-use performance standards, an owner/operator may operate a TRU meeting one of several Alternative Technology options (e.g., fueled by alternative fuels).

Cargo Handling Equipment

In December 2005, CARB adopted a rule to reduce emissions from mobile cargo handling equipment—including cranes, forklifts, tractors, and yard trucks—that operates at ports and intermodal rail yards [CCR 13, 2479]. Effective 1 January 2007, the regulation calls for the replacement or retrofit of existing engines with ones that use Best Available Control Technology (BACT), to reach 2007/2010 on-road or Tier 4 nonroad emission standards.

Commercial Harbor Craft Regulation

In November 2007, CARB approved a regulation to reduce diesel PM and NOx emissions from diesel fueled engines on commercial harbor craft (CHC) vessels [CCR 13, 2299.5][CCR 17, 93118.5]. The regulation includes requirements for both new and in-use diesel engines used on CHC operating in regulated California waters. The regulation applies to all CHC vessels such as ferries, excursion vessels, tugboats, towboats, crew and supply vessels, work boats, pilot vessels, research vessels, emergency response vessels, barges, and commercial and charter fishing boats. Mandatory upgrades to in-use engines were implemented from 2009 through 2022.

In-use requirements for ferries, excursion vessels, tugboats, and towboats include meeting US EPA Tier 2 or Tier 3 marine engine emission standards over a regulatory compliance schedule. Tier 4 is required for newly built vessels once standards come into effect. Best available control technology is required for all propulsion engines on newly built ferries.

Truck and Bus Regulation

In 2008, CARB adopted a comprehensive regulation requiring the reduction of NOx and PM emissions from in-use on-road heavy diesel trucks [5374]. The regulation, referred to as the Truck and Bus Regulation, affected more than 170,000 fleets and nearly 1 million trucks.

The regulation applied to most public and private fleets in the state, with the exception of transit buses that were subject to a separate rule. Reflecting a lack of funding, replacement of school buses was limited to 1977 models and older.

The regulation, phased in from 2012 through 2023, had two sets of requirements:

  • DPF Retrofit—Older and heavier trucks (MY 1996-2006, GVWR>26,000 lbs) were to be retrofitted with diesel particulate filters over a schedule from 2012 through 2014.
  • 2010 Compliant Engines—Nearly all trucks (MY 2009 and older, GVWR>14,000 lbs) were to be replaced with newer trucks or engines that met the 2010 new diesel engine standards (NOx = 0.2 g/bhp-hr; PM = 0.01 g/bhp-hr) over a schedule from 2015 through 2023.

Off-Road Diesel Equipment

In 2007, CARB adopted a program requiring fleets using off-road diesel equipment with more than 25 hp to retrofit engines with particulate filters or replace them with newer, lower-emitting engines [5374]. In 2010, following the 2008 recession, CARB provided a new compliance option that avoided the use of retrofit particulate filters, and delayed the start of program implementation until 2014. The in-use, off-road program is to be fully implemented by 2024 for large fleets and by 2029 for small fleets.

The revised program affects more than 150,000 pieces of diesel equipment, ranging from lawn-care tractors to road graders. It does not affect agriculture equipment, marine vessels, locomotives, or recreational vehicles. Requirements vary by fleet size. Large fleets began implementation in 2014, medium- size fleets in 2017, and small fleets in 2019. The smallest fleets have to retire only their oldest equipment (Tier 0 and 1).

Fleets must meet horsepower-weighted emission targets that become more stringent each year. If a fleet cannot meet an emissions target, it must meet a Best Available Control Technology (BACT) requirement that involves turning over about 10% of the fleet weighted by horsepower each year. Enforcement is based on regular mandatory reporting by fleets on equipment and steps taken to comply.

Marine Engines

Low Sulfur Marine Fuels

In 2005, CARB adopted a rule limiting emissions from auxiliary engines used on oceangoing ships, such as cargo ships, cruise liners and other large vessels [CCR 13, 2299.1]. The rule applied to auxiliary diesel engines and diesel-electric engines operated on oceangoing vessels located within California waters, i.e., less than 24 nautical miles from the coast. The regulation required that emissions of PM, NOx or SOx not exceed the emission rates that would result if these engines were operated on the following fuels:

  • MGO as per ISO 8217-2005 or MDO with 0.5% sulfur max. effective 1 January 2007,
  • MGO with 0.1% sulfur max. effective 1 January 2010.

Marine gas oil (MGO) is defined as DMX or DMA grades and marine diesel oil (MDO) as DMB grades as specified by ISO 8217-2005. Since ISO 8217-2005 allowed up to 1.00% sulfur in DMX and 1.50% sulfur in DMA, marine gas oil with up to 1.50% sulfur would have been allowed under the 2007 requirements.

In 2009, CARB adopted fuel sulfur limits not only for auxiliary diesel and diesel-electric engines but also for main propulsion diesel engines and auxiliary boilers on ocean-going vessels within California waters [CCR 13, 2299.2]. By adopting fuel sulfur limits rather than the emission limits specified by the earlier rule, CARB effectively prevented the use of scrubbers to meet the requirements. The definitions for MDO and MGO were expanded: MDO also includes DMB grades defined by ISO 8217-2010 and MGO also includes DMX, DMA or DMZ grades defined by ISO 8217-2010. The fuel sulfur limits are:

  • MGO with 1.5% sulfur max. or MDO with 0.5% sulfur max. effective 28 June 2009 for auxiliary engines and 1 July 2009 for main engines and auxiliary boilers.
  • MGO with 1.0% sulfur max. or MDO with 0.5% sulfur max. effective 1 August 2012,
  • MGO or MDO both with 0.1% sulfur max. effective 1 January 2014.

At-Berth Regulation

In 2007, CARB adopted a regulation to reduce emissions of auxiliary engines that provide power while vessels are at berth. The regulation applies to container and refrigerated vessels that dock at California ports 25 times or more per year and cruise ships that make five or more visits. These vessels must turn off auxiliary engines and plug in to the grid to provide power during 80% of their visits. In 2020, CARB revised the regulation requiring at-berth engine shutdown to include automobile carriers and tankers, and to require the use of shore power for every visit [5374].