California: Diesel Risk Reduction Program

California: Diesel Risk Reduction Program


Program Name

Risk Reduction Plan to Reduce Particulate Matter Emissions from Diesel-Fueled Engines and Vehicles


To reduce diesel particulate matter (PM) emissions in California by 75% by 2010 and 85% by 2020


Following the identification of diesel PM as a toxic air contaminant (TAC) in 1998, CARB developed the Diesel Risk Reduction Plan as a comprehensive strategy to control diesel PM emissions. The overall goals of the plan are to reduce diesel PM emissions by 75% by 2010 and 85% by 2020. Within the plan, reductions in PM are achieved by a combination of approaches including emission regulations for new diesel engines, a low sulfur fuel program, and measures for various categories of in-use on- and off-road diesel engines. The in-use control strategies are generally based on the following types of controls:

  • Retrofitting engines with emission control systems, such as diesel particulate filters or oxidation catalysts,
  • Replacement of existing engines with new technology diesel engines or natural gas engines, and
  • Restrictions placed on the operation of existing equipment.

Upon adoption, CARB started developing PM emission regulations for a number of categories of in-use diesel vehicles and equipment. The regulations are mandatory, requiring diesel engine operators to apply certain control measures, but in most cases include flexibilities to allow a choice of approach.1 The general objective of the various regulations is to reduce diesel PM emissions to levels of below 0.01 g/bhp-hr, equivalent to the 2007 PM emission standard for new heavy-duty highway engines. Thus, all existing engines of PM emission levels higher than 0.01 g/bhp-hr are potential targets of the regulations.

Program Components

Diesel PM control regulations cover:

  • Fleets
  • Trucks and buses
  • Solid waste collection vehicles
  • Portable diesel engines,
  • Idling regulations & auxiliary power systems (APS)
  • Transport refrigeration units
  • Marine auxiliary engines, and
  • Cargo handling equipment

CARB also negotiated an agreement with airlines operating in Southern California to reduce diesel PM emissions from airport ground support equipment. CARB has also adopted rules limiting idling of diesel school buses and commercial vehicles.


Public Transit Bus Fleet Rule

The Public Transit Bus Fleet Rule was adopted in February 2000 and modified on several later occasions (CCR Title 13, Section 1956.2). The regulation applies to urban buses, defined as vehicles powered by heavy heavy-duty diesel engines (HHDDE), capable of carrying 15 or more passengers, and intended primarily for inter-city operation. Under the rule, California fleet operators have to choose between a “diesel path” and an “alternative fuel path” for their future urban bus procurements. The alternative fuel path requires that 85% of buses purchased or leased each year through MY 2015 are fueled by alternative fuels. Transit operators who stay on the diesel path can purchase diesel fueled buses, but are required to follow a more aggressive emission reduction schedule. When the regulation is fully implemented, buses on both paths will produce the same near-zero emission levels.

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

  • A NOx fleet average limit of 4.8 g/bhp-hr is effective from 2002.10 for both diesel and alternative fuel paths.
  • Ultra low sulfur diesel fuel (15 ppm wt.) is required from 2002.07.
  • The total PM emission from the fleet must be reduced by 85% relative to the emissions in January 2002. This requirement is phased-in by 2007 on the diesel path, and by 2009 on the alternative fuel path. The PM emission reduction may be achieved by introducing cleaner engines, or retrofitting existing engines with verified, Level 1, 2, or 3 emission control equipment.
  • New model year (MY) 2004-2006 diesel bus engines have 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 formaldehyde emission standards. (A requirement for 2007-2009 bus engines meeting 0.2 g/bhp-hr NOx limit has been relaxed, and engines certified to federal standards were allowed.)
  • Zero emission bus purchase quota of 15% applies for fleets of 200 or more buses from 2008 on the diesel path and from 2010 on the alternative fuel path, through MY 2015.

In 2006, amendments were finalized to expand the transit fleet rule to include smaller diesel and alternative fuel buses, commuter buses, and heavy-duty trucks owned or operated by transit agencies. The affected vehicles are called transit fleet vehicles (TFVs) and are subject to a fleet average NOx limit and PM reduction requirement, phased in between 2007 and 2010.

Fleet Rule for Public Agencies and Utilities

On December 8, 2005, the CARB approved a regulation to reduce diesel PM emissions from fleets operated by public agencies and utilities (CCR Title 13, Sections 2020, 2022, and 2022.1). This rule is one of CARB’s efforts to reduce both criteria pollutant emissions and exposure to toxic air contaminants. The rule mandates Public Agency and utility vehicle owners reduce diesel PM emissions from their affected vehicles through the application of Best Available Control Technology (BACT) on these vehicles by specified implementation dates. Implementation is phased-in by engine model year groups.

Truck and Bus

On December 12, 2008, the California Air Resources Board approved the Truck and Bus regulation to significantly reduce PM and NOx emissions from existing diesel vehicles operating in California. The regulation applies to nearly all diesel fueled trucks and buses GVWR greater than 14,000 pounds that are privately or federally owned, including on-road and off-road agricultural yard goats, and privately and publicly owned school buses. Other public fleets, solid waste collection trucks and transit buses are already subject to other regulations and are not part of the truck and bus regulation. Trucks that transport marine containers must comply with the drayage truck regulation.

  • January 1, 2012, the regulation phases in requirements for heavier trucks to reduce particulate matter emissions with exhaust retrofit filters that capture pollutants before they are emitted to the air or by replacing vehicles with newer vehicles that are originally equipped with PM filters.
  • January 1, 2015, the regulation requires accelerated replacements of both lighter and heavier vehicles that do not have PM filters installed.
  • From 2020 to 2023 nearly all older vehicles would need to be upgraded to have exhaust emissions meeting 2010 model year engine emissions levels.

In October 2011, CARB approved amendments to the Truck and Bus regulation. The amended regulation requires PM filters for heavier trucks starting January 1, 2012 and early vehicle replacements for vehicles with a gross vehicle weight rating greater than 14,000 pounds starting January 1, 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 Title 13, Section 2020- 2021). The regulation covers 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 starts in 2004 and continues through 2011. The regulation applies to garbage truck owners, as well as to municipalities that contract waste collection services.

The rule requires that the best available control technology is implemented according to a specified schedule. Three best available control technology options are available to truck owners:

  • To use a diesel engine, alone or in combination with a verified diesel emission control strategy, that is 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:

  • A control measure to limit school bus idling became effective in 2003 (CCR Title 13, Section 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.
  • Commercial diesel vehicles with a GVWR > 10,000 lbs are subject to idling restrictions effective February 1, 2005 (CCR Title 13, Section 2485). Affected vehicles are required to limit idling to no longer than 5 minutes under most circumstances.
  • A rule to further limit idling of new and in-use sleeper berth equipped diesel trucks was adopted on October 20, 2005 (still pending approval by the Office of Administrative Law). 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.

The 2008 regulation also includes 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 will be 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 will need to comply with the fuel-fired heater emissions requirements specified in the Low Emission Vehicle Program to operate in California.

Transport Refrigeration Units

The Transport Refrigeration Units (TRU) regulation was adopted in 2003 and became effective from 10 December 2004 (CCR Title 13, Section 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). TRU units must meet in-use performance standards, shown below, 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).
In-Use Performance Standards for TRU Engines
Emission Category Engine PM Certification VDECS Retrofit
TRU engines < 25 hp
Low emission TRU (LETRU) 0.30 g/bhp-hr Level 2
Ultra low emission TRU (ULETRU) n/a Level 3
TRU engines ≥ 25 hp
Low emission TRU (LETRU) 0.22 g/bhp-hr Level 2
Ultra low emission TRU (ULETRU) 0.02 g/bhp-hr Level 3

The engine certification levels correspond to the nonroad nonroad Tier 4 PM emission standards. Thus, the TRU regulation enforces Tier 4 PM emission levels on older in-use TRU engines, and ensures that in the long term particulate filters are also used on older engines below 25 hp (which will not use filters under 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).

Marine Auxiliary Engines

In December 2005, CARB adopted a fuel quality rule for auxiliary engines used on oceangoing ships, such as cargo ships, cruise liners and other large vessels (CCR Title 13, Section 2299.1). The rule applies to auxiliary diesel engines and diesel-electric engines operated on oceangoing vessels located within California waters, 24 nautical miles of the coast. The regulation requires the use of marine diesel fuels of reduced sulfur content:

  • 0.5% sulfur limit effective 1 January 2007,
  • 0.1% sulfur limit effective 1 January 2010.

In November 2007, CARB approved a regulation to reduce emissions from diesel engines on commercial harbor craft vessels. The Commercial Harbor Craft Regulation (CHC regulation) establishes emission standards, reporting, recordkeeping, fuel, and monitoring requirements for certain categories of marine vessels operated within California waters and 24 nautical miles of the California baseline. In May 2011, CARB approved amendments to CHC regulation to add in-use engine requirements for crew and supply vessels, and barges and dredges. The amendments also addressed implementation issues.

Drayage Trucks

In December 2007, CARB approved a new regulation to reduce emissions from drayage trucks. The Drayage Truck Regulation reduces emissions and public exposure to diesel particulate matter, NOx, and other air contaminants by setting emission standards for in-use, heavy-duty diesel-fueled vehicles that transport cargo to and from California’s ports and intermodal rail facilities.

Cargo Handling Equipment

In December 2005, CARB adopted a rule to reduce emissions from mobile cargo handling equipment—including cranes, fork lifts, tractors, and yard trucks—that operates at ports and intermodal rail yards (CCR Title 13, Section 2479). Effective 1 January 2007, the regulation called 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.

In-Use Off-Road Diesel Engines

First approved by CARB in July 2007, the In-Use Off-Road Diesel Vehicle Regulation applies to in-use off-road diesel engines greater than 25 hp used in construction, mining, airport ground support, logging, and industrial equipment such as forklifts. The rule will not cover equipment used in agricultural operations, cargo handling equipment used at ports and intermodal rail facilities, or equipment already covered by other rules. The rule was amended several times in 2009 and 2010.

The Off-Road regulation:

  • Imposes limits on idling, requires a written idling policy, and requires a disclosure when selling vehicles;
  • Requires all vehicles to be reported to CARB (using the Diesel Off-Road Online Reporting System, DOORS) and labeled;
  • Restricts the adding of older vehicles into fleets; and
  • Requires fleets to reduce their emissions by retiring, replacing, or repowering older engines, or installing Verified Diesel Emission Control Strategies (VDECS) (i.e., exhaust retrofits).

The requirements and compliance dates of the Off-Road regulation vary by fleet size. For a fleet to determine their size, it must add up all of the off-road horsepower under common ownership or control in the fleet.

Requirements and Compliance Dates of the Off-Road Regulation
Fleet Size Category Description
Small Fleet or municipality <= 2,500 hp, or
Municipality fleet in low population county, captive attainment area fleet, or
Non-profit training center, regardless of total hp
Medium Fleet with 2,501 to 5,000
Large Fleet with more than 5,000 hp, or
All state and federal government fleets, regardless of total hp

Control Equipment Verification Program

To support diesel retrofit programs under the Diesel Risk Reduction Plan, CARB has developed a verification program for Diesel Emission Control Strategies. The Verification Procedure (CCR Title 13, Sections 2701-2709) 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 Level 1, 2, or 3 strategy. Optionally, strategies can be also verified for their NOx reduction effect.

Verification Classification for Diesel Emission Control Strategies
Pollutant Reduction Classification
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:

  • 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.
  • 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 below.
  • 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 below.
Minimum Durability Demonstration Periods
Engine Type Durability Demonstration Period
On-road 50,000 miles or 1000 hours
Off-road (including portable) & stationary 1000 hours
Stationary emergency standby engines 500 hours
Minimum Warranty Periods
Engine Type Engine Size Warranty Period
On-road Light heavy-duty, 70 – 170 hp, GVWR < 19,500 lbs 5 years or 60,000 miles
Medium heavy-duty 170 – 250 hp, GVWR 19,500 – 33,000 lbs 5 years or 100,000 miles
Heavy heavy-duty, above 250 hp, GVWR > 33,000 lbs 5 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 & stationary P < 25 hp, and constant speed engines < 50 hp with rated speeds ≥ 3,000 rpm 3 years or 1,600 hours
25 hp ≤ P < 50 hp 4 years or 2,600 hours
P ≥ 50 hp 5 years or 4,200 hours

Show 1 footnote

  1. ARB regulations should not be confused with the rules by South Coast Air Quality Management District (SCAQMD), adopted in 2000 and later invalidated by US Supreme Court, which attempted to ban the purchase of diesel vehicles, regardless of their PM emission level.

Contact Us

Questions or updates about policies?