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Emission Standards

USA: Cars and Light-Duty Trucks: Tier 4


Tier 4 criteria pollutant emission standards for light-duty vehicles were finalized on March 20, 2024 as part of the Multi Pollutant Emissions Standards for Model Years 2027 and Later Light-Duty and Medium-Duty Vehicles—a regulation that also includes GHG emission standards [6166]. The standards are phased-in from 2027 through 2033.

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The Tier 4 regulation is applicable to passenger cars, SUVs, pickup trucks, and vans—including chassis-certified large pickups and vans—in the following vehicle categories:

  • Light-Duty Vehicles
    • Light-Duty Vehicles (LDV), GVWR ≤ 8,500 lb
    • Light-Duty Trucks (LDT)
      • Light-Duty Trucks 1-2 (LDT 1-2), GVWR ≤ 6,000 lb
      • Light-duty Trucks 3-4, (LDT 3-4), GVWR 6,001-8,500 lb
    • Medium-Duty Passenger Vehicles (MDPV), GVWR 8,501-10,000 lb
  • Medium-Duty Vehicles (MDV)
    • Class 2b, GVWR 8,501-10,000 lb
    • Class 3, GVWR 10,001-14,000 lb

Class 2b and Class 3 vehicles were also regulated at the Tier 3 stage, but were referred to as “heavy-duty vehicles” (HDV). The new term “medium-duty vehicles” is consistent with California vehicle classifications.

The structure of Tier 4 standards is similar to the Tier 3 & Tier 2 standards—manufacturers must certify vehicles to one of several “certification bins” and must meet fleet-average NMOG+NOx emission standards in a given model year. Some key changes from the Tier 3 level include:

  • The fleet average NMOG+NOx emissions from light-duty vehicles must reach 15 mg/mi by 2032, a 50% reduction from the Tier 3 limit of 30 mg/mi. Even higher percentage reductions apply to medium-duty vehicles.
  • A PM standard of 0.5 mg/mi to be met across three test cycles, including a cold temperature (-7°C) test, is expected to force the use of gasoline particulate filters on vehicles with internal combustion engines.
  • The same emission standards (with some exceptions) must be met over the FTP cycle and other driving cycles (US06, SC03, HFET)—the prior Supplemental Emission Standards applicable to the US06/SC03 tests have been eliminated.
  • The Tier 4 standards transition back to emission caps, as opposed to bin-based standards, for all pollutants except NMOG+NOx.
  • Tier 4 standards apply nearly uniformly for both light-duty and medium-duty vehicles, in contrast to Tier 3 standards where many requirements applied differently for light- and medium-duty vehicles.

All vehicles must be certified to the Tier 4 emission standards by 2030/2031 for LDVs/MDVs, respectively, Table 1. An optional incentivized early phase-in schedule is available for heavier vehicle categories (LDT3-4, MDPV, MDV).

Table 1
Tier 4 emission standards phase-in schedules, percentage of manufacturers’ fleet
Model YearLight-Duty Vehicles
Light-Duty Trucks 1-2
Light-Duty Trucks 3-4
Medium-Duty Passenger Vehicles
Medium-Duty Vehicles
DefaultEarly OptionDefaultEarly Option

Even though several provisions of the Tier 4 program are aligned with the California LEV IV standards, the two sets of standards differ in many important respects. One example difference is the final LEV IV fleet-average NMOG+NOx emission limit that remains at the LEV III/Tier 3 level of 30 mg/mi, while the California standards limit the manufacturers’ ability to include zero emission vehicles (ZEV) in their fleet average NMOG+NOx calculation.

NMOG+NOx Emission Standards

Certification Bins

Manufacturers must certify their vehicles to one of the NMOG+NOx emission bins shown in Table 2. The higher emission bins are available to MDVs only. The same standards are applicable to all vehicles, regardless of fuel type.

Table 2
Tier 4 NMOG+NOx certification bins
BinNMOG+NOx, mg/mi
MDVs Only
Bin 170170
Bin 150150
Bin 125125
Bin 100100
Bin 8585
Bin 7575
All Vehicle Categories
Bin 7070
Bin 6565
Bin 6060
Bin 5555
Bin 5050
Bin 4545
Bin 4040
Bin 3535
Bin 3030
Bin 2525
Bin 2020
Bin 1515
Bin 1010
Bin 55
Bin 00

The above EPA bins are to some degree harmonized with California LEV IV emission categories.

Fleet Average NMOG+NOx Standards

Tier 4 standards include a fleet average NMOG+NOx limit that must be met by each manufacturer in each model year. The fleet average NMOG+NOx limit is phased-in starting from 2027, and reaches 15 mg/mi in 2032 for light-duty vehicles and 75 mg/mi in 2033 for MDVs, Table 3.

Table 3
Tier 4 NMOG+NOx fleet-average emissions standards, mg/mi
Model YearLight-Duty VehiclesMedium-Duty Vehicles
Class 2bClass 3
* Tier 3 standards

Battery electric vehicles (BEV) and other ZEVs are included in the NMOG+NOx fleet-average emissions calculation. If BEVs do not comply with the battery durability requirements specified in the regulation, manufacturers forfeit the related credits for both GHG and NMOG+NOx emissions.

NMOG+NOx Emission Testing

The above NMOG+NOx standards must be met over four test cycles: ambient temperature (25°C) FTP, HFET, US06, and SC03.

The Tier 4 standards include three provisions aligned with the CARB ACC II (LEV IV) program that are intended to address light-duty vehicle NMOG+NOx emissions from vehicle operating conditions not previously captured in EPA test procedures: (1) high power cold starts in plug-in hybrid electric vehicles, (2) early drive-away, and (3) mid-temperature engine starts. The standards are also aligned with the CARB ACC II provisions that address high load emissions from medium-duty vehicles with high gross combination weight rating (GCWR).

PM Emission Standards

The Tier 4 regulation introduces a PM emission standard of 0.5 mg/mi for both light- and medium-duty vehicles. The standard must be met across three test cycles, including -7°C FTP, 25°C FTP, and US06.

The final PM standard phases in with the general Tier 4 phase-in schedule, Table 1 (i.e., it will be fully phased in by MY 2030 for light-duty vehicles and by MY 2031 for medium-duty vehicles).

The EPA designed the 0.5 g/mi PM standard and its testing requirements, including the cold temperature test, to force the use of gasoline particulate filters (GPF) on gasoline vehicles.

CO and HCHO Emission Standards

The Tier 4 standards include the following emission limits for carbon monoxide and formaldehyde.

Table 4
Tier 4 emission standards for CO and HCHO, mg/mi
PollutantTestLight-Duty VehiclesMedium-Duty Vehicles
CO25°C FTP, HFET, SC031.73.2
CO-7°C FTP10.010.0

The 1.7 g/mi CO standard for the 25°C FTP is less stringent than some and more stringent than other Tier 3 bins.