The EPA’s heavy-duty engine emissions standards set federal limits on what new diesel and gasoline engines used in freight trucks, buses, and work vehicles are allowed to emit, and they require manufacturers to prove compliance before a single engine can be sold. The authority comes from 42 U.S.C. § 7521, which directs the agency to set emission standards for every class of new motor vehicle engine.1Office of the Law Revision Counsel. 42 USC 7521 – Emission Standards for New Motor Vehicles or New Motor Vehicle Engines No engine may enter commerce without a Certificate of Conformity, and the obligations do not end at the factory gate: standards apply for the engine’s full regulatory useful life, which for a heavy heavy-duty diesel can reach 800,000 miles.
Current Pollutant Limits
Four pollutants carry the strictest limits: nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), and non-methane hydrocarbons (NMHC). For diesel engines, the limits in force since model year 2007 are 0.20 g/bhp-hr for NOx, 0.01 g/bhp-hr for PM, and 15.5 g/bhp-hr for CO.2Environmental Protection Agency. Control of Air Pollution From New Motor Vehicles – Heavy-Duty Engine and Vehicle Standards The unit — grams per brake horsepower-hour — measures pollutant output against the work the engine actually performs, so a bigger engine doing more work gets proportionally more room to emit.
Diesel and gasoline engines follow separate testing and emission-limit tracks because their combustion chemistry produces different pollutant profiles. Vocational vehicles (refuse trucks, mixers, local delivery) and tractors (long-haul semis) are treated as distinct regulatory groups within the heavy-duty universe.
The 2027 NOx Overhaul
The biggest change in nearly two decades takes effect with model year 2027. The EPA’s final Clean Trucks Rule drops the diesel NOx limit to 0.035 g/bhp-hr on the standard Federal Test Procedure and Supplemental Emission Test cycles, roughly an 82 percent cut from the 0.20 g/bhp-hr standard that has held since 2007.3Environmental Protection Agency. Control of Air Pollution From New Motor Vehicles – Heavy-Duty Engine and Vehicle Standards The rule also introduces a Low Load Cycle with its own 0.050 g/bhp-hr NOx limit, aimed at the slow stop-and-go driving where older engines produced disproportionate emissions. A moving-average window test caps real-world NOx at 0.058 g/bhp-hr.
The practical effect is a redesign of aftertreatment systems so they perform in low-temperature, low-speed conditions that historically let NOx slip past. Manufacturers preparing 2027 engines are reworking selective catalytic reduction (SCR) chemistry, heat management, and dosing strategies to hold the tighter numbers across the engine’s useful life, not just on a certification dyno.
Greenhouse Gas Standards
Running parallel to the criteria-pollutant limits is a separate carbon dioxide and fuel-efficiency program. Phase 2 greenhouse gas rules apply to model years through 2027 and cover both vocational vehicles and tractors.4Environmental Protection Agency. Final Rule – Phase 2 Greenhouse Gas Emissions Standards Phase 3 begins phasing in with model year 2027 and tightens each year through model year 2032.5Environmental Protection Agency. Final Rule – Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles Phase 3
At full implementation, Phase 3 CO₂ limits for vocational vehicles are 30 to 60 percent tighter than Phase 2 depending on weight class, and tractor standards are 25 to 40 percent more stringent. Phase 3 also builds in expectations for zero-emission vehicle technology to enter the fleet alongside internal combustion engines.
How an Engine Gets Certified
Before selling any engine, the manufacturer assembles a technical file proving the design meets every applicable standard. Emission testing runs on multiple duty cycles: the Federal Test Procedure simulates transient on-road driving, and the Supplemental Emission Test is a steady-state ramped-modal cycle covering a matrix of speed and load points.6eCFR. 40 CFR 1036.510 – Supplemental Emission Test Beginning with model year 2027, the Low Load Cycle adds a third required test.
Durability data has to accompany the emission numbers. Manufacturers run accelerated aging protocols on aftertreatment components like SCR systems and diesel particulate filters to show the engine will hold its limits across its useful life. The application also documents combustion hardware, fuel delivery, SCR dosing strategy, and thermal management.
Products are grouped into engine families sharing combustion characteristics, and each family carries an alphanumeric code. One Application for Certification covers one engine family per model year and includes projected sales volumes and manufacturing plant locations. Filing happens through the Engines and Vehicles Compliance Information System (EV-CIS), the EPA’s centralized digital platform.7Environmental Protection Agency. How to Register for Engines and Vehicles Compliance Information System (EV-CIS)
Each application includes a fee. For calendar year 2026, a federal certificate for a heavy-duty highway engine costs $66,296, adjusted annually based on the Consumer Price Index and the volume of certificates the EPA processes.8Environmental Protection Agency. Fees Information for the Motor Vehicle and Engine Compliance Program9eCFR. 40 CFR Part 1027 – Fees for Vehicle and Engine Compliance Programs California-only certificates carry a far smaller fee. Once the EPA approves the data, it issues the Certificate of Conformity.
Labeling
Every certified engine must carry a permanent emission control information label showing the manufacturer’s name, EPA engine family designation, primary intended service class, displacement, date of manufacture, any applicable NOx family emission limit, and the statement: “THIS ENGINE COMPLIES WITH U.S. EPA REGULATIONS FOR [MODEL YEAR] HEAVY-DUTY HIGHWAY ENGINES.”10eCFR. 40 CFR 1036.135 – Labeling A unique identification number is permanently stamped on the block.
Obligations After Certification
Warranty
Manufacturers must warrant that each engine’s emission control system is free of defects that would push it over the standards. For model year 2026, the minimum warranty periods are 50,000 miles or 5 years for spark-ignition and light heavy-duty engines, and 100,000 miles or 5 years for medium and heavy heavy-duty engines, whichever comes first in each case.11eCFR. 40 CFR 1036.120 – Emission-Related Warranty Requirements Covered failures inside the window get repaired at no cost to the owner.
On-Board Diagnostics
Engines above 14,000 pounds GVWR must carry on-board diagnostic systems that monitor emission control components continuously and illuminate a malfunction indicator light when something fails. For model year 2013 and later diesel engines, the light triggers if NOx aftertreatment degrades enough to push NOx more than 0.3 g/bhp-hr above the standard, or if the particulate filter allows PM to reach 0.05 g/bhp-hr or the standard plus 0.04 g/bhp-hr, whichever is higher.12eCFR. 40 CFR 86.010-18 – On-Board Diagnostics for Engines Used in Applications Greater Than 14,000 Pounds GVWR The same systems feed the EPA data when it runs in-use compliance testing.
Maintenance Instructions
Every new engine ships with written instructions explaining how to keep the emission control system working. Federal rules set floors on how often manufacturers can schedule critical emission-related maintenance. For a heavy heavy-duty diesel engine, catalyst and particulate filter substrates cannot be scheduled for replacement more often than every 650,000 miles, and fuel injectors, turbochargers, electronic control modules, and EGR components cannot be scheduled more often than every 435,000 miles.13eCFR. 40 CFR 1036.125 – Maintenance Instructions and Allowable Maintenance The instructions must also state that any repair shop of the owner’s choosing may service emission control components; manufacturers cannot require brand-name parts or condition the warranty on franchised-dealer service unless the service is free.
Recordkeeping
All certification-related records must be kept for at least eight years after the associated application is submitted, or eight years after generating data that does not support an application.14eCFR. 40 CFR 1036.825 – Reporting and Recordkeeping Requirements Format is up to the manufacturer, but the EPA can demand organized, English-language copies at any time. Written communications with other companies as part of certification count as required records under the same retention rule.
Fleet-Wide Flexibility: Averaging, Banking, and Trading
The Averaging, Banking, and Trading (ABT) program lets manufacturers meet standards across a product line rather than engine by engine. Averaging allows engines that fall slightly above a standard to be offset by engines that beat it, provided the fleet-wide average for each engine class stays within limits. Banking lets a manufacturer save surplus credits from a strong model year for later use. Trading opens a market so credits can move between manufacturers.
Guardrails apply. Credits cannot transfer between unrelated engine subclasses, so credits earned on light engines cannot cover shortfalls on heavy ones, and banked credits expire five years after the model year that generated them.15eCFR. 49 CFR 535.7 – Averaging, Banking, and Trading (ABT) Credit Program Annual reports document credit balances and any trades.
Imported Engines
Foreign-manufactured engines get no exemption. Any heavy-duty engine imported into the United States must meet the same EPA standards as one built domestically, and the importer files EPA Declaration Form 3520-21 with Customs and Border Protection at entry.16eCFR. 19 CFR 12.73 – Importation of Motor Vehicles and Motor Vehicle Engines Engines not already covered by a U.S. Certificate of Conformity must go through an Independent Commercial Importer, a certified entity that modifies and tests them into compliance. Engines that cannot be brought into compliance are denied admission and cannot be sold to consumers in any form.
Penalties and Tampering
Under 42 U.S.C. § 7524, selling an uncertified engine or violating the Clean Air Act’s prohibited-acts provisions carries a civil penalty of up to $25,000 per engine at the statutory baseline.17Office of the Law Revision Counsel. 42 USC 7524 – Civil Penalties With inflation adjustments, that figure reached $57,617 per engine as of the EPA’s most recent update.18Environmental Protection Agency. Amendments to the EPA Civil Penalty Policies to Account for Inflation Each non-conforming engine is a separate violation, so a production run of thousands of units can generate nine-figure exposure quickly. Information-gathering violations carry a separate penalty of up to $57,617 per day.
The EPA also runs in-use testing on engines already in service. If a family exceeds its certified limits in the real world, the agency can order a recall or stop-sale, with the manufacturer paying the full cost of repair or replacement.
Tampering with emission controls is flatly prohibited under 42 U.S.C. § 7522, both before and after the engine reaches the end buyer. It is equally illegal to manufacture, sell, or install any part whose principal effect is to bypass or defeat an emission control device.19Office of the Law Revision Counsel. 42 USC 7522 – Prohibited Acts That language covers aftermarket delete kits and tuner software that strip out DPF and SCR systems. The only exceptions are temporary removal for legitimate repairs, with the component reinstalled afterward, and conversions to approved clean alternative fuels that still meet emission standards.
Liability runs down the whole chain. Manufacturers and dealers face the same per-engine civil penalties above. Repair shops, aftermarket parts sellers, and vehicle owners face penalties of up to $5,761 per engine or per component after inflation adjustment.20Environmental Protection Agency. EPA Fact Sheet – Aftermarket Defeat Devices and Tampering Settlements against defeat-device sellers have run into the tens and hundreds of millions of dollars, and the EPA continues to treat this area as an enforcement priority.