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Key Takeaways
- The 2027 NOx emissions standard is not changing. EPA is proposing revisions to compliance requirements — not the emissions limits themselves.
- If finalized, the proposal could lower truck costs and improve uptime. Changes to warranty requirements, DEF derates and certification flexibility are intended to reduce costs while maintaining the 2027 emissions standard.
- The proposal is not yet final. EPA is accepting public comments through August 29, 2026, before issuing a final rule.
The U.S. Environmental Protection Agency (EPA) has proposed several significant revisions to the implementation of its Model Year (MY) 2027 heavy-duty NOx emissions rule originally released in 2023. While the new proposal would roll back or delay several compliance requirements introduced in the original rule, one thing remains clear: the 2027 NOx emissions standard itself is not changing.
If finalized, these proposed updates could reduce compliance costs for truck manufacturers, improve vehicle uptime and provide additional flexibility during the transition to the new emissions requirements. However, the proposal is not yet final and must first complete the federal rulemaking process, including a public comment period.
Here's what fleets and truck owners need to know.
What Is Not Changing: The 2027 NOx Emissions Standard
The biggest takeaway from EPA's proposal is that the 2027 heavy-duty NOx emissions standard itself is not changing.
Under the original MY2027 rule, new heavy-duty diesel engines were required to meet a NOx emissions limit of 0.035 grams per brake horsepower-hour (g/bhp-hr), a reduction of more than 80% compared to the previous federal standard. The rule also requires engines to meet those emissions standards over a wider range of real-world operating conditions — not just during laboratory testing.
Manufacturers will still be required to develop engines and aftertreatment systems capable of meeting the 2027 standard. However, the newly proposed updates focus on reducing the cost and complexity of complying with the rule through changes to warranty requirements, durability timelines, certification procedures and other implementation provisions.
In short, EPA is proposing changes to how manufacturers comply with the rule — not the emissions standard they must ultimately achieve.
What Is Changing Under EPA's NOx Proposal?
The proposal focuses on reducing regulatory burden while preserving the environmental benefits of the 2027 program.
1. Emissions Warranty Requirements Would Return to Current Levels
One of the largest proposed changes involves reverting emissions-related warranty requirements to the current MY2026 warranty periods rather than implementing the significantly longer warranty periods adopted under the original MY2027 rule.
According to EPA, the expanded warranty requirements represented the single largest compliance cost driver in the 2023 regulation — accounting for up to 50% of total program costs. EPA also cited manufacturer estimates showing that warranty costs for heavy-duty engines (HDE) could increase by more than $15,000 per engine for certain low-mileage vocational applications.
Under the new proposal, manufacturers would continue providing today's emissions warranty coverage instead of transitioning to the longer warranty periods originally scheduled for MY2027.
| Service Class | 2023 Rule Requirements (MY2027+) | Proposed Requirements (Today's) |
| Spark-Ignition HDE | 160,000 miles / 10 years / 8,000 hours | 50,000 miles / 5 years |
| Light HDE | 210,000 miles / 10 years / 10,000 hours | 50,000 miles / 5 years |
| Medium HDE | 280,000 miles / 10 years / 14,000 hours | 100,000 miles / 5 years |
| Heavy HDE | 450,000 miles / 10 years / 22,000 hours | 100,000 miles / 5 years |
EPA estimates these reduced warranty obligations could lower the cost of new diesel trucks by $4,000 to more than $6,000, depending on vehicle class. Below is a breakdown of potential savings:
| Regulatory Class | Diesel | Gasoline | Compressed Natural Gas |
| Light HDE | $6,152 | $2,783 | |
| Medium HDE | $4,130 | $2,838 | |
| Heavy HDE | $5,358 | $6,270 | |
| Urban Bus | $6,004 | $6,878 |
2. Nonconformance Penalties Would Become Available
EPA proposes allowing manufacturers of medium-heavy-duty and heavy-heavy-duty diesel engines to use Nonconformance Penalties (NCPs) beginning with MY2027.
What are Nonconformance Penalties (NCPs)? Authorized under the Clean Air Act, NCPs allow manufacturers to continue selling engines that narrowly miss a new emissions standard by paying an EPA-established penalty while continuing to work toward compliance.
Under this provision, manufacturers whose engines certify above the 0.035 g/bhp-hr NOx standard, but at or below 0.200 g/bhp-hr, could continue selling those engines by paying an EPA-established, per-engine penalty while continuing to develop fully compliant engines. The farther an engine's certified NOx emissions exceed the 0.035 g/bhp-hr standard, the larger the penalty becomes.
| Engine Compliance Level (g/bhp-hr) | Medium HDE Penalty | Heavy HDE Penalty |
| 0.045 | $851 | $904 |
| 0.100 | $3,799 | $5,875 |
| 0.200 | $4,330 | $6,781 |
EPA also proposes allowing certain existing engines that are carried over into MY2027 to use the NCP program. This provides manufacturers with additional flexibility during the transition and helps reduce the risk of engine shortages while new emissions-compliant engine platforms are introduced.
3. SCR Inducements Could Be Replaced with Driver Notifications
Perhaps the most noticeable change for fleets involves Selective Catalytic Reduction (SCR) emissions systems, which use Diesel Exhaust Fluid (DEF) to reduce NOx emissions.
Under the MY2027 rule, certain SCR system faults — such as low DEF, poor-quality DEF or system tampering — can trigger increasingly severe vehicle inducements, including:
- Engine power derates
- Vehicle speed limitations
- Restart restrictions
- Vehicle immobilization
What is a derate? A derate is a manufacturer-programmed reduction in engine performance designed to encourage corrective action when an emissions system fault is detected. Depending on the severity of the condition, a derate can reduce engine power, limit maximum vehicle speed, restrict engine restarts or, in some cases, prevent the vehicle from operating until the issue is addressed.
EPA now proposes replacing many of those mandatory derates with visible and audible driver notifications.
The agency says this change is intended to improve vehicle uptime while reducing roadside breakdowns, towing events and operational disruptions that fleets have associated with SCR-related derates.
4. Longer Useful-Life Requirements Would Be Delayed
The proposal would delay implementation of the extended useful-life requirements until MY2030.
Unlike a warranty period, useful life is a regulatory requirement that determines how long an engine must continue meeting federal emissions standards for certification purposes. Under the proposal, engines produced between 2027 and 2029 would remain subject to today's durability requirements before transitioning to the longer useful-life standards beginning in 2030.
| Primary Intended Service Class | Current Standard (MY2027-MY2029) | Proposed Extended Standard Beginning MY2030 |
| Spark-Ignition HDE | 110,000 miles / 10 years | 200,000 miles / 15 years |
| Light HDE | 110,000 miles / 10 years | 270,000 miles / 15 years |
| Medium HDE | 185,000 miles / 10 years | 350,000 miles / 12 years |
| Heavy HDE | 435,000 miles / 10 years / 22,000 hours | 650,000 miles / 11 years / 32,000 hours |
EPA says the additional lead time will allow manufacturers to further validate new emissions technologies under real-world operating conditions before the more stringent durability requirements take effect.
5. Additional Flexibility for Specialized Vocational Vehicles
The original MY2027 rule recognized that certain specialized vocational vehicles — such as refuse trucks, fire trucks, emergency vehicles, concrete mixers and heavy-haul tractors — often require additional engineering time to package larger aftertreatment systems.
To help with the transition, EPA allowed manufacturers to certify a limited percentage of these engines to the previous emissions standard. However, manufacturers could only use that flexibility by spending NOx emission credits earned under EPA's Averaging, Banking and Trading (ABT) program.
Several manufacturers told EPA they did not have enough credits to make meaningful use of the provision. As a result, EPA now proposes removing the NOx credit requirement while keeping the production limits and other safeguards in place, making the flexibility more accessible for manufacturers of specialized vocational vehicles.
6. Technical and Certification Requirements Would Be Simplified
The proposal also includes numerous technical revisions designed to simplify certification and testing requirements for manufacturers.
These updates primarily affect engineering and regulatory compliance processes by:
- Clarifying testing procedures
- Correcting regulatory language
- Removing inconsistencies
- Aligning certain heavy-duty certification procedures with existing light-duty processes
While these revisions primarily affect manufacturers' certification processes, they are intended to simplify implementation of the MY2027 program and reduce unnecessary administrative burden.
Remember: The Proposal Is Not Yet Final
It's important to remember that these changes are only proposed at this time.
The proposal must complete the federal rulemaking process before any changes take effect. EPA is accepting public comments from manufacturers, fleets, industry organizations and other stakeholders through August 29, 2026, after which the agency will review feedback before issuing a final rule.
What Does This Mean for Fleets and Truck Owners?
If the proposal is finalized largely as written, fleets could see several potential benefits:
- Lower purchase prices (up to $6,000) if manufacturers pass along reduced compliance costs.
- Improved vehicle uptime by replacing mandatory SRC-related derates with driver notifications.
- Greater engine availability through the addition of Nonconformance Penalties.
- More time for manufacturers to validate new emissions technologies before longer durability requirements take effect.
At the same time, fleets should recognize that shorter emissions warranty requirements could shift more emissions-related repair costs to owners later in a truck's life.
The proposal represents an effort to balance lower ownership costs, improved reliability and continued progress toward cleaner heavy-duty engines.
How Rush Truck Centers Can Help
As emissions regulations continue to evolve, understanding what changes apply and when can be challenging.
Rush Truck Centers works closely with leading truck and engine manufacturers to stay informed on regulatory developments and product availability. Whether you're planning future truck purchases, evaluating replacement timelines, comparing available engine options or preparing for upcoming emissions requirements, our team can help you understand how proposed and finalized regulations may impact your fleet.
If the EPA finalizes these proposed changes, Rush Truck Centers will continue providing updates and guidance to help you make informed equipment decisions with confidence.
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