Comments close on EPA’s DEF rule; which way will it go?

Listening and seeing signals

The key Federal Register proposal on deratements–which falls under the category of “inducements”–is to replace them with “audible and visual signals.” Those signals would alert the user of the DEF-mandated equipment–which is more than just truck engines–of depleting supplies or a fully empty DEF tank. It would replace a gradual slowdown of an engine.

The EPA already had loosened some of the earlier deratement requirements in guidance released to engine manufacturers in August 2025. That was followed by more guidance released in March. 

“Guidance” was defined in the March letter as “not (having) the force and effect of law” and seek to “provide clarity to the public regarding existing requirements under the law or Agency policies.”

The August 2025 guidance widened the use of acceptable inducements. The March 2026 guidance opened the door to more use of NOx sensors rather than the reportedly troublesome Urea Quality Sensors (UQS). But both documents saw the continued use of deratements.  

Those guidance letters to OEMs did not contain a reference to the “audible and visual signals” as a substitute for deratements in the EPA proposal. 

Many of the other specific proposals in the Federal Register proposal are highly technical, including changes to various aspects of the selective catalytic reduction system (SCR) that uses DEF as a key component.

Warranties and useful life

There also are proposals to shorten the required emission-related warranty periods that were part of the earlier rule creating the tighter nitrogen oxide (NOx) standards beginning in model year 2027. The proposal also calls for delaying implementation of the definition of “useful life” for engines to begin in model year 2030 rather than 2027. 

Another proposal that is less technical is the introduction of nonconformance penalties (NCPs) that an engine builder could pay, allowing them to “continue to sell their current products while they complete the development of engines compliant with the model year 2027 standards.” 

Those 2027 model year standards call for a significant reduction in NOx emissions. DEF usage is a key component of reduced NOx emissions, which is why many of the comments from environmental groups submitted to the EPA criticized the proposal.

“Weakening provisions related to useful life requirements, emissions warranties, SCR inducement systems, and nonconformance penalties undermines the EPA’s statutory responsibility to protect public health,” a group called California Environmental Voters said in its comment submitted to the EPA. 

Not surprisingly, the most complex comments come from manufacturers of the equipment that would be impacted by the changes.

DEF does its job

DEF, working inside an SCR system, is seen as highly successful in reducing NOx emissions. That success also allows engines to run “hotter” and still operate within the boundaries of NOx emission limits, which provides greater fuel efficiency, a fact pointed out by the Truck and Engine Manufacturers Association (EMA) in its comments.

EMA, whose members include such companies as Cummins (NYSE: CMI), Daimler Truck North America (OTCPK: DTRUY) and PACCAR (NASDAQ: PCAR) , said SCR “is highly effective at reducing NOx, enabling manufacturers to optimize combustion to increase engine performance and reduce fuel consumption.”

“EMA appreciates that EPA is mitigating the derate impacts on truck and equipment operators in a way that can still ensure that operators maintain the onboard DEF supply necessary for engine efficiency and long-term engine component health,” the association said in its comments. “We applaud EPA for responding to commercial vehicle and equipment operators who have complained about the real-world impacts of the previous overly-stringing SCR inducement requirements.”

August 2025 guidance looks good

But it was lukewarm on the latest proposal. “Regarding the applicability of additional guidance for in-use engines and vehicles, we believe that the existing August Guidance is sufficient,” the EMA said. 

It also expressed concern that some of the changes might be problematic for users of SCR systems that aren’t part of a truck.

Relative standardization of truck technology contrasts with “literally hundreds of nonroad equipment manufacturers producing thousands of different types of nonroad machinery,” the EMA says in its comments.

Many of those applications are targeted at the agricultural sector, which has been one of the loudest voices criticizing deratements. 

But the EMA says there needs to be “global harmonization” with other countries that non-truck equipment manufacturers are supplying. 

“EPA’s regulatory requirements for SCR-related inducements harmonize and align as much as possible with the corollary inducement-related requirements” in other parts of the world, stressing that alignment with the EU is “critical.”

The solution is in the guidance provided by the first EPA proposal a year ago, the EMA said. 

An existing rule that required nonroad engines to move to an “idle condition” after a tank is empty or four hours after an “SCR fault” should be scrapped in favor of the EPA guidance that would allow up to 100 hours of operating equipment before deratement would begin, “and even then with the derate capped at 50%.”

Given that the guidance on nonroad engines is more than a year old, the EMA warned against “changing course…in a way that abandons the guidance going forward.”

The Engine Technology Forum (ETF), which was previously known as the Diesel Technology Forum, said in its comments that it estimates about half the off-road engine and equipment sector uses SCR/DEF technology.

One concern with the rule that has been discussed is that DEF users will use the reduced stringency to operate as much as possible without DEF. 

Lots of bad info out there

The ETF addressed that indirectly by saying it has needed to respond to media reports “seeking to verify casual comments from government officials suggesting that ‘we are going to get rid of DEF,'” ETF said in its comments. “This statement has created a false expectation.”

In this video, President Trump says he “terminated” the DEF program (see 4 minutes in), with the video producer’s “research” leading him to suggest that the end of DEF is possible.   

DEF consumers stretching or reducing their use of DEF under the less stringent deratement rules have caught the eye of a trio of trade groups whose members sell DEF: the National Association of Truck Stop Owners, the Society of Independent Gasoline Marketers of America and the National Association of Convenience Stores. 

The three groups, in a joint comment to the EPA, conceded there are legitimate issues of concern.

Current deratement requirements “have proven overly punitive and often counter-productive,” the groups said. “Improperly triggered derates – frequently the product of sensor malfunctions – have imposed real costs on drivers and, understandably, eroded goodwill toward SCR systems generally.”

But the trio does not want the deratement rule eliminated. 

In its comments, the associations praised the guidance of August 2025 and March 2026. “Those measures are directed at the underlying sources of SCR-related challenges, and they should be afforded the requisite time to take effect,” the trio wrote. “Engine manufacturers have only recently begun implementing the guidance across the existing fleet. The full measure of their benefits will not be observable until that process is complete.”

But they also said they “generally oppose any revisions of the inducement requirements that would ultimately undermine the comprehensive, nationwide system supplying DEF.”

The associations “support the concerns of manufacturers as they continue to build in strategies to protect their engines against damage, including by using derates,” they added.

They also expressed a fear that the message of possible changes is becoming distorted, which could ultimately affect supply.

“Whichever inducement framework the Agency ultimately adopts will only function properly if drivers understand that DEF remains required as a matter of law and necessary to protect their equipment,” they wrote. 

Regulations won’t be enough, they added. It will also depend on “the communications and signals that surround it. The (EPA) should take care to ensure that sensible reforms to the derate schedule are not misperceived by diesel engine users as repealing DEF requirements or encouraging removal of SCR systems.”

Buses are interested

The list of commenters has a notably high number of bus companies, suggesting a coordinated effort by members of that industry to let the EPA know that deratement has been a particularly large issue for motor coaches.

Trans-Bridge Lines, a motor coach company that operates in the Northeast, said derates “can create significant safety and operational issues that can ultimately work against that goal.”

The company, in its comment to the EPA, recalled an incident when one of its buses “went into derate and was unable to continue operating normally, resulting in a significant traffic backup.” To add insult to injury, the incident occurred just outside the Lincoln Tunnel that connects New Jersey to Manhattan.

The end result was a massive traffic jam filled with cars sitting with idling engines. “In other words, a system intended to reduce emissions ultimately contributed to a situation that increased emissions while also creating a safety hazard,” Trans-Bridge said in its comment.

The relative brevity of the comment period drew criticism from some environmental groups. Atid Kimelman, an attorney with the  Natural Resources Defense Council, quoting a legal precedent, said a 45-day comment period is “is…inadequate because the proposed rule is complex and based on scientific or technical data.”

“Commenters will be effectively denied their legally guaranteed opportunity to fully respond to EPA’s health-harming Proposal and may be unfairly hampered in their ability to preserve potential legal claims in the rulemaking record as the Clean Air Act requires,” Kimelman said in the NRDC’s comments.

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