The public comment period has closed on the federal proposal widely referred to as the “DEF rule,” giving the Environmental Protection Agency a broad range of views on potential changes to its diesel exhaust fluid requirements.
Nearly 200 submissions were filed during the comment period, covering everything from strong support for regulatory changes to concerns that the proposal could weaken protections intended to reduce emissions.
The EPA published the proposal in the Federal Register on July 14, following earlier indications that the agency was preparing to revisit the issue. The formal comment period lasted just 45 days and officially ended last week, although additional submissions continued appearing on the federal comment website through Tuesday.
The proposal has become known as the DEF, or Diesel Exhaust Fluid, rule because one of its central objectives is to address a long-standing source of frustration among diesel equipment operators: engine “deratement.”
Under current requirements, an engine can progressively reduce its power when sensors determine that DEF levels are too low or that the fluid does not meet required quality standards.
The latest proposal is the third EPA action in just over a year aimed at addressing deratements and other problems raised by DEF users. It follows guidance issued in August 2025, additional guidance released in March 2026 and, most recently, the more formal proposal published in the Federal Register.
Listening and seeing signals
The most significant proposed change concerning deratements — classified by the EPA as “inducements” — would replace them with audible and visual warnings.
Those signals would notify operators when DEF supplies are running low or when a tank is completely empty. Rather than progressively reducing engine performance, the system would alert the equipment user to the condition.
The proposed change applies to DEF-equipped equipment beyond truck engines as well.
The EPA had already relaxed some of its earlier deratement requirements in guidance sent to engine manufacturers in August 2025. That was followed by another guidance document in March 2026.
The March letter specifically described guidance as not having “the force and effect of law,” while explaining that it was intended to provide clarity regarding existing legal requirements and agency policies.
The August 2025 guidance expanded the range of acceptable inducements. The March 2026 guidance subsequently allowed greater use of NOx sensors as an alternative to the Urea Quality Sensors, or UQS, that have reportedly created problems in the field.
Neither guidance document, however, proposed replacing deratements with the audible and visual warnings now included in the Federal Register proposal.
Other elements of the proposal are considerably more technical, including changes to various components of selective catalytic reduction, or SCR, systems in which DEF plays a central role.
Warranties and useful life
The EPA is also proposing shorter emission-related warranty periods. Those warranties were established under the earlier regulatory framework that introduced tighter nitrogen oxide, or NOx, standards beginning with model year 2027.
Another proposal would delay implementation of the “useful life” definition for engines. Rather than beginning with model year 2027, the requirement would be pushed back to model year 2030.
The agency is also proposing a less technical but potentially significant change involving nonconformance penalties, or NCPs.
Under the proposal, an engine manufacturer could pay an NCP and continue selling its current products while completing development of engines capable of meeting the 2027 standards.
Those standards are designed to deliver a substantial reduction in NOx emissions. Because DEF is a critical component of the SCR systems used to achieve those reductions, environmental organizations have been among the strongest critics of the proposed changes.
California Environmental Voters argued in its submission that weakening useful-life requirements, emissions warranties, SCR inducement systems and nonconformance penalties would undermine the EPA’s responsibility to protect public health.
Manufacturers of equipment affected by the proposal, unsurprisingly, submitted some of the most detailed and technically complex comments.
DEF does its job
DEF remains widely regarded as highly effective at reducing NOx emissions when used as part of an SCR system.
Its effectiveness also allows engines to operate at higher temperatures while remaining within NOx emission limits. According to the Truck and Engine Manufacturers Association, that can improve fuel efficiency.
The association’s membership includes Cummins (NYSE: CMI), Daimler Truck North America (OTCPK: DTRUY) and PACCAR (NASDAQ: PCAR).
EMA said SCR technology is “highly effective at reducing NOx,” allowing manufacturers to optimize combustion in ways that can improve engine performance while reducing fuel consumption.
The association also welcomed the EPA’s efforts to address the real-world effects of deratements while maintaining the DEF supply needed for engine efficiency and long-term component health.
EMA said the agency was responding to complaints from commercial vehicle and equipment operators who had experienced the consequences of what it described as overly stringent SCR inducement requirements.
August 2025 guidance looks good
Despite supporting the direction of the earlier guidance, EMA was considerably less enthusiastic about the EPA’s latest proposal.
“Regarding the applicability of additional guidance for in-use engines and vehicles, we believe that the existing August Guidance is sufficient,” the association said.
EMA also warned that some of the proposed changes could create difficulties for operators of SCR-equipped equipment outside the trucking sector.
Truck technology is relatively standardized compared with the off-road equipment market, the association noted. That sector includes “literally hundreds of nonroad equipment manufacturers producing thousands of different types of nonroad machinery.”
Agricultural equipment represents a significant portion of those applications and has been among the sectors most vocal in opposing deratements.
EMA nevertheless argued that any regulatory framework should account for the international nature of the nonroad equipment market.
The association called for “global harmonization” and said EPA requirements should align as closely as possible with corresponding rules in other countries. It specifically emphasized that alignment with the European Union is “critical.”
For EMA, the solution lies largely in the guidance issued by the EPA a year earlier.
An existing requirement says nonroad engines must enter an “idle condition” after a DEF tank is empty or four hours have passed following an SCR fault. EMA wants that provision removed in favor of the August 2025 guidance, which would allow equipment to operate for up to 100 hours before deratement begins, with the eventual derate capped at 50%.
Because that guidance is already more than a year old, EMA cautioned the EPA against “changing course…in a way that abandons the guidance going forward.”
The Engine Technology Forum, previously known as the Diesel Technology Forum, also weighed in. ETF estimates that SCR/DEF technology is used by roughly half of the off-road engine and equipment sector.
One concern surrounding the proposal is that less stringent deratement requirements could encourage DEF users to reduce or avoid DEF consumption wherever possible.
Lots of bad info out there
ETF addressed that concern indirectly by pointing to what it described as inaccurate public messaging about the future of DEF.
The organization said it has had to respond to media reports attempting to verify casual remarks from government officials suggesting that “we are going to get rid of DEF.”
According to ETF, that message has created “a false expectation.”
In a video, President Trump says he “terminated” the DEF program, around the four-minute mark. The video’s producer subsequently used what it described as research to suggest that DEF requirements could ultimately disappear.
The possibility of reduced DEF use under less stringent deratement requirements has also attracted the attention of three trade organizations representing businesses that sell DEF: the National Association of Truck Stop Owners, the Society of Independent Gasoline Marketers of America and the National Association of Convenience Stores.
In a joint submission to the EPA, the groups acknowledged that operators have legitimate complaints about the existing system.
They said current deratement requirements “have proven overly punitive and often counter-productive,” adding that improperly triggered derates, frequently caused by sensor malfunctions, have imposed real costs on drivers and damaged confidence in SCR technology.
But the organizations do not support eliminating deratements altogether.
Instead, they praised the August 2025 and March 2026 guidance and argued that the measures should be given time to work.
“Those measures are directed at the underlying sources of SCR-related challenges, and they should be afforded the requisite time to take effect,” the groups wrote.
They noted that manufacturers have only recently begun implementing the guidance throughout the existing vehicle and equipment fleet. As a result, the full benefits have yet to become apparent.
At the same time, the three associations said they “generally oppose any revisions of the inducement requirements that would ultimately undermine the comprehensive, nationwide system supplying DEF.”
They also said they support manufacturers’ efforts to protect engines from damage, including through the continued use of derates when necessary.
The groups expressed another concern: that public messaging surrounding the proposed changes could become distorted enough to affect DEF 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.
The associations argued that regulation alone would not be enough.
The effectiveness of any new framework, they said, would also depend on “the communications and signals that surround it.” They urged the EPA to make sure that reforms to the deratement schedule are not misunderstood by diesel users as eliminating DEF requirements or encouraging operators to remove SCR systems.
Buses are interested
The list of organizations submitting comments includes a notably large number of bus companies, suggesting that the motor coach industry has made a coordinated effort to highlight the operational problems created by deratements.
Trans-Bridge Lines, a motor coach operator serving the Northeast, told the EPA that derates “can create significant safety and operational issues that can ultimately work against that goal.”
The company cited one incident involving a bus that entered derate mode and could no longer operate normally.
The breakdown occurred just outside the Lincoln Tunnel, which connects New Jersey with Manhattan, and resulted in a significant traffic backup.
The consequences extended beyond the affected bus.
Vehicles were left sitting in traffic with their engines idling, creating additional emissions. Trans-Bridge argued that the incident illustrated a fundamental contradiction in the current system.
“In other words, a system intended to reduce emissions ultimately contributed to a situation that increased emissions while also creating a safety hazard,” the company said in its comment.
Environmental groups challenge the timeline
The EPA’s relatively short 45-day comment period also came under fire from environmental organizations.
Atid Kimelman, an attorney with the Natural Resources Defense Council, cited legal precedent in arguing that the period was inadequate given the complexity of the proposal and the scientific and technical information involved.
Kimelman said the limited timeframe could prevent commenters from fully responding to what the NRDC described as a health-harming proposal.
He also argued that commenters could be unfairly restricted in their ability to preserve potential legal claims within the rulemaking record, as required under the Clean Air Act.
With the comment period now closed, the EPA faces competing demands from manufacturers, truck and equipment operators, DEF suppliers, bus companies and environmental organizations.
At the center of the debate is a difficult balance: reducing the operational disruptions caused by deratements without weakening the DEF and SCR systems that have played a major role in cutting NOx emissions.
The next step will be for the EPA to weigh those competing positions and determine whether its final approach will retain the existing deratement framework, modify it further or move toward the proposed system of audible and visual warnings.
