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

The public comment period on the federal proposal that has come to be seen as the “DEF rule” has come to a close, with almost 200 submissions spanning the gamut of opinions on possible changes in the Environmental Protection Agency (EPA) regulation.

The proposal, published in the Federal Register by EPA July 14 after earlier signaling of its intentions, had a relatively short window for comment, just 45 days. That ended last week though some comments continued to be posted on the comment’s website through Tuesday.

It’s come to be known as the DEF (Diesel Exhaust Fluid) rule because among other goals, the EPA proposal would seek to eliminate or significantly change one aspect of the use of DEF that has come in for criticism: the “deratement” of an engine, where it slows the vehicle if sensors pick up that DEF supplies are inadequate or are of poor quality.

There have been three EPA actions in a little more than a year that are attempt to tackle the issue of deratements and other issues that users of DEF have criticized: the August 2025 guidance, the March 2026 guidance and the more formal proposal in the Federal Register that was the basis for the recent comments.

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.”