EPA Proposes Eliminating DEF Limp Mode and Rolling Back Emissions Warranty Requirements, Projects $12 Billion in Trucking Savings

Since the EPA's first deregulatory action targeting DEF system failures in August 2025, the Trump administration has moved in a series of steps to dismantle the enforcement and compliance architecture built around diesel emissions equipment. The July 9 proposal is the most sweeping step yet.

What the Proposal Actually Does

The EPA's announcement lays out three distinct changes. First, it would eliminate deratements and vehicle speed restrictions entirely for newly manufactured highway engines, vehicles, and nonroad equipment, including farm equipment. Instead of a truck losing power or seizing up when a DEF sensor fails, operators would receive visible or audible alerts and be allowed to keep driving until they can safely address the problem, according to the EPA's own published summary.

Second, the agency proposes scaling back the emissions warranty requirements introduced in the 2023 Biden-era rule, which the EPA describes as the largest single cost driver for the trucking industry. The underlying NOx emissions standard stays in place. According to the EPA, the revised approach would still preserve nearly 90% of the nitrogen oxide reductions the 2023 rule was designed to achieve.

Third, the EPA would extend manufacturing lead time for NOx compliance applying to Model Year 2027 and later heavy-duty highway engines and would create nonconformance penalties for diesel engines that temporarily cannot meet the new standards, rather than halting truck sales outright.

The total projected savings: $12 billion, or roughly $6,000 per new truck, according to the agency's own estimate.

The DEF Problem, Explained

Diesel Exhaust Fluid is injected into the exhaust stream to break down nitrogen oxide pollution. The problem is that the sensors monitoring the system fail regularly. When they do, under current rules, the truck automatically derate—slowing or stopping—regardless of whether the sensor reading is accurate.

CDLLife reported that the July 9 proposal also opens a related public comment period on whether manufacturers should be allowed to apply the same deratement elimination to existing in-use engines, not just new production. That would extend the relief to the current fleet.

Zeldin announced the proposal at the Great American State Fair. "When they fail, the engine does something called a derate. In plain English, the truck or tractor slows down or stops running altogether," he said, as quoted by the Daily Signal. "So, we protect the air and we protect your pocketbook," he added, describing the warranty rollback.

Small Business Administration Administrator Kelly Loeffler tied the announcement to a broader interagency effort. "The EPA, SBA, and [the U.S. Department of Agriculture] have made steady progress toward ending DEF deratements, removing unreliable sensor requirements, and offering operators more practical compliance options," she said, according to the Daily Signal.

The Broader Deregulatory Context

This proposal did not arrive in isolation. The Autopian documented the full timeline: in August 2025, Zeldin directed manufacturers to stop forcing trucks into immediate limp modes for low DEF levels. In February 2026, the EPA said it was seeking to hold manufacturers accountable for unreliable DEF systems. In March 2026, the EPA dropped the requirement for DEF urea quality sensors on all diesel-powered equipment. In January 2026, the Justice Department announced it would no longer pursue criminal charges under the Clean Air Act for tampering with emissions devices. The Autopian also reported that the administration has fully pardoned nine diesel tuners previously convicted of emissions equipment tampering.

The Strongest Counterargument

Environmental groups and public health advocates argue that NOx emissions from heavy-duty diesel trucks contribute to ground-level ozone and particulate pollution, which are linked to respiratory illness, disproportionately in communities near freight corridors. Their argument is that the 2023 warranty requirements existed precisely because manufacturers had a history of deploying emissions equipment that worked in testing but degraded in real-world use. Removing the warranty backstop, they contend, creates an incentive to ship trucks with emissions systems that meet standards on paper but fail in practice, with no binding obligation to fix them.

The EPA's answer is that the underlying NOx standard stays intact and that 90% of the nitrogen oxide reductions survive the warranty rollback. Whether that 90% figure holds in real-world fleet operation over years—rather than in new-vehicle certification testing—is a question the proposal does not fully resolve. The comment period is where that fight will play out.

What Happens Next

The proposal is not final. It is currently open for public comment, and according to the EPA's announcement, implementation is targeted for 2027 if finalized. The nonconformance penalty structure for Model Year 2027 engines, which would allow trucks to be sold even if they temporarily cannot meet the new NOx standard, is a specific element likely to draw legal challenges from environmental organizations that successfully litigated against the first Trump administration's emissions rollbacks. Whether courts treat the warranty revision as a procedural rule change or a substantive weakening of the Clean Air Act's enforcement mechanism will determine how much of the $12 billion estimate actually reaches truckers.