Under the Biden administration's truck emissions rule a diesel truck's engine could automatically force itself into degraded performance mode — cutting speed to 5 miles per hour — if its Diesel Exhaust Fluid system detected a malfunction.
They called it "deratement." Everyone else called it limp mode.
On July 9, EPA Administrator Lee Zeldin stood at the Great American State Fair and announced the Trump administration's proposed rollback. "At the Great American State Fair, I just announced a proposed roll back of the Biden admin's overburdensome truck emission rules," Zeldin said. "This proposal, which includes a permanent and total elimination of Diesel Exhaust Fluid deratements, would save American truckers $12 billion."
The EPA's own announcement put it plainly: "The Trump EPA is proposing to fix the last administration's unworkable truck rule — saving truckers $12B, up to $6,000 per new truck." The proposal applies to model year 2027 engines and later. The 477-page proposed rule replaces the automatic engine shutdowns with warning lights and audible alerts — the same approach used in passenger vehicles. Your car doesn't force itself to 5 mph when an emissions sensor trips. The Biden EPA decided trucks should.
The data behind the rollback is substantial. Fourteen major truck and engine manufacturers — representing 80% of the market — provided EPA with real-world data on their Selective Catalytic Reduction and DEF systems. The information showed that the forced deratement requirements were, as the administration characterized them, unworkable. Trucks were being sidelined not because they were polluting but because sensors were malfunctioning, fluid levels were misread, or systems simply glitched during operation.
The Biden EPA's position was that forcing trucks into limp mode would ensure emissions compliance by making noncompliance physically impossible. Which sounds reasonable until you consider that a fully loaded semi doing 5 on an interstate is a rolling traffic hazard. The regulation treated every DEF system hiccup as a compliance violation requiring immediate engine intervention — no distinction between an actual emissions problem and a faulty sensor reading.
The Trump proposal preserves 90% of the planned NOx emission reductions from the original rule while eliminating the deratement provisions entirely. It also scales back emissions-warranty requirements and extends the timeline for manufacturers to test new emissions technology. The docket number is EPA-HQ-OAR-2026-0728 for anyone who wants to participate.
This is a $12 billion regulatory rollback that preserves the vast majority of the environmental gains while eliminating the provision that could force a truck to crawl at walking speed on a freeway.
Some regulations are about the environment. Some are about control. The name usually tells you which.