EPA Administrator Lee Zeldin called "derating", the software-imposed restrictions of sudden speed and power losses due to low diesel exhaust fluid levels, “unacceptable” and “unnecessary.”

“We have heard loud and clear from small businesses across the United States that the current DEF system is unacceptable. It is unacceptable that farmers, truckers, construction workers, and many other small businesses continually experience failures of diesel-powered equipment when they need it most—costing millions of dollars in lost productivity,” Zeldin insisted. “Today, we are responding to those concerns by calling on manufacturers to take action to update their software and eliminate the unnecessary sudden loss of power and frustrating shutdowns that too many Americans have experienced.”

Even though DEF-related “limp mode” was never mandated by the federal government, companies use the method to prevent people from running their vehicles and equipment without the solution, because of the restrictions put in place by past versions of the EPA.

New EPA requirements taking effect in model-year 2027 state that new diesel trucks for road use cannot be engineered to force sudden and severe power loss after running out of DEF.

https://www.thedrive.com/news/epa-tells-truck-makers-to-stop-forcing-limp-mode-for-low-def