If you have ever watched a truck’s dash light up over a diesel exhaust fluid fault that turned out to be a $400 sensor lying to you, the EPA’s latest proposal is aimed squarely at your blood pressure.
On July 14, the agency published a proposed rule at 91 FR 43154 that would gut the derate. Specifically, EPA wants to replace engine performance derates and vehicle speed reductions used as SCR inducements with visible and audible notifications instead. It covers newly manufactured diesel highway engines and vehicles, light-duty and medium-duty included, plus nonroad engines and equipment. The agency’s own fact sheet calls the 2023-era requirements “unnecessary and unworkable.”
Read that carefully, because the word doing the heavy lifting is “newly.”
This does not un-derate your truck
Nothing in the proposal touches the engine already sitting in your yard. EPA says plainly on its DEF landing page that by law it cannot mandate field fixes for in-use vehicles and nonroad equipment, which is why every action aimed at the existing fleet has come as voluntary guidance rather than regulation. The proposal only floats the possibility of new guidance letting manufacturers push the same change into engines already in service.
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So what does an in-use truck actually get today? EPA’s August 2025 guidance letter spells out an acceptable inducement schedule manufacturers may adopt. For heavy-duty truck and tractor engines, the first step arrives at 650 miles or 10 hours after fault confirmation as a 15 percent ramped torque derate. At 4,200 miles or 80 hours it becomes 30 percent. Final inducement waits until 8,400 miles or 160 hours, and it’s a 25 mph speed limit rather than the 5 mph crawl that made this a national grievance in the first place. Motor coach engines get a gentler curve ending at 50 mph. Chassis-certified vehicles skip torque derates entirely and go straight to a 45 mph cap at 4,200 miles.
Nonroad is different again: a 25 percent torque derate at 36 hours, then at 100 hours either an idle-only condition or a 50 percent derate. Gensets and constant-speed engines like agricultural pumps are exempt from the initial step. There’s also an override that restores full power for 30 minutes, usable three times per incident, with the full half hour preserved across shutdowns. If you farm, that override is the single most useful sentence in the document.
None of this is automatic. Your engine builder has to write the software and you have to go get it.
Why a healthy truck derates anyway
Here’s the part almost nobody explains, and it’s the actual root of the problem.
EPA treats DEF quality as an “adjustable parameter.” That’s regulatory language for something an operator can physically change in a way that moves emissions, and the rules require a manufacturer to install some restraint that keeps it from being adjusted out of compliance. The derate is that restraint. It was never primarily a diagnostic tool. It was the stop on the dial.
To police the dial, the truck needs to know what’s in the tank. DEF is meant to be 32.5 percent urea by weight per ISO 22241-1, and most manufacturers police it with a Urea Quality Sensor. In March 2026, EPA issued guidance on alternate sensors noting that the Truck & Engine Manufacturers Association has flagged the UQS as having among the highest failure rates of any SCR component, and that EPA’s own review of manufacturer warranty data points the same direction. The sensor meant to prevent cheating has become a leading cause of punishing people who weren’t.
The alternative is inferring quality from a NOx sensor, watching conversion efficiency across the SCR catalyst and flagging deviation from the modeled rate. Only one manufacturer stuck with that approach after 2016. EPA has now made clear that route is still legal and that pushing such software into existing products isn’t tampering.
The physics is why it isn’t a clean swap. NOx-based detection requires DEF dosing to be happening, which generally means exhaust temperature above roughly 190°C. Idle a truck in a yard on a cold morning and there’s nothing to measure. Meanwhile a UQS can misread when the tank is full of bubbles, and neither technology can read frozen fluid at all. EPA also acknowledges that a determination may reasonably take longer than an hour and that a system shouldn’t declare a fault just because the clock ran out.
That last line matters. A lot of phantom derates were manufacturers being conservative with a timer, not sensors detecting real dilution.
The money, and the catch nobody’s flagging
The DEF headline is riding along with a bigger deregulatory package. EPA proposes rolling MY2027 and later emissions warranty periods back to the MY2026 values, delaying the longer regulatory useful life to MY2030, and making nonconformance penalties available to medium and heavy heavy-duty diesel engine makers who won’t have compliant products ready in January 2027. The agency estimates up to $6,000 saved per new truck and roughly $12 billion in net savings, while still keeping about 90 percent of the projected NOx reductions from the 2023 rule.
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Buyers should think about that warranty rollback for a second longer than the press release invites. Shorter federal emissions warranty on a MY2027 truck means the second owner is buying more aftertreatment risk, and MY2027 hardware is new hardware. If you shop three-to-five-year-old trucks, the coverage you inherit in 2031 will be materially thinner than what the 2023 rule promised. Price accordingly, or budget for an extended warranty the way fleets already do.
There’s also an honest counterargument that deserves airtime. If nothing eventually slows the truck down, the incentive to top off the tank with actual DEF rests entirely on the driver caring about a chime. Some industry groups have argued the recent guidance and better diagnostics should be allowed to work before the derate is deleted outright. EPA’s answer is that the notification does the job with a fraction of the collateral damage. Whether that holds up in the real world is exactly what a final rule has to defend.
Comments closed August 29 under docket EPA-HQ-OAR-2026-0728, with well over 13,000 filed. None of this is binding yet.
The practical move if you own a post-2010 diesel: call your dealer and ask specifically whether an updated inducement calibration is available for your engine serial number, and ask separately whether a NOx-sensor-based quality strategy has been released for it. Those are two different updates. Nobody is going to mail them to you.
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