Every driver who has watched a truck crawl to 5 mph on the shoulder because a sensor did not like the diesel exhaust fluid has an opinion about this rule, and by the time the docket closed on Saturday, August 29, a lot of them had filed it. The Federal Register's counter for docket EPA-HQ-OAR-2026-0728 showed 22,988 comments received by Friday; regulations.gov had posted 795 of them for public view, with hundreds more appearing this week. The proposal, published July 14 as 91 FR 43154, does three big things to the 2027 heavy-duty engine program, and truckers are for one of them and against the other two. It ends the engine derate, the "limp mode" that has stranded trucks over bad, low or empty DEF since 2010. It cancels the longer emissions warranty that 2027 trucks were supposed to get. And it delays, by three model years, the point at which an engine has to keep meeting its standards for 650,000 miles instead of 435,000.
What the EPA is actually proposing
Start with the part everyone likes. Since the agency's 2013 guidance, an engine that detects poor-quality DEF, a low tank or a tampered system has been required to "induce" the operator to fix it by cutting performance, ending in a speed cap of 5 mph after four hours under the old guidance or 25 mph after 30 to 164 hours under the 2023 rule. The July proposal replaces the derate for newly built engines with "visible and/or audible notifications": a dash warning, then an alarm that sounds on a schedule as the DEF level falls and keeps sounding after the tank runs dry. The preamble asks whether each alarm should last more or less than 90 seconds and how often it should repeat. The truck keeps running. The EPA cites seat-belt research showing that long, repeated audible warnings change behavior about as well as a speed interlock does, without the interlock. The agency had already loosened the derate twice through guidance to manufacturers, in August 2025, when it gave in-use trucks more time before the final 25 mph cap, and in March 2026, when it let makers use NOx sensors instead of the failure-prone urea quality sensor. The rule makes the change permanent.
The two changes nobody in a cab asked for
Emissions warranty: the 2023 rule raised the required emission-related warranty on 2027 and later heavy heavy-duty engines from 5 years or 100,000 miles to 10 years or 450,000 miles. The proposal keeps it at 5 years or 100,000 miles. Useful life: the period over which an engine must keep meeting its standards was to rise from 435,000 to 650,000 miles for 2027 engines. The proposal keeps 435,000 miles through model year 2029 and moves the 650,000-mile requirement to model year 2030. Both changes reduce what a manufacturer has to stand behind; the EPA says they give makers "more time to evaluate their new technologies in the real world."
The fourth piece is the one with the least driver attention and the most money in it: nonconformance penalties. Under the Clean Air Act, when a standard gets harder to meet and some manufacturers cannot get there in time, the EPA can let them sell non-compliant engines if they pay a per-engine penalty. The proposal makes that option available for medium and heavy heavy-duty engines starting in model year 2027, the first year of the tighter nitrogen-oxide standard. In plain terms, an engine builder that is not ready for 2027 can keep shipping its current engine and write a check, rather than stop production.
What OOIDA told the agency
The Owner-Operator Independent Drivers Association filed on both sides. On the derate it is unequivocal: an audible warning, it wrote, "would allow drivers to properly address DEF problems in a safe manner without having to drastically reduce speeds, miss deliveries or be sidelined for days/weeks at a time." On the warranty it is just as blunt. A 100,000-mile warranty, the association wrote, "may be suitable for fleets with hundreds or thousands of trucks that are constantly purchasing new vehicles, but does not provide the necessary protection for small-business truckers that are making a long-term investment in their equipment." Big fleets, it added, have the staff and the systems to chase warranty claims; a one-truck owner has a phone and a dealer's service desk. OOIDA called the shorter warranty "simply a cost-cutting measure" at the expense of small carriers, and said the useful-life delay turns drivers into test pilots: "OEMs can sit back and see if their engines perform while truckers will be left on the side of the road if and when the engines fail during this 'real world' testing phase."
“ Any emissions rule should ensure that drivers and carriers who are purchasing new equipment are getting a fair deal and will not be constantly sidelined from their profession due to costly and repeated breakdowns. ”
Environmental groups took the opposite line on the same provisions. California Environmental Voters told the agency that weakening useful life, warranty, inducement and penalty provisions "undermines the EPA's statutory responsibility to protect public health." The 2027 NOx standard depends on the SCR system working over the engine's whole life, and DEF is what makes SCR work; a truck that keeps running with an empty tank emits like a truck with no aftertreatment at all. Expect that argument in court if the final rule keeps the derate repeal.
Why the numbers in the press do not match
Trade coverage this week quoted everything from "almost 200" comments to "more than 13,500." Both were read off a moving counter. Regulations.gov posts submissions for public view in batches after the deadline, so the visible count was about 125 in late July, 795 by Friday and still climbing. The Federal Register's tally of comments received, which includes form letters and campaign submissions before they are consolidated, stood at 22,988. The right way to read it: this is a docket with a few hundred substantive filings from carriers, OEMs, states and advocacy groups, and tens of thousands of drivers who took the time to say what limp mode has cost them.
What happens next, and how fast
The EPA now has to read the docket, write a final rule, and send it to the White House Office of Management and Budget for review before it can be published. Nothing in the proposal commits the agency to a date. The problem is the calendar: the 2027 standards the rule amends apply to model year 2027 engines, and the 2023 rule's requirements start applying to them late this year, when 2027 production begins. If the final rule is not out by then, engine makers will be certifying 2027 products against the warranty and useful-life numbers the proposal is trying to repeal, and the derate provisions of the 2023 rule will stay in force for new engines until it is. For trucks already on the road nothing changes either way: the August 2025 and March 2026 guidance letters cover in-use engines, and the proposal only says the EPA is "considering" further in-use guidance.
What this means if you are buying a 2027 truck
Do not assume the 10-year, 450,000-mile emissions warranty. It exists in the 2023 rule, but if this proposal is finalized, a 2027 engine carries the same 5-year, 100,000-mile emissions warranty as a 2026. Price the extended warranty from the dealer accordingly.
Limp mode is not gone yet. The repeal applies to newly manufactured engines once the rule is final. A 2026 truck keeps its derate logic; relief for in-use engines comes only through manufacturer software updates under the 2025 and 2026 guidance, so ask your dealer whether yours has been reflashed.
Watch for nonconformance-penalty engines. If a manufacturer uses NCPs for 2027, its engine is legally sold but does not meet the 2027 NOx standard, and the penalty is baked into the price. Ask which standard the engine is certified to before you sign.
The useful-life delay matters for resale. A 2027 to 2029 engine is certified to 435,000 miles of compliance; a 2030 engine to 650,000. Buyers of five-year-old trucks in 2032 will know the difference.
The DEF rule so far
2013 — EPA guidance formalizes the derate that arrived with SCR engines in 2010: warning, then performance cuts ending at 5 mph within four hours of a DEF fault. Urea quality sensors become standard equipment.
January 2023 — Final heavy-duty NOx rule codifies inducements (final speed 25 mph after 30 to 164 hours), extends warranty to 10 years/450,000 miles and useful life to 650,000 miles for model year 2027.
August 11, 2025 — EPA guidance to manufacturers gives in-use trucks more time before the final 25 mph derate. Administrator says the agency will reassess whether derates are needed at all.
March 26, 2026 — Guidance lets manufacturers use NOx sensors in place of urea quality sensors, the source of most false derates.
July 14, 2026 — Proposed rule published at 91 FR 43154: derate replaced by notifications, warranty rolled back, useful life delayed to 2030, nonconformance penalties for 2027. Virtual hearings July 29 and 30.
August 29, 2026 — Comment period closes. 22,988 comments received per the Federal Register; 795 posted by September 4.
Next — Final rule drafted, OMB review, publication. No date announced; 2027 engines are already in production planning.
Buying before or after the rule? Know what you are paying for.
Qrylo lists trucks by model year and engine with real sellers and real prices, so you can compare a 2026 with a derate against a 2027 without one.