EPA 2027 Truck Emissions Rules: NOx Limits, Warranties and DEF Derates
The 2027 NOx standard still stands, but EPA has proposed to keep today's warranty periods and drop DEF power derates, and it repealed the truck greenhouse gas rules in 2026.
EPA’s 2027 heavy-duty engine rule limits new diesel truck engines from model year 2027 to 35 milligrams of NOx per horsepower-hour on the main test cycles, down from 200 mg. As of November 2026 that limit stands, but a July 2026 EPA proposal would keep current warranty periods and replace DEF power derates with warnings.
What does the EPA 2027 NOx standard require?
The standard sets lower limits for NOx, particulate matter, hydrocarbons and carbon monoxide for heavy-duty highway engines starting with model year 2027. EPA adopted it in a final rule published January 24, 2023, and the limits are now in 40 CFR 1036.104. For compression-ignition (diesel) engines:
| Test cycle | NOx (mg/hp-hr) | PM (mg/hp-hr) | HC (mg/hp-hr) | CO (g/hp-hr) |
|---|---|---|---|---|
| FTP and SET (transient and steady-state) | 35 | 5 | 60 | 6.0 |
| LLC (low-load cycle) | 50 | 5 | 140 | 6.0 |
The low-load cycle is new. It targets the idling and slow, stop-and-go driving where today’s selective catalytic reduction (SCR) systems run cold and clean up less NOx, which matters for drayage and urban delivery trucks. The 2023 rule also lengthened useful life periods, meaning engines have to meet the limits for more miles, and lengthened the emission-related warranty that manufacturers must pay for.
What did EPA propose to change in 2026?
On July 14, 2026, EPA published a proposed rule titled “Amendments and Nonconformance Penalties for Model Year 2027 and Later Heavy-Duty Highway Engines and Amendments to Inducement Provisions for SCR-Equipped Diesel Engines.” It does not change the 35 mg NOx limit. According to the proposal, EPA would:
- keep the model year 2026 emission-related warranty periods for 2027 and later engines instead of the longer periods adopted in 2023;
- delay the longer useful life periods until model year 2030;
- offer nonconformance penalties for medium and heavy heavy-duty diesel engines from model year 2027, so a maker whose engine family is not ready can still sell it by paying a per-engine penalty;
- replace engine power derates linked to DEF problems with visible or audible warnings on newly built highway and nonroad diesel engines.
Comments closed August 29, 2026. As of early November 2026 no final rule had been published in the Federal Register, so the 2023 provisions remain the law until EPA finalizes changes. The proposal is in the Federal Register notice of July 14, 2026.
Will DEF derates go away on trucks already on the road?
Not automatically. The proposed rule covers newly manufactured engines. For engines already in service, EPA said it is considering new guidance that could let manufacturers get approval to modify inducement software without breaking the federal rules against tampering and defeat devices. In August 2025 EPA said it would assess whether DEF-related derates were still necessary, after complaints from drivers, farmers and several states about trucks losing power when the DEF system failed.
Until a manufacturer releases an approved update for a specific engine, deleting or bypassing the SCR system or its sensors is still illegal tampering. A DEF fault that derates a truck is also a breakdown issue for the driver’s hours of service, since repair delays eat into the 14-hour window.
What happened to the greenhouse gas Phase 3 rules for trucks?
They were repealed. In a final rule published February 18, 2026 and effective April 20, 2026, EPA rescinded its 2009 endangerment finding and repealed all greenhouse gas emission standards for light-, medium- and heavy-duty vehicles and engines, including the heavy-duty Phase 3 standards adopted in 2024. Phase 3 had set tighter CO2 limits for new trucks starting with model year 2027.
The repeal is being challenged in the U.S. Court of Appeals for the D.C. Circuit by health and environmental groups and by a coalition of states. If the court overturns it, earlier standards could come back into force, so the GHG picture is not settled. The NOx standard is a separate criteria-pollutant rule and was not part of the repeal.
What does this mean for new truck prices and orders?
EPA’s 2026 proposal says the longer warranty periods were the largest single driver of the per-truck cost increase it projected in 2023, which is why keeping the 2026 warranties is the main cost-cutting step. EPA also said most engine makers told it they will have a full lineup of 2027-compliant engines, while a few diesel engine families were at risk of not being ready on time.
For buyers, three points follow from the official record:
- Model year 2027 diesel trucks will carry new emissions hardware to meet the 35 mg NOx limit, whatever happens to the warranty rules.
- Prices for 2027 trucks depend partly on whether the warranty changes are finalized before production, which was still open in November 2026.
- Buyers comparing a late 2026 truck with a 2027 model should get the emissions warranty terms in writing for each, since they may differ depending on the final rule.
California sets some of its own truck emissions rules on top of the federal ones. Battery-electric trucks, which have no tailpipe emissions, are covered in electric semi trucks.
Bottom line
The NOx limit is the piece of the 2027 rule that has held through every change in Washington, and it is what will change the engines. Warranty length and derates are still in rulemaking, so read the spec sheet and warranty terms of any 2027 truck rather than relying on headlines from 2025 or early 2026. Watch the Federal Register for EPA’s final rule.
FAQ
Is the EPA 2027 truck emissions rule cancelled?
No. As of November 2026 the 35 mg/hp-hr NOx standard for model year 2027 still applies. EPA has proposed changes to warranty, useful life and DEF inducements, not to the NOx limit itself.
Will my 2027 truck still derate for low DEF?
Under the 2023 rule, yes. EPA’s July 2026 proposal would replace those derates with warnings on new engines, but it was not final as of early November 2026.
Do greenhouse gas standards still apply to new trucks?
EPA repealed them effective April 20, 2026. The repeal is under legal challenge in the D.C. Circuit, so that could change.
Can I legally delete my DEF system now?
No. Removing or disabling emissions controls is still tampering under the Clean Air Act. Only manufacturer updates that EPA approves can change how inducements work on existing engines.
