Diesel engine and equipment manufacturers including John Deere, AGCO, Cummins, Mack Trucks and Volvo Trucks are rolling out software changes across the United States that make diesel exhaust fluid faults far less likely to bring a machine or truck to an abrupt crawl or idle.
The changes follow revised guidance from the U.S. Environmental Protection Agency on selective catalytic reduction systems and DEF-related inducements. The objective is not to remove SCR or make DEF optional. Instead, manufacturers are being given considerably more flexibility in how quickly an engine must reduce power after a DEF or emissions-system problem is detected.
For farmers, contractors and truck fleets, that distinction is significant. A faulty sensor, contaminated DEF or an SCR-related diagnostic code can still require repair, but it no longer has to turn a repairable emissions issue into an immediate productivity problem.
DEF Derates Get Longer Repair Windows
SCR has been widely used on U.S. diesel engines since the introduction of increasingly stringent emissions standards around 2010. The system injects DEF, a urea and water solution, into the exhaust stream so nitrogen oxides can be converted into less harmful compounds.
The hardware itself is only part of the emissions-control strategy. Engine control software also monitors DEF level, quality and SCR performance. When the system detects certain failures, an escalating series of warnings and operating restrictions known as inducements can follow.
Under earlier strategies, some nonroad machines could progress from a warning to effectively idle-only operation in roughly four hours. On highway trucks, severe inducement could eventually restrict road speed to as little as 5 mph.
That created a problem that had little to do with whether SCR chemistry actually worked. A failed sensor or related fault could leave a mechanically sound tractor stranded during planting or harvest, or force a heavy truck to slow dramatically before it could safely reach a repair facility.
The newer software strategies are designed to preserve the incentive to repair the emissions system without making the penalty disproportionately disruptive.
John Deere Extends DEF Operation to 20 Hours
John Deere provides one of the clearest examples of what the new approach looks like on agricultural and off-highway equipment.
Previously, certain DEF and SCR faults could eventually restrict Deere machines to idle after approximately four hours. Under the company’s updated software strategy, operators receive approximately 20 hours before reaching the final operating restriction.
Even then, the machine is no longer necessarily limited to idle. Deere says approximately 30% of rated engine power can remain available. The company has also revised its DEF quality diagnostics to reduce unnecessary interruptions associated with system monitoring.
In agricultural terms, increasing the repair window from four to 20 hours is more important than it may initially sound.
Five times as much operating time can be the difference between abandoning a field operation in the middle of a working day and finishing that day’s planting, spraying or harvest before scheduling a repair.
Deere began releasing compatible software in November 2025 and expects nearly 80% of the updates within the program’s scope to be available by the end of 2026, with the remainder following during 2027.
AGCO Begins U.S. DEF Software Rollout
AGCO is taking a similar approach.
The company began incorporating the revised EPA guidance into engine software updates during the third quarter of 2026 for U.S. customers. AGCO says the software increases the time available before a derate occurs and makes the final restriction less severe, allowing machines to retain limited productive capability where previous strategies could reduce them to idle.
AGCO has not publicly attached the same 20-hour and 30% figures to its entire equipment portfolio, so those John Deere figures should not automatically be assumed to apply to every Fendt, Massey Ferguson or other AGCO machine.
Installation of the optional AGCO update requires a qualified service technician to connect to the machine, and independent dealers may charge for the service.
Trucks Move From 5 MPH to 25 MPH
The changes are also substantial in heavy-duty trucking.
Cummins has begun revising calibrations for more than 1.5 million engines used in medium-duty and heavy-duty trucks. Eligible applications include certain truck engines dating to model year 2017 and motorcoach applications dating to model year 2018.
For applicable heavy-duty engines, Cummins increases the time before certain DEF quality and tampering-related derates and raises the final restricted speed from 5 mph to 25 mph.
Some eligible motorcoach applications receive even greater flexibility, including a final restricted speed of 50 mph rather than 5 mph and the removal of a secondary derate stage under certain conditions.
Mack Trucks and Volvo Trucks are following a similar timetable.
Their revised software increases final inducement speed from 5 mph to 25 mph and can extend the repair window from four hours to as much as 160 hours.
Both manufacturers are progressively extending availability to older vehicles, eventually covering eligible post-2010 models by the end of 2027.
DEF Is Still Required
The software changes should not be confused with DEF deletes or emissions-system bypasses.
DEF remains required on engines certified to use SCR, and manufacturers are still required to meet applicable nitrogen oxide emissions standards. Removing SCR hardware or installing software specifically designed to defeat an emissions-control system remains prohibited.
Cummins, for example, explicitly says its DEF tank-level inducement stages remain in place. The revised calibration primarily changes how certain faults involving DEF quality, tampering or system performance are handled.
EPA has separately clarified that owners and independent repair providers can receive access to service information and that temporary overrides used legitimately for diagnosis and repair can be permitted without turning emissions tampering into an accepted practice.
That is an important distinction because the policy is moving toward repairability, not toward eliminating emissions control.
EPA Could Go Further
There is another regulatory development worth watching.
On July 14, 2026, EPA formally published a proposed rule that would go substantially further for newly manufactured diesel engines and vehicles. Instead of mandatory engine derates and speed reductions after SCR-related faults, EPA is proposing that future systems could rely on audible and visual warnings.
The proposal covers both highway and nonroad diesel engines. EPA is also considering updated guidance that could allow comparable strategies to be applied to existing equipment.
As of August 27, however, that broader change remains a proposal rather than a finalized rule. Public comments remain open through August 29, 2026.
That makes it important to separate what is already happening from what could happen next. Current Deere, AGCO, Cummins, Mack and Volvo software updates are already giving operators more flexibility under EPA guidance. The proposed rule could eventually remove mandatory power and speed inducements much more broadly on new engines.
Why the DEF Change Matters
The most important aspect of this shift is not that environmental requirements are disappearing. They are not.
What is changing is the assumption that an emissions-system fault must quickly make an otherwise functional machine almost unusable.
That approach made sense as an enforcement mechanism on paper, but it often produced poor outcomes in actual operation. A tractor sitting in a field with a failed DEF sensor does not become cleaner because it has been restricted to idle. A truck unexpectedly limited to extremely low speed may create an additional safety problem without immediately solving the emissions fault.
Extending the repair window is a more practical engineering compromise.
Operators still receive a clear incentive to repair the SCR system, but the engine management strategy recognizes that parts, diagnostic equipment and technicians are not always available within a few hours. This is particularly important in agriculture, where a single lost day during a narrow planting or harvesting window can be worth considerably more than the component that triggered the fault.
The next logical step should be better fault discrimination. A genuinely empty DEF tank, contaminated fluid, a failed level sensor and an SCR efficiency problem are different technical conditions. Treating them all with nearly identical consequences has never been an especially sophisticated control strategy.
More accurate diagnostics combined with longer, graduated inducement periods should reduce downtime without turning emissions compliance into an optional feature.
DEF Update by Manufacturer
John Deere
- Previous final restriction: approximately 4 hours.
- New repair window: approximately 20 hours.
- Final fault-related operation: about 30% of rated engine power instead of idle-only.
- Nearly 80% of applicable software updates expected to be available by the end of 2026.
- Remaining updates expected during 2027.
- Updates began rolling out in November 2025.
AGCO
- U.S. rollout began in Q3 2026.
- Longer operating period before derate.
- Final derate retains limited productive capability instead of idle-only operation.
- Applies through AGCO’s equipment and engine software program, including machinery sold under brands such as Fendt and Massey Ferguson.
- Installation requires a qualified service technician and may involve a dealer fee.
- AGCO has not published one universal 20-hour or 30% figure covering every affected machine.
Cummins
- Revised calibrations already offered for more than 1.5 million engines.
- Certain truck applications supported back to model year 2017.
- Certain motorcoach applications supported back to model year 2018.
- Heavy-duty final inducement speed increases from 5 mph to 25 mph.
- Eligible motorcoaches can increase final inducement speed from 5 mph to 50 mph.
About Mack Trucks
- Updated software introduced on Pioneer and new Anthem models with MP13 engines in May 2026.
- Final inducement speed increases from 5 mph to 25 mph.
- Maximum repair window increases from 4 hours to as much as 160 hours.
- Model year 2024 and newer coverage targeted by the end of summer 2026.
- All eligible post-2014 vehicles targeted by Q2 2027.
- Eligible post-2010 vehicles targeted by the end of 2027.
Volvo Trucks
- Updated D13 VGT and D13 TC engine software entered production in May 2026.
- Final inducement speed increases from 5 mph to 25 mph.
- Maximum repair window increases from 4 hours to as much as 160 hours.
- Model year 2024 and newer remote updates targeted by the end of summer 2026.
- Eligible post-2014 models targeted by Q2 2027.
- Eligible post-2010 trucks targeted by the end of 2027.
