Good News for Detroit-Powered Fleets:
Daimler’s Software Update Softens the DEF "Limp Home" — 25 MPH Instead of 5
What it means for your trucks, and how TAG Truck Center and Lonestar Truck Group can help.
If you run Freightliner or Western Star trucks with Detroit engines, a recent software change from Daimler Truck North America (DTNA) is worth a few minutes of your attention. It reworks how the truck’s diesel exhaust fluid (DEF) inducement system behaves when there’s a fault or a low-fluid condition, and the headline takeaway is that a truck in the final stage of a derate can now keep moving at up to 25 mph instead of being throttled all the way down to 5 mph.
For anyone who has ever had a unit crawl to a near-standstill in traffic over a DEF issue, that’s a meaningful difference in safety and in getting the truck somewhere it can be serviced.
A Quick Refresher: What DEF Derate Actually Is
Selective catalytic reduction (SCR) systems have been standard on diesel trucks since the 2010 model-year EPA mandates. They use DEF — a nontoxic, colorless, odorless mix of roughly 32.5% urea and 67.5% deionized water — to convert nitrogen oxides into harmless nitrogen and water so the truck meets emissions standards.
To keep those systems working the way regulators require, trucks are built with safeguards called inducements. When the system detects a problem — the DEF tank running dry, a failing sensor, or a component fault, for instance — it progressively cuts engine power and speed. The idea is to “induce” the driver to correct the issue, whether that means topping off DEF, replacing a bad sensor or component, or running a regeneration cycle.
The catch has always been that a legitimately healthy, compliant truck could get dragged into a severe derate by nothing more than a flaky sensor.
What Changed with the New Derate Stages
DTNA’s update follows revised guidance the EPA issued in August 2026. Here’s how the inducement steps look under the updated software:
- Stage 1 – Low/Empty DEF: Engine torque is reduced by 15% for 650 miles or 100 hours.
- Stage 2 – Still Empty: If the tank isn’t addressed, torque is derated 30% until another 4,200 miles or 80 engine hours pass.
- Stage 3 – Final Inducement: The engine management system caps road speed at 25 mph until 8,400 miles or 160 engine hours.
That final stage is the big one. Previously, a truck in final inducement was held to 5 mph and would not restart once it was shut down. The revised guidance also extends the time it takes to reach that final stage for component-related issues, giving operators more runway to get the truck in for service before it’s severely limited.
Which Trucks Are Covered?
DTNA began rolling this out in February 2026 to roughly 330,000 in-service vehicles, covering:
- DD15 engines: Model years 2021–2025
- DD13 engines: Model years 2022–2025
The rollout continues throughout 2026. New Freightliner and Western Star trucks equipped with Detroit engines are already leaving the line with the updated software.
Important: The update is optional, and it isn’t an over-the-air push. It must be installed on-site through a hard plug-in using a diagnostic link, which is exactly where Lonestar Truck Group and TAG Truck Center come in.
The Bigger Regulatory Picture
The change has drawn broad support. Federal officials, including the Small Business Administration, welcomed the more flexible inducement approach as relief for the owner-operators, farmers, and small fleets who’ve absorbed the cost of unreliable DEF systems through repairs and downtime.
There’s also related movement on the sensor side. In March 2026, the EPA approved guidance allowing manufacturers to replace urea quality sensors with nitrogen oxide sensors, a change the American Trucking Association backed, citing how often faulty or unreliable DEF quality sensors have sidelined otherwise safe, compliant trucks. Legislation to codify that March 2026 guidance, the Diesel Engine Flexibility Act, was introduced in the House in early June 2026.
For now, DTNA has said it does not plan to remove DEF sensors from current Freightliner or Western Star vehicles and is coordinating with the EPA to keep its approach aligned with regulatory requirements while maintaining system performance.
How Lonestar Truck Group and TAG Truck Center Can Help
Is one of your units derating right now?
Don’t let a DEF issue keep a truck parked or crawling. Our factory-trained Detroit service technicians can diagnose the fault, determine whether it’s a fluid, sensor, or component issue, and get your unit back to full power. Bring it to your nearest Lonestar Truck Group or TAG Truck Center dealership, and let’s work your unit in.
Want this software update pushed to your fleet?
Because the update installs on-site through a diagnostic connection, it’s a natural fit to bundle with routine service. If you run DD13- or DD15-powered Freightliner or Western Star trucks in the covered model years, contact us and we’ll confirm eligibility and get the revised DEF inducement software loaded to your units.
Keeping your trucks compliant and on the road is what we do. Let Lonestar Truck Group and TAG Truck Center help you take advantage of this update before your next DEF headache becomes downtime.
The developments in this article were reported by Transport Topics, "Daimler Truck update raises DEF derate speed to 25 MPH from 5 MPH," by Keiron Greenhalgh, July 2026. Facts and figures in this article are based on reporting by Transport Topics. This piece is provided by TAG Truck Center and Lonestar Truck Group for informational purposes; contact your dealer for guidance specific to your equipment.