DEF Act Moves Fast in Congress, But Limp-Mode Fix Isn't New

What does the DEF Act actually change for carriers?

H.R. 9618, the Diesel Engine Flexibility Act, does not create a new limp-mode fix. It protects EPA guidance issued in August 2025 that already lets truck makers raise diesel exhaust fluid (DEF) limp-mode speed from 5 mph to 25 mph and extend fault timelines to 160 hours. The bill, introduced July 9, 2026, by Rep. Julie Fedorchak (R-N.D.) and Rep. Don Davis (D-N.C.), moved out of the House Energy and Commerce environment subcommittee by voice vote July 14. It is bipartisan. Reps. Brad Finstad, Jack Bergman, and Michelle Fischbach signed on as cosponsors.

The bill is moving fast because the pain is real. A quality sensor throws a false code, the dash lights up, and a compliant truck counts down to 5 mph. The load is late, the tow is $1,200, the shop cannot get the sensor for three weeks, and the emissions system strands a safe truck on the shoulder. Farmers have the same story with a combine and a harvest window. Motorcoach operators have it with a bus full of passengers.

Why Congress is codifying guidance that already exists

The EPA guidance from August 2025 is administrative. It can be reversed by a future EPA administrator without congressional action. H.R. 9618 writes the 25-mph limp-mode speed and 160-hour fault timeline into statute. Once the bill becomes law, EPA cannot roll back the relief without Congress repealing it.

PACCAR already raised limp-mode speed to 25 mph under the August 2025 guidance. Other truck makers are expected to follow. The bill does not require manufacturers to adopt the higher speed or longer timeline. It permits them to do so without violating Clean Air Act selective catalytic reduction (SCR) tampering rules.

How selective catalytic reduction created the limp-mode problem

Heavy-duty NOx standards that took effect in 2010 effectively married every new diesel to selective catalytic reduction. SCR systems inject DEF into the exhaust stream to convert nitrogen oxides into nitrogen and water. The system works. NOx emissions from new trucks dropped sharply after 2010.

The problem is enforcement. EPA required manufacturers to program engines to enter limp mode when DEF quality sensors detect a fault or when the DEF tank runs dry. The intent was to prevent operators from running without DEF and exceeding NOx limits. The result is that a single faulty sensor can strand a truck that is otherwise compliant and safe to operate.

Quality sensors fail. DEF crystallizes in cold weather. Aftertreatment systems are complex and expensive to repair. A false code in Nebraska can cost a carrier a load, a tow, and three weeks of downtime waiting for a part. The emissions system that was supposed to protect the air ends up punishing carriers for sensor failures they cannot control.

What the bill does and does not do

H.R. 9618 protects the August 2025 EPA guidance that lets manufacturers raise limp-mode speed to 25 mph and extend fault timelines to 160 hours. It does not eliminate limp mode. It does not change DEF quality requirements. It does not change NOx emission limits. It does not change SCR system design requirements.

The bill gives carriers and drivers more time and speed to reach a repair facility when a fault occurs. A truck in limp mode at 25 mph can reach a shop or a safe parking area without a tow. A 160-hour fault timeline gives a carrier time to schedule a repair without stranding the truck mid-route.

The bill does not address the root cause, which is sensor reliability and aftertreatment system complexity. It does not change the fact that a false code can still force a truck off the road. It does not change the fact that DEF quality sensors fail more often than they should. It does not change the fact that aftertreatment repairs are expensive and parts are hard to get.

What carriers should do now

Carriers with PACCAR trucks can already take advantage of the 25-mph limp-mode speed and 160-hour fault timeline. PACCAR implemented the change under the August 2025 EPA guidance. Carriers with other truck brands should contact their OEM to ask when the higher limp-mode speed will be available.

Carriers should not disable DEF systems or tamper with SCR components. Tampering is a Clean Air Act violation. EPA can fine carriers and drivers. Tampering does not fix the underlying problem. It creates a compliance risk and a CSA risk if FMCSA finds evidence of tampering during a roadside inspection or audit.

Carriers should track DEF quality sensor failures and aftertreatment system faults. Document every false code, every tow, every repair delay. If H.R. 9618 becomes law and manufacturers adopt the higher limp-mode speed, carriers will still need to prove that sensor reliability is a problem if Congress considers further relief.

The bill is moving fast because the pain is widespread and the fix it protects is already on the books. But the fix is partial. A truck in limp mode at 25 mph is better than a truck in limp mode at 5 mph. A 160-hour fault timeline is better than an immediate shutdown. Neither solves the sensor reliability problem that creates the fault in the first place.