DEF Derate and Limp Mode: How the Speed Limit Works and What Changed

The direct answer: a truck with selective catalytic reduction (SCR) is built to limit its own power or speed when it runs out of diesel exhaust fluid (DEF), detects poor-quality fluid, or loses part of the DEF system. Regulators call that an inducement; drivers call it derate or limp mode. On many trucks built to the original rules the last step is 5 mph. The rules are being loosened: EPA guidance in August 2025 asked manufacturers to release software with a far slower schedule for trucks already on the road, and 2027-and-later engines follow a schedule that ends at 25 mph, not 5. Whether your truck has the slower schedule depends on its manufacturer and its software. The repair is the same either way: find and fix the fault that started the clock.

Why the engine does this

SCR cleans nitrogen oxides out of the exhaust only while DEF is being dosed. A truck that kept full power with an empty DEF tank would run with its emissions control switched off, so the engine is programmed to make that impractical. The limit is deliberate software behaviour. It is not a mechanical failure, and it lifts when the system confirms the fault is gone.

What starts the clock

The federal rule for 2027-and-later engines, 40 CFR 1036.111, lists the triggers plainly, and they match what drivers see on current trucks:

• Low DEF — the tank down to 2.5 percent of capacity, or about three hours of running left
• DEF quality outside the engine maker’s concentration specification — diluted, contaminated or aged fluid
• A missing catalyst signal
• Open-circuit faults in the DEF level sensor, pump, quality sensor, dosing valve, tank heater, tank temperature sensor, NOx sensors, the SCR wiring harness or the aftertreatment control module

The last group is why a truck with a full tank of good fluid can still derate: a failed sensor or a chafed harness looks the same to the software as a system that is not working. The DEF warning light page covers how to read the dash message before you call.

Trucks already on the road: 5 mph, and EPA’s 2025 guidance

EPA’s own description of the systems in service is that vehicles can be “limited to as little as five miles per hour within hours of a DEF-related fault.” On August 11, 2025 the agency issued guidance urging engine and equipment manufacturers to revise that software on existing vehicles. Its published comparison for heavy-duty trucks:

Stage	Before	Under the 2025 guidance
First step	5 mph within about 4 hours	15% torque reduction at 650 miles
Second step		30% torque reduction at 4,200 miles (80 engine hours)
Final step		25 mph at 8,400 miles (160 engine hours)

Two cautions. This is guidance to manufacturers, not a change that reaches a truck on its own: the engine maker has to release the revised calibration and it has to be installed. Ask the dealer for your engine make whether an update exists for your model year. And the exact steps on any truck are the ones in its own software, so treat the table as EPA’s target, not a promise about your dash.

2027-and-later engines: the schedule in the rule

For model year 2027 and later, the derate is a maximum road speed that steps down with hours of non-idle running. For a typical highway truck (average speed of 25 mph or more) the rule’s table reads:

Hours of non-idle operation after the fault is detected	Maximum speed
0	65 mph
6	60 mph
12	55 mph
20	50 mph
86	45 mph
119	40 mph
144	35 mph
164	25 mph

Slower-running vehicles have their own columns that start lower. Each step comes in gradually — 1 mph for every five minutes of running — and the rule requires that a generic scan tool be able to clear the inducement codes while the truck is stopped. If the same fault comes back within 40 hours of running, the derate resumes where it left off instead of starting over.

What changed in 2026

• March 2026: EPA guidance said manufacturers may meet the requirements with NOx sensors instead of the urea quality sensor, and that approved NOx-sensor-based software updates can be installed on existing engines without being treated as tampering.
• July 9, 2026: EPA proposed removing DEF-related derates and speed restrictions entirely from newly manufactured engines, and asked for comment on allowing the same for engines in use. That is a proposal. Until a final rule is published, the schedules above are the law for the engines they cover.

None of this makes it legal to remove or disable the DEF system. The updates EPA describes are the manufacturer’s approved software, installed through the manufacturer’s channel. We do not do deletes.

Getting out of a derate

1. Low fluid: fill with sealed, in-date DEF. Many trucks clear the warning after a key cycle and a short drive once the level reads correctly.
2. Quality fault: fresh fluid alone may not clear it. The tank may need draining and the system has to see good NOx conversion again.
3. Component or wiring fault: the part or the circuit has to be repaired. Clearing codes without fixing the cause buys a short drive before the derate returns at the step it left.
4. Already speed-limited: call before driving far. A truck at 5 mph on a highway shoulder is a safety problem first; see mobile repair or a tow.

We read the fault history, test the accused parts and repair the actual failure — see DEF and SCR system repair. Call (916) 571-8888 with the exact dash wording and whether a countdown or speed limit is showing.

Common questions

How fast will my truck go in DEF limp mode?

It depends on the engine’s software. Trucks on the original calibration can end at 5 mph. Trucks with the manufacturer’s revised software follow a slower schedule that EPA’s guidance ends at 25 mph, and 2027-and-later engines end at 25 mph under the rule.

Can a scan tool clear a DEF derate?

It can clear the codes, and on 2027-and-later engines the rule requires that a generic tool be able to. The derate stays away only if the fault is actually fixed; otherwise the system detects it again and picks up where it stopped.

Will filling the DEF tank fix it?

If the cause was low fluid, usually yes. If the message is about quality or a system fault, filling the tank does not address it.

Has EPA ended DEF derates?

Not as of the review date on this page. EPA has issued guidance for existing trucks and proposed ending derates for new engines; the proposal was not final when this page was last reviewed.

Sources: 40 CFR 1036.111 (read via eCFR); EPA news release of August 11, 2025 on DEF guidance; EPA news release of March 26, 2026 on DEF sensor guidance; EPA fact sheet 420-F-26-012 (July 2026). This page summarizes them and is not legal advice.