That DEF Warning Light Isn’t a Suggestion. Here’s What Happens Next

It's a situation on no driver or fleet wants to deal with: the DEF gauge hits empty. But what actually happens next? Does the truck shut off immediately? Can it keep running? And why does DEF Warning matter so much in the first place?

In today's article, we will discuss what happens when a truck runs out of DEF, how engines respond, and what it means for day-to-day operations.

First, why DEF matters

Diesel Exhaust Fluid (DEF) is crucial to modern diesel engines. It works with the truck's Selective Catalytic Reduction (SCR) system to reduce nitrogen oxide (NOx) emissions.

When DEF levels start dropping, the truck doesn't stay quiet about it. Warnings show up early and often, giving drivers plenty of notice that a refill is coming due. That's by design, because once DEF is gone, the engine can't operate the way it's intended to.

What if DEF actually runs out?

If the truck is already running when the system determines the DEF tank is empty, it will usually continue operating long enough to allow the vehicle to be safely positioned for a refill. But that grace period comes with strings attached. Most trucks are programmed to enter a reduced-power or "limp" mode, where speed is severely limited, and acceleration is noticeably restricted. The truck is still moving, but it's no longer operating normally.

The real impact often shows up after the engine is shut off. Once turned off, some systems will not allow the truck to restart until DEF has been added. That's when a manageable situation can quickly turn into unexpected downtime.

This isn't just a manufacturer preference; it's federally mandated. The Environmental Protection Agency (EPA) currently requires diesel engines equipped with SCR systems to operate normally without DEF. It's part of a broader effort to control emissions, and truck manufacturers are required to design their vehicles to enforce that compliance. Because of these regulations, DEF tanks never truly "run empty," and the system intervenes before that point is reached.

Changes Ahead

Starting with 2027 engine models, the EPA will permit reduced torque instead of full engine shutdown when its levels run too low. This adjustment is intended to minimize disruptions while maintaining compliance with emissions standards. At the same time, the agency is reviewing older endangerment findings, signaling that broader policy shifts may be ahead.

Why running out of DEF causes bigger problems

Beyond the immediate slowdown, running out of DEF can:

• Disrupt delivery schedules.
• Increase downtime.
• Create safety concerns if derating occurs in traffic.
• Add stress for drivers trying to find DEF on the road.

All of that from a tank that's relatively easy to keep full.

Centralizing Procurement

Fuel procurement is often centralized, but many fleets still handle DEF locally or location-by-location. This decentralized approach creates inconsistencies in price, storage, and supply risk. By consolidating procurement, fleets can unlock economies of scale, reduce delivery costs, and simplify compliance across all locations.

Mansfield helps customers design optimal programs by:

• Right-sizing storage (totes or tanks)
• Inventory management with tank monitors
• Consolidating supply contracts
• Providing national delivery performance backed by logistics expertise
• Advice for Fleet Managers

DEF is no longer just a box to check for emissions compliance; it's a key cost center that can be optimized for better performance and savings. Centralizing your program, upgrading to the right storage solution, and partnering with an experienced supplier can transform DEF from a routine purchase into a strategic advantage.

Ready to take control of your DEF strategy? Reach out to the Mansfield team to learn how a more efficient, scalable program can help you prepare for the road ahead.