If you’re a diesel truck owner, you’re likely familiar with diesel exhaust fluid (DEF) and probably wondered what happens when the truck runs out of DEF. Diesel exhaust fluid is an essential component in modern diesels built after 2010 to reduce nitrogen oxide (NOx) emissions through a system called Selective Catalytic Reduction (SCR). But what actually happens when you run out of DEF? Will your truck shut down? Is it dangerous for the engine?

Will My Diesel Truck Keep Running?

Yes—and no. If your truck is already running when the truck determines the DEF tank has hit empty, it will usually continue operating to allow you to safely get the truck into a position to refill the DEF. However, most trucks are programmed to enter a limp mode or reduced power mode, where speed is severely limited. The real impact comes when you shut the engine off. Once off, some systems will not allow a restart until the DEF tank is refilled.

This isn’t just an OEM preference—it’s federally mandated. The Environmental Protection Agency (EPA) requires that diesel engines using SCR must prevent normal operation without DEF. It’s part of a broader effort to control emissions, and manufacturers design their vehicles accordingly. Because of these federal regulations, DEF tanks never truly run empty.

Will I Know When My Truck’s DEF Is Low?

Absolutely. Your truck gives you plenty of warning. You’ll see dashboard alerts when DEF levels start to drop—usually first at around 2,500 miles remaining, then again at 1,000, 500, and finally critical warnings under 100 miles. What if you ignore these warnings? You’ll get a final warning before the truck won’t restart or is limited to 5 mph. It’s nearly impossible to “accidentally” run out unless you’re willfully ignoring these very disruptive messages appearing on your gauge cluster.

For example, a 2018 GMC Sierra 2500HD was limited to 5 mph 1,014 miles after the first low-DEF dash warning. Here are some of the dashboard alerts along the way:

• 1,000 Miles Until Empty
• 500 Miles Speed Will Be Limited
• 75 Miles Until Speed Limited to 55 MPH
• 75 Miles Until Speed Limited to 5 MPH

Is It Bad to Run Out of DEF?

Mechanically, if a truck runs out of DEF it isn’t the same as running out of oil or coolant—your engine won’t suffer immediate damage. However, it’s definitely bad for business, and it’ll eventually leave you with a “dead” truck. You’ll face power loss or speed restrictions until the DEF is refilled. Whether you’re working or recreating, these limitations are inconvenient at best and costly at worst.

Just Don’t Run Out

Like it or not, DEF is as essential to your diesel truck as fuel and oil. Keep an eye on your DEF levels and top-off regularly. It’s wise to simply fill the DEF tank at every third or fourth fuel stop, if convenient. If you’re towing, anticipate notably above-average DEF usage. If you’re towing in remote areas, carry extra DEF. Actually, always stash a little DEF tote somewhere in your diesel truck.