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What Happens if Your DEF Runs Out?

Driving a diesel truck has numerous advantages, including high towing capacity and improved fuel economy. However, it entails certain responsibilities, one of which is to monitor your Diesel Exhaust Fluid (DEF) levels. DEF is essential for reducing harmful emissions, and running out is not something to take lightly.

So, what happens when your DEF runs out?

We will look into the various aspects of DEF’s role and the consequences of ignoring it, as highlighted by Fuel Logic.

Early Warnings and Indicators

Modern diesel vehicles have systems that provide ample warning before you run out of DEF. The alerts usually start when you have about 1,000 miles of DEF left. Your dashboard is likely to show a message like “DEF level low” or “DEF needs to be refilled.” These messages become more urgent as your DEF levels drop.

So, what happens if you ignore all of these warnings and run out of DEF?

The short answer is, your vehicle will not function normally.

Restricted Performance

When your vehicle runs out of DEF, its performance suffers significantly. Initially, you may discover that your truck limits your speed to around 5 mph. Imagine driving down a highway and suddenly being limited to a speed of 5 mph; it’s not only inconvenient, but also dangerous.

No Engine Restarts

If you completely deplete your DEF while driving, your diesel truck will not let you restart the engine after you turn it off. This means you could end up stranded somewhere where refilling is inconvenient.

Potential Repairs

Running out of DEF not only causes immediate problems while driving, but it can also cause significant damage. The engine and exhaust systems are under constant strain due to forced shutdowns and low-speed operations. This may result in additional repair costs that could have been avoided with proper DEF levels.

Refilling DEF

The good news is that refilling DEF is simple. It’s available in various sizes, ranging from small jugs to large drums, at most auto parts stores, truck stops, and even gas stations.

Most vehicles have a dedicated filler cap, usually blue, to distinguish it from other fluids. Check your owner’s manual for the correct DEF specification to ensure vehicle compatibility.

Pro Tips for Maintaining DEF Levels

1. Regular Checks

Make it a habit to check your DEF levels during routine maintenance checks. Staying ahead is easier than being caught off guard.

2. Bulk Purchase

If you own a fleet of trucks, consider purchasing DEF in bulk. This not only ensures a consistent supply but also saves money.

3. Professional Maintenance

During routine maintenance, have your mechanic check the DEF levels and top it up as needed.


Running out of DEF is not an ideal situation. The warnings are there for a reason, and you must heed them to avoid reduced performance and potential vehicle damage. Regular checks and timely refills will keep your diesel truck running smoothly and in compliance with environmental regulations.

It’s not only about extending the life of your vehicle; it’s also about contributing to cleaner air. Pay attention to the dashboard warnings and keep your DEF tank full. A little precaution goes a long way toward ensuring a trouble-free driving experience.

Keeping these points in mind will help you avoid the pitfalls of running out of DEF and keep your diesel vehicle reliable and roadworthy. Drive safely!

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