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What happens if you run out of DEF?

Author

Rachel Ellis

Published Jan 14, 2026

The Cost of Running Out of DEF‍

In a best-case scenario, running out of DEF simply means refilling the tank and restarting your machine. If you have DEF nearby, you could be back to work within minutes. But sometimes it isn't so quick and easy.

What happens if my diesel runs out of DEF?

If the truck is allowed to run out of DEF the engine's power is reduced, a solid red warning will be displayed and the vehicle speed will be limited to 5 mph until the DEF tank is refilled.

What happens if you run without DEF?

If you ignore the DEF warning the truck will cease to work. Some diesel engine manufacturers allow the engine to go into reduced power mode so the truck can “limp home” or limit the number of times you can turn the engine over. At some point though the diesel engine will not start.

What happens if you don't refill DEF fluid?

If the reservoir is not replenished with DEF and runs low, vehicle speed will be limited, but as soon as DEF is added, the engine will resume normal speed levels. A prudent measure would be to have a top-off gallon jug of DEF available on each piece of equipment with an EPA 2010 engine with an aftertreatment system.

Can you use water instead of DEF fluid?

Subject: RE: Anyone replace DEF with water? Water won't hurt the system but it will de-rate you pretty quick. ECM compares NOx values pre/post, if conversion is low it will start throwing codes and will eventually derate.

34 related questions found

Can I pee in DEF tank?

Please do not urinate in your DEF tank, the SCR system will be damaged and it will cost thousands of dollars to repair. Here at Transliquid Technologies we specialize in providing the best quality of Diesel Exhaust Fluid.

What can you substitute for DEF?

DEF is urea or otherwise known as aqueous ammonia. Windshield washer fluid that contains ammonia will do in a pinch.

Should I delete my DEF system?

The catch about a DEF system is that you have to keep its fluid reservoir filled regularly to keep it functioning properly. This doesn't just take time – it costs money, too. Deleting your DEF system can help save you money. Plus, your exhaust might have a bit of a beefier growl once you're done.

How much does it cost to replace a DEF system?

Professionally clean approximately $1,500. Recommended replacement after 200,000 miles. All in DEF / SCR Filter adds approximately $ . 20 per gallon of diesel.

Can you bypass DEF fluid?

If you're thinking of bypassing any of your diesel's emission controls, remember that that it's highly illegal and the fines can be steep. The chances of getting caught may be low, but the consequences are costly.

Is it illegal to bypass DEF system?

It is illegal to remove or inoperable an emission control device from a motor vehicle under the EPA Clean Air Act.

Will DEF freeze in my truck?

Diesel Exhaust Fluid or DEF Fluid

Whether in storage tanks or in equipment, DEF can freeze and possibly cause issues. As temperatures drop below 12°F, DEF will begin to solidify and not work as intended.

Is there a shortage of Def fluid?

Across the board, major DEF manufacturers are struggling to maintain supplies. Most of the major manufacturers have changed annual contracts to monthly spot-priced allocations. Many are reducing their DEF output to increase fertilizer production.

Can you fill DEF while engine is running?

DEF is vaporized and decomposes into ammonia and carbon dioxide in the exhaust pipe. In order to ensure that owners take their clean-air responsibilities seriously, the Environmental Protection Agency requires manufacturers of new light-duty diesels to interrupt the normal startup routine when the DEF runs out.

What happens if DEF is low?

Low DEF levels will trigger a decrease in engine performance. The use of improper fluid will trigger a decrease in engine performance. In the empty and ignored state, vehicle speed will be limited to 5 mph until DEF is detected in the tank. The parked regeneration may take up to 40 minutes.

What causes DEF pump failure?

Topping up your DEF system with hard water can also result in scale or deposits forming. Dirt / Metal Contamination: Contaminated DEF can cause abrasion of the DEF pump. Dirt, rust or tank scale can occur with improper of DEF storage or handling (e.g. res-using disposable DEF containers).

How long is DEF good for?

Shelf Life

Because it doesn't include any preservatives, DEF does come with a shelf life. The good news is, it's a fairly long shelf life; DEF can usually be stored up to one year without any issues.

What does Blue DEF?

BlueDEF® Diesel Exhaust Fluid is a mixture of high purity synthetic urea and deionized water that is used in Selective Catalytic Reduction (SCR) systems in diesel vehicles. DEF is used to help convert NOx to harmless nitrogen and water.

Is DEF fluid made from urine?

DEF is a mixture of (typically) 2/3 deionized water and 1/3 urea. It's carefully regulated by the American Petroleum Institute. Technically, urea is derived from one of the byproducts of urine. But it's synthetically made, so no cats are ever harmed in the production of the fluid.

Is deleting DEF illegal?

Can You Get In Trouble For Deleting Def? It is illegal to tampering with DEF, but it happens anyway. Operators were prompted to get rid of the DEF systems or to use DEF delete kits to avoid using them. It's not just the risk of putting someone in jail if you violate a DEF system that's worth taking.

Is DEF delete illegal in California?

There is no way around this; it is 100% illegal to tamper with or modify the emission system on your truck in any way. It isn't a state or local law (although those exist, as well), but Federal law.

Is deleting emissions illegal?

Under the EPA Clean Air Act, if an emission control device is removed from a motor vehicle or rendered inoperable, that's illegal.

Can you mix DEF?

Mixing DEF and diesel is never a good idea. Despite the fact that diesel fuel is separated from DEF by safeguards, it still happens: DEF is accidentally mixed with diesel fuel. This can lead to significant damage and costly repairs if it occurs frequently.

What is a DEF delete?

DEF Delete

The DEF (diesel engine fluid) system operates similarly to the DPF, removing soot from your engine and storing it in a canister within your exhaust system. Just like the DPF, the system will clear out excess soot by going into a regen cycle and using your vehicle's fuel to burn off and blow out the soot.

Is Blue DEF cow urine?

Contrary to popular myth, cow and bat urine and DEF are not a thing. The urea used in the manufacture of DEF is synthetically derived from ammonia and carbon dioxide, and urea production plants are often adjacent to other sites where ammonia is produced, such as coal and natural gas refineries.