How Far Can You Drive with DEF Light On?

Nowadays, cars are getting more and more complicated. Before, you only had to worry about having sufficient oil, gas, and water in your car. This list gets even longer with today’s requirements, including the DEF. But what most people are concerned with is how long you can drive with the light on.

On that note, how far can you drive with the DEF light on? Once the DEF light comes on, you only have about 50 to 100 miles to drive. This depends on your vehicle and the size of the DEF tank. If you continue driving beyond the 100-mile mark, you risk your vehicle getting into limp mode, where the engine limits the speed to prevent further damage.

Check out the information below to understand more about the driving distance when the light comes on and the best speed to drive. I have also gone deeper to explain how to avoid it and much more.

How Far Can You Drive with the DEF Light On?

DEF fluid level check

Driving with the DEF light on is possible, but you will not go very far. Usually, when the light comes on, it is accompanied by the warning message “5mph Max Speed in 100 mi, Service DEF System, See Dealer.” This implies that you have just about 100 miles to refill your DEF—failure to do so and your vehicle goes into limp mode. This is its defense measure, where the engine limits the speed to prevent damage.

Driving limp mode can be very dangerous on the highway or a city street.  If this happens, you must get to the nearest DEF station as soon as possible, or you will be forced to contact a tow truck. The situation can get even more complicated if you happen to be cruising through regions that do not have DEF stations on them. You must plan by bringing along some DEF fluid or ensuring your tank is full to avoid getting into these situations.

Is It Safe to Drive with the DEF light blinking?

Initially, it may seem safe to drive with the Diesel Exhaust Fluid light on, but when looking at the bigger picture, you will be putting yourself in a dangerous situation. There are several consequences of ignoring the warning light and failing to top up the fluid. DEF is put in place to ensure the smooth operation of diesel engines. When it is low amount, it adversely affects the overall performance of the engine.

For one, driving in limp mode is not ideal, and you will not likely get far. If you continue driving, you may end up with the engine shutting off. It can also clog the exhaust system and damage the DEF filter—it will cost you around $1800 to get a replacement. Hence, driving around with the def light blinking is unsafe and can also get very costly.

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How Often Should I Fill the DEF Tank?

DEF fluid reservoir location
DEF fluid reservoir location

This depends on your engine type, the usage of your truck, the MPG you drive at, and the DEF tank size. For example, if your vehicle is a ¾ ton pickup equipped with a 6.7L engine running lightly, you may only need to top up the DEF tank every 8000 miles. On average, light-duty trucks tend to consume approximately 2 to 3 gallons of DEF for every 800 miles. A diesel truck with 20+ mpg can go 8000 to 10,000 miles on 10 gallons of DEF.

How Do I Prevent the DEF Light from Coming On?

The simple and short answer to this is to refill the DEF tank regularly. This way, you will not have to worry about the DEF warning light coming on or suffering the consequences of the same. Hence, inspecting the DEF levels and regularly refilling them when necessary is advisable. While at it, ensure that you carefully review the owner’s manual for instructions on locating the DEF filler port to avoid contaminating your fuel.

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Frequently Asked Questions (FAQs)

What are the Consequences of Not Filling Out DEF in Time?

Low DEF levels directly impact the engine performance, prompting the car’s engine to activate the limp mode. This means you cannot drive your car until you refill the DEF efficiently. Moreover, it is likely to cause the car to break down due to the reduced engine performance. There are also chances of reduced fuel economy and increased emission production.

Why is My DEF Light On, But the Tank is Full?

DEF light blinking when the tank is full is a sign of trouble with the level sensor. You must check the float as it could be stuck at one spot while the fluid moves. This is what may have brought about this error. If that is the case, slightly shake it loose or re-install it to deactivate the light. You can also inspect the DEF pump.


If your DEF light comes on, you cannot go very far, only about 100 miles. Hence, refilling is advisable as soon as the warning light appears. This will spare you a lot of other issues, such as increased dangerous emissions, a clogged exhaust system, or the engine shutting off completely.

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