How Far Can You Drive with DEF Light On and How to Reset DEF Light

DEF, or Diesel Exhaust Fluid, reduces the environment’s toxic gases (NOx).

DEF levels are maintained in coordination with fuel levels.

DEF light comes on when the DEF fluid-to-fuel ratio changes or when the DEF quantity exceeds the required level.

It is not recommended to drive with the DEF lights on.

This article discusses possible solutions to the problem.

What does DEF level warning light mean?

DEF level warning light works with the DEF sensor to indicate the fluid level in the DEF tank.

At the low level of DEF fluid, DEF level warning light turns on. You can refill the DEF tank and it will turn off automatically.

Continuous blinking of the warning light means below critical level of DEF fluid. It means the fluid is gonna run out soon.

You should refill the fluid from a nearby pump to avoid complications including low performance and limp mode.

Why DEF Fluid is Used in a Vehicle?

As part of its environmental protection mission, the Environmental Protection Agency (EPA) takes various steps.

An example of such a step came in 2010 when harmful emissions from diesel vehicles were curtailed.

A DEF solution is added to the exhaust system to reduce harmful emissions, such as NOx.

Since then, DEF fluid has been used as a toxic gas inhibitor.

Can I Drive with DEF Light On?

Yes, you can drive with DPF light on. It indicates the fluid level of the DPF tank.

Still illuminating DEF warning light means low level and blinking light means below critical level of DEF fluid in the tank.

When the tank becomes empty, your vehicle performance becomes limited. It can lower to 5 mph which will force you to fill the tank.

You can find any gas station within 50 to 100 miles. So you won’t need to drive long without DEF fluid.

Why DEF Fluid Warning Light Comes on a Vehicle?

how far can you drive with def service light on

Diesel vehicles are required to use a certain amount of DEF fluid.

From model to model, the amount varies.

A warning light appears if the DEF fluid level is less than the required level.

Medium-class trucks usually hold 3 to 5 gallons of DEF fluid (or 11.35 to 18.92 liters).

The warning light will illuminate when the DEF fluid level is below 2% or three gallons per 100 gallons of fuel.

In general, one gallon of DEF lasts about 300 to 500 miles.

What Happens When the DEF Light Comes On?

DEF lights indicate that your vehicle does not have enough DEF.

It reduces the performance of the engine and the speed of the vehicle.

Limp mode is activated on the vehicle.

How Far Can You Drive with DEF Light On?

Low-level DEF fluid will allow you to drive about 50 to 100 miles, but it will affect the performance of your engine.

Your truck will go into limp mode if you don’t add DEF within this range.

The average modern truck can travel 7,500 to 10,000 miles on a full tank of DEF (9-10 gallons).

The DEF light appears when the DEF level drops below the required level.

In a medium-sized truck, DEF consumption ranges from two to three gallons every 700 to 1200 miles.

Diesel engines are strongly connected to DEF.

Driving with the DEF light on causes the following problems:

  • The exhaust system may be clogged with debris
  • Reduces the engine’s overall performance
  • Reduces fuel economy
  • Increases the harmful emissions
  • Limits the speed of the vehicle to 05 mph
  • The engine shuts down

What to Do If DEF Light Comes on the Dashboard?

What to Do If DEF Light Comes on the Dashboard

An illuminated DEF light indicates that the DEF fluid level is too low.

It reduces your vehicle speed.

You will need to refill the DEF fluid in this case.

As soon as you add DEF, your vehicle automatically returns to its normal speed.

So, you need to add DEF fluid when the DEF light comes on.

How Much DEF Is Used Per Mile?

Most engines have a DEF dose rate of 2%. However, it can change from 1 to 3% depending on the manufacturer.

Every 50-gallon fuel needs 1 gallon of DEF in general. As a matter of fact, one gallon DEF can last 300 to 500 miles.

So, the average usage of DEF fluid is 7.56-12.6 ml per mile.

What happens if the DEF sensor goes bad?

DEF sensor is important for detecting fluid levels and reporting it to the ECM.

Without an operational DEF sensor, there can be many complications including incorrect DEF fluid reading, communication with the ECM failure etc.

Generally, it lowers the vehicle’s performance and shifts to limp mode.

But it can also send error data and shut down the engine completely causing total engine stalling.

Will a Diesel Truck Run Without DEF?

In some cases, diesel engine companies allow drivers to drive without DEF until they reach home or the garage, but it reduces engine power.

But if you ignore the warning light continuously, your engine may stop working or even shut down.

Is It Safe to Drive with DEF Light On?

Driving with the DEF light on does not affect your safety.

But it will be harmful to your vehicle.

So, take the necessary steps when the warning light turns on.

Does Low DEF Cause Check Engine Light?

No, a low DEF cannot cause a check engine light.

However, ignoring the DEF warning light can cause damage to the engine in the long run and may cause the check engine light to illuminate.

How Do I Reset My DEF Light?

How Do I Reset My DEF Light

There are times when the DEF warning light flashes, even when the DEF fluid tank is full.

Resetting the warning light is necessary in this case.

To reset the DEF light, follow these steps:

  1. Update the ECM and scanning tool to the latest version.
  2. Install the scan tool on the vehicle.
  3. Enter ECM and select Reductant System Data from the Data Display menu.
  4. There are three DEF sensors in Duramax; make sure they are all active.
  5. Turn the ignition key “off” and then “on.”
  6. Now reset the fluid level with the help of the scan tool. Reset systems are different for different models and years. So, follow the owner’s manual for the exact way.
  7. The DEF system reprogramming and reset is done when the Reductant Level Warning shows “none.”

To learn more, check out this video.


What does DEF mean on dashboard?

DEF on your dashboard indicates that your vehicle’s fluid level is low and must be refilled immediately.

The fluid level indicator indicates low DEF fluid and warns you to refill.

Can I run water instead of DEF fluid?

Some people say that water is a good replacement for DEF fluid. Well, it is not. DEF has a chemical compound that reacts with the exhaust and converts the NOx to Nitrogen gas. Water doesn’t have that ability and a lot of impurities too. So, there is no way that you can run water instead of DEF.

How many miles can you drive with DEF fluid?

It depends on the size and capacity of the DEF fluid tank in your truck.

If your truck is medium-sized, it can contain 3-5 gallons of DEF fluid.

Within this capacity, you can drive around 900-1200 miles.

In a full-size modern truck with 10-gallon DEF fluid, you may reach 6,000-8,000 miles.

How much does it cost to replace a DEF sensor?

Replacing a DEF sensor costs around $250 to $300.

In cases where DEF is incorrect, many people choose to clean the DEF sensor, which General Motors do not recommend.

I would suggest replacing it rather than cleaning it because it is more efficient.

Why is the DEF light flashing?

The vehicle requires a certain amount of DEF fluid to run properly.

A DEF light may flash on your dashboard if your vehicle’s DEF fluid level is below a certain level.

Ensure that the DEF tank has the correct level of fluid by checking the DEF tank.

Refill the fluid if the level is below the recommended level.

How much DEF fluid can a Duramax 6.6 hold?

Duramax 6.6 holds 5 gallons or 18.92 liters of DEF fluid.

It can last approximately 1200 miles.


Driving with a DEF light on is not a smart idea.

Low-DEF fluid allows you to drive to your home or garage while reducing your truck’s speed.

If the warning light flashes, fill up the DEF tank for better and healthier driving.

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