How Far Can You Drive with DEF Light On?

Though car engine doesn’t run on diesel exhaust fluid, a low or empty DEF tank can prevent you from keep driving. Do you know how far can you drive with DEF light on? You can’t drive more than 50 to 100 miles with a DEF light on.

If you delay refilling the DEF tank, it can reduce the engine performance, activate limp mode, and shut off the engine. A modern diesel car consumes 2 to 3% DEF of fuel consumption. 10 gallons of DEF last around 8,000 to 10,000 miles in a vehicle with 20+mpg speed.

How Far Can You Drive with DEF Light On?

You can drive around 50 to 100 miles with the DEF light on. However, it’ll affect the engine performance and the car’s computer can activate the limp mode. So you can’t drive the vehicle efficiently and at more than 5 mpg speed with the DEF light on.

It can also impact the engine’s ability to run efficiently and cause the car to break down. Running with DEF light also reduces the car’s fuel economy and increases dangerous emission production. It’ll reduce the engine overall performance and end up shutting down the engine.

It can clog the exhaust system and damage the DEF filter due to producing more emission gasses. The exhaust system can also get if you keep driving with DEF light on.  

It may also end up shutting the engine off.  So, you may cost around $1800 to replace the DEF pump if it gets damaged for your delay. Most mid-size diesel vehicles consume 2 to 3 gallons of DEF per every 700 to 1200 miles.

Is It Safe to Drive with DEF Light On?

No, driving with DEF light isn’t dangerous as it doesn’t affect the car performance immediately. But it can be harmful to the car in the long run. If the DEF light appears on with unusual frequency, your car causes serious problems. If you keep driving with a low DEF level, it can clog the exhaust system with debris.

Is It Safe to Drive with DEF Light On

DEF ensures the smooth operation of diesel engines. So a vehicle can’t run for a long time with a low DEF level. If you let the tank empty, it can cost you serious consequences and costly repairs.

What Happens if You Ignore the DEF Warning Light?

A car’s computer continuously monitors the emission level and helps keep it at a safe level. It turns on the DEF light if detects any shortage in the fluid level. If the DEF level drops under a certain point, the vehicle’s computer will activate the limp mode to maintain a safe emission level.

It’ll also cripple the ride performance and reduce the driving speed. So you may need to call for a tow to reach the home or a filling station. Your car mightn’t start until you refill the DEF tank.

Is Running out of DEF Bad for the Engine?

The diesel exhaust fluid (DEF) or AdBlue doesn’t go even near the engine. So the engine won’t be directly affected if there is low or no DEF in the tank. However, it’ll affect the anti-pollution system of your vehicle directly and the sensors may stop the pollution system.

It may also de-rate the engine and stop it. For example, if DEF runs out in a Volvo Bdouble truck, its engine will de-rate from 550HP to 200HP. In this case, you have to refill the tank with DEF to restart the engine.

Low DEF will also turn on the warning light and the vehicle will count the miles until the engine stop. DEF helps break down the combustion product and carbon particles. Running out of DEF may also plug the particulate filter in some exhaust and cause poor economy and lack of power.

No DEF means trying to run diesel without an ignition timer or spark plugs. So diesel won’t work effectively and efficiently as the exhaust gases won’t push through a muffler.

How often should you Fill the DEF Tank?

It depends on the engine, the size of the DEF tank, the MPG you drive at, and the usage of your diesel truck. For example, if you have ¾ ton pick with a 6.7L diesel engine and run it lightly, you may have to top up the DEF tank every 8000 miles.

How often should you Fill the DEF Tank

However, a light-duty truck consumes around 2 to 3 gallons DEF per 800 miles on average regarding the tow loads. A diesel truck with 20+mpg can run around 8,000 to 10,000 miles on 10 gallons of DEF. while dodge ram gauge readout and GM has a digital readout to show the remaining DEF in the tank, Ford turns on the DEF light if the fluid level drops down on the tank.

Medium and heavy-duty vehicles consume more DEF which is around 2 to 3% of fuel consumption. It means these vehicles consume 1.2 to 2 gallons of DEF per 65 gallons of diesel fuel burning. So if your vehicle has 5 gallon DEF tank, you’ll have to refill it every 3 to 4 times of refilling the fuel tank.

How to Reset the DEF Sensor?

Driving with a DEF light on can activate the limp mode and limit the vehicle’s speed to 5 mph. it can also hamper mobility and makes driving inconvenient. In this case, resetting the DEF sensor can be helpful that can be done following these methods:

1, Refilling the DEF tank

In most cases, refilling the DEF tank will reset the warning light automatically as it mostly turns on due to lack of fluid. Consult your car’s owner’s manual to fill the tank to the proper level and then restart the vehicle to turn off the warning light.

Refilling the DEF tank

2. Disconnect the Battery

If the warning light is still on after refilling the tank, disconnect the battery’s negative terminal for around 30 seconds. Then reconnect the battery’s terminal and turn on the engine. it’s the simplest way to reset the DEF light of most diesel vehicles. If it doesn’t work, your vehicle needs a diagnosis.

Disconnect the Battery

3. Use an OBD2 Scanner

Connect an OBD2 scanner to your vehicle to clear the error code that may trigger the DEF warning light. If so, you’ll notice the P203F code on the scanner. Erase the code, disconnect the scanner, and restart the car. The DEF light should disappear.

Use an OBD2 Scanner

4. Reset the Dashboard

You can also reset some vehicle’s dashboards to clear all the error messages and warning lights including the DEF light. If your vehicle allows it, check its owner’s manual and follow the procedure to clear the error light.

5. Cycle the Ignition

After refilling the DEF tank and/or trying any resetting methods, you may need to cycle the key a couple of times to reset the DEF sensor. To do so, turn on and off the ignition using a key three to five times and then the DEF light should deactivate.

6. Bring the car to the service center

If neither of those methods works, the damaged DEF filter or clogged exhaust system can be the root of the problem. If so, inspect the car by an expert mechanic to solve the underlying problem and deactivate the warning light.

Bring the car to the service center

How to Prevent the DEF Light from Illuminating?

You can easily keep the DEF light off simply by topping the DEF tank regularly. So, inspect the diesel exhaust fluid level regularly and refill it whenever needed. However, you must read the car’s owner’s manual carefully before attempting to refill it to find the DEF filler port and refill the right type of fluid and avoid contaminating the fuel.

A vehicle’s computer monitors the exhaust gases and insets sufficient DEF to reduce NOx level. The engine creates more gases when works hard. So it’ll burn more DEF if you run the vehicle at a higher speed or carry heavier items. In this case, you have to top up the fuel tank more frequently. If you run out of fuel while driving on the road, visit the dealership or any parts store.


Why is the DEF light on but the tank is full?

If your car shows a DEF light when the tank is full, check the float as it can stuck in one spot while the fluid moving and cause this error. If so, shake it loose or re-install it to deactivate the warning light. You should also check the Doser valve or Def pump.

Can low DEF fluid cause a check engine light?

Recently, engine manufacturers add diesel particulate filters (DPFs) to their after-market standards to maintain exhaust levels more efficiently. So, low diesel exhaust fluids trigger the check engine light. Ignoring this warning light can put you in trouble while driving on the road.

What happens if you put water in a DEF tank?

A car’s SCR system is designed to use DEF, not water. If you accidentally put water in the DEF tank, it can damage the SCR system, and the catalytic conversion won’t be able to reduce NOx emission.

Final Words

You can’t drive very far with DEF light on particularly safely and efficiently. However, your car will let you drive a few miles slowly so you can refill the DEF tank before it shut down the engine.

Ignoring the DEF light can pollute the environment, clog the exhaust system and trap you in the middle of the road. So, keep some gallons of DEF on the car if you don’t want to tow your vehicle or spend thousands on engine repair.

My name is Henry Roberts and I am an experienced car mechanic with over 15 years of expertise in the automotive industry. I am currently running a Auto Repair Shop in Denver, Colorado. I have been servicing and repairing vehicles at my shop, where i have built a solid reputation for my skill and knowledge. I am passionate about sharing my automotive knowledge through blogging and my goal is to provide accurate and helpful information to car owners to keep their vehicles in perfect condition.

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