How to Reset DEF Sensor? And When Will You Need This?

Diesel exhaust fluid (DEF) is a solution used in a vehicle's selective catalyst reduction (SCR) system to break down pollutants and harmful substances produced by a diesel engine. DEF helps to protect the vehicle and the environment by reducing harmful emissions. The DEF light lets you know when the level of DEF in your car is low. Sometimes the light will stay on even though you've refilled the tank. When this happens, you will need to reset the DEF sensor.

Emission system electrical problems highlights

• Level of urgency:medium
• DIY inspection:possible but complicated
• DIY repairs:impossible
• Can you drive?yes
• Price of repairs:$200 - $800
• If ignored:car stalling, getting stranded, various errors
• Ways to fix:OBD2 scanner check, sensor replacement

What Does the DEF Sensor Do?

The DEF sensor measures the level of DEF in the tank. When the sensor detects that the level in the tank is below the minimum acceptable amount, it sends a message to the ECU which turns on the DEF warning light on the dashboard.

Without a DEF sensor, your vehicle wouldn't be able to alert you when the DEF level is low, which would result in a damaged catalytic converter once it runs out. So it's easy to see how important the DEF sensor is in making sure your car runs safely.

Can You Drive With the DEF Light On?

When the level of DEF in your vehicle is too low, it might enter limp mode. When this happens, the engine power is severely reduced to protect your engine from damage. This allows you to drive your car to a safe location where you can refill the tank.

Your truck's exhaust system will slowly start to get damaged the further you drive with no DEF. This is why it is important to stop driving as soon as possible to refill the DEF tank.

If refilling the DEF doesn't remove the light, your car will remain in limp mode. In this case, you will need to reset your car's DEF sensor to turn off the light and get it out of limp mode.

When Do You Need to Reset the DEF Sensor?

You need to reset the DEF sensor when the DEF warning light on the dashboard won't go off. Normally the light will go off once the DEF tank has been filled to the appropriate level.

If you've refilled the DEF tank and the warning light hasn't gone off, you will only have a limited distance you can drive your car before it goes into limp mode. In limp mode, your vehicle will have severely reduced power, and will not be of much use to you. You'll want to reset your DEF sensor before this happens.

How to Reset the DEF Sensor

If the DEF warning light is lit up on your dashboard, you'll definitely want to reset it. If you continue to drive with the light on, your car will eventually go into limp mode, limiting the car's speed to 5mph. This is very inconvenient and hampers your mobility.

To reset the DEF sensor before this happens, you can try the following methods.

1) Fill the DEF Tank to the Appropriate Level

Under normal circumstances, you will only need to refill the DEF tank to reset the DEF warning light on your dashboard. This is the first thing that should be done when the DEF light comes on.

In some cases, filling the DEF tank might not make the light go away. This might mean that you haven't filled the DEF tank to an appropriate level. To find out how much DEF you need to add to the DEF tank, you can consult the owner's manual for your vehicle.

After filling the DEF tank, it might take a few restart cycles for the DEF warning light to go off.

If the light doesn't go off after a few restart cycles, then you will need to try one of the following steps to reset the DEF light.

2) Disconnect the Battery

One of the simplest ways to reset the DEF sensor is to disconnect the negative terminal of the battery for about 30 seconds. Doing this will reset the ECU, clearing any codes that have been stored. This is the easiest way of resetting the DEF and will work on most diesel trucks.

Once you have reconnected the battery after 30 seconds, start the engine and check whether the DEF light is still on. If you have done this correctly, the DEF sensor should be reset, and the light should disappear. If the light is still on, then it is likely that the problem is coming from somewhere else.

3) Use an OBD2 Scanner

If filling the DEF tank hasn't caused the light to disappear, then you can manually clear the DEF warning from your ECU using an OBD2 scanner.

• Plug your scanner into the OBD2 port of your car.
• Turn the ignition to accessory mode and read your car's codes.
• You might see a code like P203F, which is a code that indicates an issue with the level of DEF in the tank. If you see a different code, search for what it means to make sure it is a DEF code.
• Once you've found the relevant trouble code, hold down "erase" to remove the code from your car's memory.
• If the scanner asks you to, confirm that you want to erase the code.
• Read the codes again to ensure that the DEF code has been erased.
• Unplug your OBD2 scanner.
• Restart the vehicle.
• Check whether the light has turned off.

4) Reset the Dashboard

Some vehicles will allow you to reset the dashboard. Resetting the dashboard will clear all error codes from the dash, including the DEF warning. This option is not available in all vehicles.

The procedure for resetting the dashboard is different depending on your vehicle. Consult the owner's manual of your car to find out if you can reset your dashboard, as well as how to do it.

Once you have reset the dashboard, start your engine to verify whether the light has been successfully cleared.

5) Cycle the Ignition

Sometimes after filling the DEF tank, the warning light will remain on for a few more key cycles. To fix this, you can try cycling your key a few times after filling the tank.

You can also try cycling the key after attempting any of the previous methods, in case they didn't immediately reset the warning light. In some cases, you might need to cycle the key a few times after attempting to reset the DEF sensor.

Performing a key cycle is a straightforward process. To cycle the key, simply turn on the ignition, and then turn it off. This counts as a single key cycle.

To reset the DEF sensor, perform 3-5 key cycles. Make sure to check the DEF light after each cycle to see if the warning light has been reset.

6) Take the Vehicle to a Mechanic

If you've attempted every method of resetting the DEF sensor and none of them have worked, you should take the car to a trustworthy mechanic. There is a possibility of an issue in the selective catalyst reductant system.

If this is the case, the mechanic will inspect the system and let you know what repairs, if any, need to be made. They can also help you reset the DEF sensor.

How Often Should I Change My DEF

While it is important to refill your DEF whenever it runs out, the fluid in the tank will also need to be replaced fully once in a while. This is important to do because dirty or contaminated DEF fluid can cause damage to the catalytic converter. Debris or clumps can accumulate in the tank after a while and cause clogs or blockages in the system. This will result in very expensive repairs.

It is recommended that you change your DEF every 200,000 miles.

Conclusion

The DEF sensor in your vehicle measures the amount of diesel exhaust fluid (DEF) remaining in the tank. DEF breaks down toxic substances and pollutants in the vehicle's exhaust and helps to protect the environment and passengers from harmful emissions.  When the level of DEF in the tank is too low, the DEF warning light will come on.

Sometimes you will need to reset the warning light or the DEF sensor. The process might be different depending on the make and model of your vehicle, but there are some methods that commonly work for most cars.

You can start by refilling the DEF tank to the recommended level. If this doesn't work, you can try to clear the DEF warning from the ECU by disconnecting the battery for 30 seconds. Another way to clear the code from the ECU is by erasing the code with an OBD2 scanner.

After attempting any of the previous methods, you might need to perform 3-5 key cycles before the DEF sensor is reset.