How to Reset DEF Light (Sprinter, Peterbilt, Kenworth, F350)

Modern technology is full of surprises. DEF is supposed to help the engine as well as the environment which is why it is used in diesel engines

The reset process may be different in different brands of diesel trucks. For example, the diesel Dodge Ram has 3 ways to reset it. One of those ways is to simply refill the tank. After adding between 2 and 2 1/2 gallons of DEF fluid, start your engine. This should reset the error code light. Then again, it may not.

To learn more about DEF and resetting the lights on different vehicles, just continue to read our article. It has that information and more. Take a few minutes to see how this important information can solve your DEF light error code issues.

What is DEF?


For those of you who do own a gas engine, the letters DEF may be speaking a foreign language. You shouldn’t encounter this as gas engines deal with emissions in different ways than diesel engines do.

DEF stands for Diesel Exhaust Fluid and it is a liquid emission control product that is required on all modern diesel engines. The good news is that it is not injected into the cylinders with your diesel fuel. Instead, it is injected into the exhaust stream.

Made from 32.5% urea in 67.5% de-ionized water the solution is colorless and has a smell of ammonia to it. Plus, it is non-hazardous. Its purpose is to remove harmful NOx emissions and is found in Selective Catalytic Reduction (SCR) technology.

DEF Warning Light Reset

Most of the time, all you have to do is refill the DEF tank to reset the light that has been illuminated on your dashboard. This is the go-to option and the first one that should be tried. But it is important to understand the different signals and what they are saying to you.

1. DEF light is on and steady- the amount of this fluid is below the minimum required and you need to refill the tank.

2. DEF light is flashing- your reservoir is critically low and to avoid bigger issues, you need to stop everything and refill the tank

3. DEF light is flashing & the check engine light is on- you went too long before refilling the tank and now you will lose power in your engine. Normal engine power will be restored once you refill the tank

4. DEF light is flashing, check engine light is on as is the stop engine light- You are out of DEF fluid and your vehicle will be limited to 5 mph until you refill the tank.

In all cases, the DEF light should be reset once you refill the tank.

Sprinter Def Light Stays On

This light will remain on if you do not add enough DEF fluid to the tank. The tank is supposed to hold 6.5 gallons of fluid and when the light turns on, it means that you are down to 1 1/2 to 2 gallons of fluid in the tank.

To turn the light off, you are supposed to add a minimum of 2 gallons of DEF fluid. Then, it may take a couple of restart cycles to fully clear the light and have it shut off. When the light stays on, it is also possible that the DEF pump module has failed or a NOx sensor failed on you.

When the light stays on, you should go to a dealer and let them diagnose the problem. You may be in over your head if you try to do it on your own.

DEF Warning Light Reset Peterbilt


The general consensus is that all you have to do to reset this model’s DEF light is to refill the tank. It is that simple and there are not supposed to be any complicated procedures involved.

However, if the light stays on after refilling, you should take it to your nearest Peterbilt dealer and have them look at it. This system is not something you can really do on your own as you may, like Dodge, need a scanner to read the fault codes.

The key is to make sure you use the right DEF fluid. There may be a difference between truck makers but we are not sure about that issue. if there is a difference, you need to make sure you use the fluid made for a Peterbilt vehicle.

How To Reset DEF on Ford F350

Resetting a DEF light indicator is not always as simple as filling the tank. You will find, as one owner did, that simply refilling his Ford F350 DEF tank did not reset the light and he was facing a different problem.

He was running out of the mileage grace period before the engine derated to idle mode and he would only be allowed to travel at 5 mph. When this happens there may be a fault in the fluid level sensor circuit. As technology would have it, you would not be able to reset the DEF light or fix the problem.

This is a dealer-only fix and you need to be near a dealer and get to it before your truck’s engine shuts down and you are stuck. There is really nothing for you to reset when the computer does not recognize the fact that you refilled the tank to proper levels.

DEF Light on But The Tank is Full International

This problem is a little bit more complicated to solve but it is possible to fix it if you have the right part. But before we get to that, you should check to see if the float is stuck. If it is that would be the reason why the light remains on even though the tank registers as full

Another possibility would be a dosing pump. It has 3 bolts securing it in place and you need to trace the line from the DEF to get to the pump. It should only take about 2 minutes to change the part and it may solve your problem.

Just do not confuse the dosing valve with the pump. They are very close to each other and it is easy to make this mistake. There are 3 lines attached to the pump as well so that should help you distinguish it from the valve.

Resetting DEF Fluid on Duramax


Like every other model of diesel truck we have already mentioned in this article, if the light does not go off when you refill the tank, then it is a dealer problem. On the Duramax, there is a check button that is supposed to reset everything but it may not work for the DEF sensors.

The key to resetting the light is to make sure you added enough fluid to the tank. If you do not add enough, then the light will probably stay on. The illuminated light is not indicating another more serious problem. it is just telling you that you did not add enough.

This makes diagnosing the problem a little more difficult to do and you should let the dealer handle the problem. The problem with letting the dealer handle it is that you may be charged a lot of money for a simple fix.

DEF Light on But The Tank is Full Kenworth

This could be the result of a sensor failing. For some reason, these parts are not the best quality and they can fail at any time on you. When that happens, and it is only a small part, your big truck and load are stuck wherever you are until you can get it repaired.

Or if you happen to have a PACCAR engine in your Kenworth, you may only need a software update to get the light to turn off. Technology has made life a little bit more difficult in these circumstances. These updates cover the following parts- Dosing Control Unit, Common Rail system, and Aftertreatment System.

In other words, you may not be able to reset the light because of these malfunctions or out-of-date parts.

How To Reset Def Light on Hino 268

It is not all about the light. While refilling the tank is supposed to reset the light, there may be another issue that stops you from completing that task. For example, this model of vehicle.

A mechanic changed the DEF filter, the fuel filter for the burner, and so on and it drove fine for 30 minutes. However, the vehicle came back right away to the same mechanic with the same problem.

This problem could be sourced in the DEF injector or in the box sensor. If you do not have the proper tools to pinpoint the issue, then the vehicle has to be sent off to the dealer to be fixed.

As we said, for non-dealer resets, all you have to do is fill the tank up. If the light stays on, that indicates that there is another problem beyond your skill set to solve.

How To Reset Def Light on Ram 2500


The first step in this process is to fill the DEF tank up with 2 to 2 1/2 gallons of DEF fluid. When the computer does not detect the addition of the fluid, it won’t reset the DEF warning light.

That is when you can go to step two and use the dashboard reset button. Just press the button located on the bottom corner of the dashboard, then while pressing the button turn the key in the ignition to the on position but do not start the truck.

Now release the reset button and you should see the word ‘done’ on your dash. At this point start your engine and look to see if the DEF light is out. If these two methods do not work, unhook the negative wire from your battery and leave it off for 30 minutes.

This should do the trick but if it doesn’t, you will need a scanner to read the codes which you may not have. This means a trip to the dealer and letting them handle the problem.

DEF Warning Light Reset Freightliner

In this case, it could be something as simple as the NOx sensor going bad on you. If the code is 4364 then your light is going to be tripped and have you thinking it is the fluid level.

Resetting is easy as all you do is pour fluid into the tank and the light should go off. However, if the fluid level is not the problem, you have to spend time searching for the source of why the light is not turning off.

In one case, it was a bad NOx sensor that cost one trucker over $7000 to find and fix. A rarer problem is when the DEF pump starts sucking air. It may be rare but it does happen.

You may not see a problem when you do a parked regen or an SCR performance test but when you drive it and the NOx box or sensor is faulty, it will show up when you drive the truck. Again, this is a trained technician’s territory and not a DIY fix.

Why is My DEF Light Flashing?

This is not a good sign when you are driving far from home. When you see your DEF light flashing it means that your DEF fluid is critically low. If you do not stop right away and fill it you can lose power to your motor.

That situation means that you will be traveling at much slower speeds than you would want to be going. The solution to this situation is that you need to stop right away and refill the tank as soon as possible. Don’t delay or you may cause yourself more trouble down the road that may become costly to fix.

When Does The DEF Light Come On?


The DEF light will come on when your DEF tank has low fluid in it. There is a minimum level that you should maintain at all times if you want to avoid seeing that DEF warning light.

While you can still drive at normal power and speeds it is best not to ignore the light. As the fluid gets lower, your car gets penalized and it may be reduced to idling speed. The light should be flashing, the check engine light should be on as well as the stop engine light will be illuminated.

Some people avoid this problem by removing the DEF system from their vehicles.

How Many Miles When DEF Light Comes On?

This may depend on the type of truck you own. One Duramax truck lasted only for 3200 miles before the DEF light came on. The calculation is that you will use approx. 1 1/4 to 1 1/2 gallons of DEF fluid for every gallon of diesel you own.

If you get 15 mpg then you should be down approx. 1 1/2 gallons of DEF after 15 miles. You can figure it out from there according to your personal mpg on your vehicle. However, the standard is between 1200 and 1500 miles before you start to see the light.

Towing will use a lot of DEF fluid.

What Happens When DEF Light Comes On?

With the first two warnings, illuminated and flashing light, you are being given a grace period. Nothing will happen to your vehicle if you keep driving with those lights on. When you get a flashing DEF light and a check engine light then you start to see the results.

This situation will derate your vehicle so that you will not go as fast or have the power to go up hills that well. If you have an illuminated DEF light & check engine with the stop engine light on, then you may only go as fast as 5 mph until you refill the tank.

What Does a DEF Light Look Like?

On the Duramax, this light looks kind of weird. Maybe it is designed that way so you will not mistake it for another warning light. It is hard to describe but underneath the upper symbol, there is a picture of waves.

To the right is 3 half circles and above are what looks like raindrops falling on a stapler. Your model’s DEF light may look a bit differently than this

Some Final Words

Technology may say it is upgrading engines so that the environment is taken care of. But what it is really doing is causing a lot of trouble for diesel truck owners. DEF is supposed to clean up emissions but the reality is it may clean out wallets when the system needs repairing.

It is up to you to decide if this is a good addition to a diesel motor or not.

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