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DEF Head Problems Are Disabling Diesel Vehicles

DEF heads are failing at alarming rates, leaving vehicles disabled in masses because replacement parts are not available. If you drive a diesel truck, car, or RV made after 2017, then your vehicle probably requires diesel exhaust fluid (DEF). You need to understand the definition of a DEF head and how to avoid being stranded by it.

Join us as we learn more about DEF and the major problems plaguing the DEF Head, as well as some clever solutions if you end up with this problem.

DEF Head Problems Are Disabling Diesel Vehicles

The DEF unit in a diesel vehicle is a part of the engine maker’s push to build a more environmentally sound engine. It helps to lower a diesel engine’s emissions by converting some of the bad into better, like nitrogen and water. 

While these systems have made huge advances in reducing dangerous diesel emissions, they have caused many drivers a huge headache.

Sensors used in the DEF systems have been failing and causing major downtime for operators and owners because replacement parts are back-ordered due to the global chip supply shortage.

When a part does become available they are usually selling at extremely high prices 200-400% higher than normal because of the shortage.

Pro Tip: We took a closer look to uncover Do You Need to Bulletproof Your Diesel Engine?

What Is Diesel Exhaust Fluid?

Diesel exhaust fluid (DEF) cleans up the exhaust from a diesel engine. It consists of 32.5% high purity urea and 67.5% pure deionized water. It’s injected into diesel exhaust before the Selective Catalytic Reduction (SCR systems)

Once injected DEF becomes ammonia vapor before it enters the SCR and reacts with nitrogen oxide to form simple nitrogen and water before it dispels into the atmosphere. If your diesel engine has an SCR, you will need to add DEF to your system regularly. DEF enters the exhaust stream after passing through the DEF head on the vehicle. 

DEF Fluid bing poured into car
Close-up filling of diesel exhaust fluid from a canister into the tank of a car.

Nearly all vehicles manufactured after 2010 will have DEF head technology. In 2008, the Environmental Protection Agency (EPA) started cracking down on the pollution generated by all diesel trucks on the roads. 

The law required all diesel truck manufacturers to incorporate SCR technology into their vehicles. From there, the standards fanned out to include cars, smaller trucks, and RVs with diesel engines. 

DEF always has its own tank on the vehicle and is indicated by a blue cap. Diesel exhaust fluid should never be added to diesel fuel, and diesel should never be in the DEF tank.

DEF is nonhazardous to handle however it is corrosive to some metals and leaves a crystalized residue when it dries. It can also cause staining to clothes.

How Does a Def Head Work?

There are checks and sensors for many things under your vehicle’s hood. The diesel exhaust fluid is one of them and needs to be regulated too.

A DEF head is the part that draws DEF out of the tank to send it to the exhaust. The DEF head is inserted into the top of a diesel vehicle’s DEF tank. This part includes a heating element, filter, and monitoring system for the quality of the engine’s diesel exhaust fluid. 

It’s the monitoring system that has been so problematic.

DEF Head
This is a DEF Head, it includes sensors, heaters and sometimes filters.

The DEF head monitors the quality, quantity, and temperature of your fluid. If the monitoring system determines that the DEF isn’t up to par in some way, it will send a warning signal to the engine. The vehicle operator also received a notification. If the problem persists, the monitoring system will impede the function of the engine or even force it to be shut down. 

The vehicle will shut down or be limited to only 5 miles per hour when the diesel exhaust fluid isn’t correct.

For the driver, there will be a warning light on the dash accompanied by a sort of countdown. Listen to what your vehicle tells you, and correct the problem as soon as possible. If you don’t, your vehicle may go into limp mode.

Join The Debate: We compared diesel versus gas trucks to decide which is better. Find out which won!

SCR Filter System
This shot from a Cummins Filter Manual Shows The SCR System in the exhaust.

What Are the DEF Sensor Issues?

Just like your fuel level, you need to know the level of DEF in your tank. So, the DEF head includes a level sensor.

However, in 2017 the EPA mandated additional sensors that would check for the quality of DEF fluid. This was because some owners were adding diluted or incorrect fluids to the tank either to save money or just as an accident.

These new sensors have begun to fail at a rapid rate. With the global chip shortage in the early 2020s, this sensor failure has caused major problems for diesel operators.

Many times the solution to a failed sensor is to replace the entire DEF head, but usually, there is nothing wrong with it. In fact, we have heard reports of some users clearing the codes repeatedly to keep the engine running. Usually, the system will operate just fine, it just thinks that there is something wrong with the fluid.

Sometimes these shutdowns will require a dealer to reset the sensors to be able to get the vehicle back on the road, so it can be a real headache.

Porcupines chewed through our diesel truck’s DEF system wires in the middle of nowhere Canada once and had to repair the wires before we kept driving. Otherwise, we may have ended up with a dead truck that would have needed a very, very long tow.

You can watch that story in the video below.

Trouble with Wildlife, Canol Road, Top of the World Highway & Crossing into Alaska! | Go North Ep 9

What Happens If Your Def Head Fails?

When the DEF head on a diesel vehicle fails or finds a problem with the diesel exhaust fluid, you will get a notification via the vehicle’s dashboard. It will vary from vehicle to vehicle but usually, it will be the check engine light, DEF fluid light, or even a message on the dash.

When the sensors fail the vehicle will usually display a fault code that says “DEF Quality”, “DEF Tank Level” or “DEF Tank Temperature” because these are the things it monitors.

If the problem persists, the monitoring system will impede the function of the engine or even force it to be shut down. Frequently the vehicle will shut down or be limited to only 5 miles per hour when the diesel exhaust fluid isn’t correct.

For the driver, there will be a warning light on the dash accompanied by a sort of countdown to when the engine will shut down.

DEF Head Simulator
Some have come up with simple electronic solutions to simulate the DEF Sensors to get the engine to run properly

Some are Taking Advantage of the Shortage 

Replacing a DEF head is expensive, but with the global shortage some are selling DEF heads for thousands more than normal. This is an unfortunate situation because some are looking to alternative options or removing the systems completely.

The industry is looking into other solutions; therefore, the supply of replacement DEF head parts is very slim. Manufacturers are somewhat stingy in their shipping practices, but try to assure that everyone who breaks down due to DEF head failure can get the piece they need, even if it takes a while. 

Lets take a look at some alternative options to get you back on the road.

What Can You Do If Your DEF Head Fails? 

As of this article, the EPA has been working with manufacturers to come up with a temporary software solution. It might get your DEF head working again because it’s usually just fine, just a sensor. It’s extremely frustrating to replace an entire module like the DEF head for a non-essential part.

As mentioned above, we have heard reports of some owners using code readers to continuously clear the DEF codes before the engine shuts down. This is, however, unverified by us and would still be a risky move to continue operating with the condition.

The correct course of action is, of course, to replace the DEF head or the sensor. But they may not be available.

So, to get back on the road, many tech-savvy users have figured out a way to fool the vehicle into thinking the tank is fine. They are doing it by creating a simulator that sends the correct info to the engine.

Keep in mind that it’s technically illegal to modify a vehicle’s emissions. But in this case, the system is usually operating normally, except for this one sensor.

Some DIY enthusiasts build their own simulators and run some custom software themselves. Others are selling entire devices that are ready to go for your particular vehicle. Below is a video about one user who bought one of these devices.

Recover From A DEF Head Failure | Spartan AND NOW Freightliner Chassis

Pro Tip: Need to do some maintenance on your RV? Keep an eye out for these 5 Red Flags to Watch for With an RV Repair Shop.

Don’t Let A DEF Head Failure Be A Death Blow

While extremely frustrating, we are all about the simulation fixes that users are coming up with. Hopefully, the chip shortage will resolve, and manufacturers will come up with better, more reliable sensors.

In the meantime, it’s a great idea to have an understanding of this issue. While we hope you never deal with this, it’s better to have the knowledge to get back on the road quickly in case it comes up.

Have you had a DEF issue or know someone who has? Tell us in the comments!

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About Tom Morton

Tom, a Pacific Northwest native, is our technical genius. Born in Washington and raised in Alaska before settling in Michigan. He's the man who keeps our operation running, both figuratively and literally.

With a background in Electrical Engineering, Tom specializes in RV solar systems and lithium batteries. He made history as the first documented individual to use a Tesla battery module as an RV battery. Tom has personally assisted countless RVers with system installations and has educated thousands more through his videos and articles.

Cinematography is another of Tom's passions, showcased in his work on the Go North series. You can see his camera skills on display in The RVers TV show on Discovery Channel and PBS where he also stars as a co-host.

Tom's mechanical expertise extends beyond RVs to boats, planes, and all things mechanical. He's renowned for taking on maintenance and repair projects single-handedly and is often spotted underneath RVs, making him the technical backbone of our endeavors.

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Wednesday 19th of October 2022

I have just retired from the transportation industry. DPF systems are the most costly thing our nation has ever incurred. You think Covid was bad? The total cost is staggering. You can't even quantify the total damage caused. Between down time, "limp time", excess driver labor, crazy high parts costs, maintenance costs, un-diagnosible systems,and over a 150% increase in new trucks that started well before covid. This BS has been going on since 2008. It's no wonder many operators are "deleting" these systems. Imagine making everyone use a toilet that worked properly only half of the time, costs over $2,000 each, the plumbers would have to use a computer that tells them to replace part after part until it worked and then a short time later repeat the process. We would all be as full of shit as the EPA.


Tuesday 18th of October 2022

Remove def head. Wash with VERY HOT PLAIN WATER. Viola. Works. Now pay dealer to reset ur truck. Parts biz is a scam for these sensors ps. EPA allows you to drive with bad sensor until 10 days after it's in stock.

Dave Miller

Wednesday 31st of August 2022

Good afternoon to the Mortons! I have a 2022 F350 diesel with a Bigfoot 10.6 on it. Just got back from a trip up the North Shore of Lake Superior. Averaged 15.3 MPG so was very happy. Any additional information on sources of a simulator would really be appreciated. I also have some questions on upgrading my camper to better solar if you wouldn't mind sharing an email address. Thank you, Dave Miller


Sunday 28th of August 2022

Yes, I had the sensor fail on my Ram 2016 2500 with about 40k mi. Dealer replaced the tank/sensor, it came as 1 unit. They charged me $2100 for a 5gal plastic tank so the sensor must have cost $2k!!! I told the dealer I wanted the old tank so I saved it for when the sensors become available. Just wondering if outside freezing temps effected the sensor? Should I not keep the tank full in hot or freezing conditions? I’ve been trying to keep DEF fluid level low so I can put new fluid in for a long trip. Any thoughts from others, am I doing the right thing?


Tuesday 18th of October 2022

@Jeff, keep FULL. use straight from pump. Or inform yourself on the half life of def and what sitting in the heat does to it. It spoils in the container. YouTube is a wonderful source of information.