DEF Head Issues? Everything RVers Should Know About DEF (Diesel Exhaust Fluid).

The EPA has authored numerous regulations in vehicle emissions levels in an effort to improve the quality of the air we breathe. Increased regulations on diesel engine emissions are one area that has affected owners of diesel powered motorhomes since 2003. The EPA 2007 regulations were a big change requiring ultra-low sulphur diesel fuel and the addition of a Diesel Particulate Filter (DPF) to the exhaust system to reduce the level of soot in the exhaust.

The next big step was the Tier IV EPA 2010 emissions standards which took effect for the most part in model year 2011 motorhomes. These regulations were a significant step in reducing pollutants from diesel engines and required some major changes in the design of diesel engine emission systems. The prevalent method of achieving this was Cummins’ implementation of Selective Catalytic Reduction technology, commonly referred to as SCR. In addition to the Diesel Particulate Filter, SCR technology adds more complexity to the emissions system but significantly lowers emissions below the EPA 2007 specifications.

The SCR aftertreatment system resides above the muffler on the exhaust manifold side of the engine.
The SCR aftertreatment system sits above the muffler on the exhaust manifold side of the engine.

The SCR system adds a decomposition reactor where Diesel Exhaust Fluid (DEF) is injected into the exhaust stream where it forms ammonia vapor. The ammonia and nitrogen oxides in the exhaust flow together and pass into the SCR Catalyst, where they react to form nitrogen and water vapor and reduce emissions to near-zero levels. SCR is aftertreatment technology so it destroys these harmful emissions after combustion, which gives the engine manufacturer the ability to fine tune their engines to produce maximum power, efficiency and fuel economy. Other than clean exhaust, the biggest impact upon coach owners is the necessity of having to maintain a supply of  DEF in the coach’s storage tank .

However, changes to the EPA requirements in 2017 have resulted in a large number of DEF issues involving failed DEF sensor problems, resulting in forced engine shutdowns that have caused major downtime and cancelled trips for some owners. Before we get into the specifics of that and how to deal with it, let’s first begin with a better understanding of the components of this system and DEF itself.

Refilling a DEF tank from a 2-1/2 gallon jug.
A shot of refilling a DEF tank from a 2-1/2 gallon jug.

What Is DEF?

Diesel Exhaust Fluid, or DEF, is a product designed exclusively for use in diesel engines using SCR emissions technology. It’s basically a non-hazardous solution of 32.5% urea and 67.5% water. It’s clear and colorless and has a slight smell of ammonia. DEF isn’t something you can make yourself and engine manufacturers specify that any DEF used should by certified by the American Petroleum Institute (API), the same people who rate engine oils and other petroleum products. The API has a Diesel Exhaust Fluid Certification Program that allows DEF producers to display the API certified label on their DEF packaging.

The production of DEF is governed by the ISO 22241 standard. This ensures that DEF is produced with an exacting 32.5% urea concentration. This concentration is also designed to offer the lowest freezing temperature of DEF, 12o F. Contaminated fluid can damage SCR injectors and catalysts so the level of impurities is limited to strict tolerances and DEF quality is also monitored. Urea used in manufacture of fertilizer is not allowed and only distilled or deionized water may be used in DEF production. Even the containers are regulated by ISO 22241 because DEF is corrosive to carbon steel, copper and aluminum so these containers may not be used. If your DEF container bears the API DEF Certification Mark, you can be assured that the product meets the ISO 22241 standard and is safe to use in your vehicle.

Just how much DEF your coach will use varies with the size of the engine and how hard you are working the engine. A common statement is that DEF usage will equal 2% of your diesel fuel usage but this is just a generalization. Lighter RVs with small displacement engines that are driven more leisurely have seen figures as low as 1.5% while larger heavy coaches with the 15 liter X series engines have gone as high as 4%, depending on how hard you are working the throttle and how much weight you are towing.

Instrument panels include a DEF level indicator
Instrument panels include a DEF level indicator as well, such as this common 4 LED bar graph display built into the fuel gauge.

Handling DEF

DEF isn’t overly difficult to handle but there are a few considerations to keep in mind. Bulk DEF is available at truck stops from dedicated DEF pumps located on the driver’s side fuel island, oftentimes right next to the diesel fuel pump and located behind a rubber flap to prevent freezing in cold weather. DEF is also available in 2.5 gallon containers at auto parts stores, gas stations and most large retailers, such as Wal-Mart. As long as the container bears the API Certification Label it will meet the ISO 22241 standards so paying more for certain name brands of DEF won’t give you a better quality of DEF. It’s only ammonia water and doesn’t contain additives like a sophisticated engine oil or high tech lubricant.

DEF can evaporate if stored at high temperatures for prolonged periods because it is 67.5% water but field tests have shown that there isn’t a significant risk of evaporation from DEF tanks as long as you keep your tank or container securely closed. DEF does have a shelf life of two years but this can be reduced if the DEF is exposed to direct sunlight or if the temperature remains above 86o F for sustained periods. DEF packaging does have an expiration date so keep that in mind if you plan on stocking up on DEF. Ideally your DEF should be stored in a location where temperatures do not drop below freezing or exceed 85 degrees and will be consumed within one year of purchase.

DEF Tank
The tanks are equipped with a blue filler cap that is appropriately labeled. The filler necks are designed to prevent the larger sized diesel fuel nozzles from accidentally being used in these tanks.

DEF is always stored in its own tank and should never be put into a diesel fuel tank nor is diesel fuel ever to be put into a DEF tank. Fortunately there are a few safeguards in place to help prevent this from happening. Diesel fuel nozzles are 0.87” (22mm) in diameter while DEF nozzles are 0.75” (19mm) in diameter so this should prevent anyone from accidentally inserting a diesel fuel pump nozzle into a DEF tank. DEF tank caps are also blue in color to help identify them and separate them from diesel fuel tank caps. Diesel fuel is lighter than DEF and will float on the top of the DEF if it somehow managed to get into the tank but even a small amount of diesel fuel will damage the SCR system so do not run the engine. Instead, call a service center immediately and do not drive the vehicle until they remove the diesel fuel from the DEF tank.

On the other hand it is possible to insert the smaller DEF nozzle into a diesel fuel filler neck. If this happens, do not start the engine. DEF contains 67.5% water and this can have disastrous effects if mixed with your diesel fuel, including exploding fuel injector tips. If this happens, do not drive the vehicle and call for help to have the fuel system drained or cleaned to remove the DEF. Some bulk DEF pumps have magnetic switches built into the nozzle to help prevent this from happening. The DEF tank has a magnet placed in the filler neck that allows the DEF nozzle to open up and dispense DEF. The nozzle will not allow any flow without that magnet, such as when inserting the DEF nozzle into a fuel tank filler. However, not every DEF pump has these magnetic switches and neither do any of the 2.5 gallon jugs so you do need to pay close attention to which tank you are adding DEF to.

DEF is not hazardous to handle but it can stain clothes if you spill any on your clothing. If you do spill any DEF on your clothing just wash it away with water. If you spill a small amount on the ground just rinse it with water or wipe it up with a paper towel or rag. Once any residue dries out it will turn to crystals, which can also be rinsed away with water. As mentioned earlier, DEF can be corrosive to carbon steel, copper or aluminum so if you spill any on those metals you may want to rinse that off fairly quickly.

Pump DEF nozzles
Pump DEF nozzles are stored behind a rubber flap to help keep it from being confused with diesel fuel nozzles and to help prevent it from freezing.

DEF In Your Coach

Now that you know what DEF is let’s take a look at how it is implemented in your coach’s emissions system. It begins by storing the DEF into the typical 10 to 15 gallon polypropylene storage tank, which is required in order to prevent corrosion between the DEF and any metals. There are limits on the length of the hoses that connect the DEF tank to the engine’s emissions systems so you’ll find the tank located at the rear of the coach on a diesel pusher chassis or at the front if a front engine vehicle such as a Super C or Sprinter type chassis. Most side radiator chassis don’t have enough room to place the DEF tank on the driver’s side of the coach so the tanks will be located on the curbside. Unfortunately, pump DEF at truck stops is always on the driver’s side to accommodate the driver’s side DEF tanks on trucks so this doesn’t work well for an RV but in recent years the chassis manufacturers began to add additional driver’s side DEF fills to make it more convenient for a motorhome owner to use pump DEF when refueling. This does require moving the coach forward after pumping fuel so that the pump nozzle can reach the driver-side DEF filler at the rear but this a small inconvenience that is worthwhile because it allows you the ease of refilling your DEF tank and receiving the bulk pump price versus the higher cost of retail DEF jugs.

DEF freezes at 12o F so it needs to be kept warm enough to allow tit to flow. The engine’s cooling system passes heated engine coolant through a heating element in the DEF tank to warm the DEF enough to allow this to happen. In extremely cold temperatures the DEF will not initially flow until the heat in the engine coolant has warmed up the DEF, which happens fairly fast, and the emissions controls will allow enough time for this to happen without throwing an error code. Whenever the engine is shut down you may hear a buzzing noise coming from the rear of the coach. This is an electric purge pump that will run for approximately 60 seconds and will drain all of the DEF from the hoses and return it to the tank to prevent any freeze damage to the lines and valves should the temperature drop below freezing. DEF expands about 7% when frozen so you also need to keep a bit of air space above the DEF in the tank to allow for expansion during cold weather. The filler neck in DEF tanks is generally low enough to prevent over-filling but if your curbside tank also has a second driver’s side filler cap you will want to keep an eye on this. Don’t fill it all the way up or else the DEF won’t have room to expand and damage will occur.

DEF Tank Cap
The tanks are equipped with a blue filler cap that is appropriately labeled. The filler necks are designed to prevent the larger sized diesel fuel nozzles from accidentally being used in these tanks.

The DEF is then sent to a dosing valve. This valve is electronically controlled and sprays DEF into the decomposition chamber, which is located immediately after the Diesel Particulate Filter, and is both a filter and a catalyst that removes carbon particles from the exhaust gas and traps them into a wall flow filter. At the same time nitric oxide in the exhaust gas is then converted to nitrogen dioxide in the diesel oxidation catalyst. As the nitrogen dioxide flows through the wall-flow filter it reacts with the carbon to produce carbon dioxide. As the exhaust flows out of the DPF and into the decomposition chamber a light mist of DEF is sprayed from the dosing valve into the decomposition chamber. It then forms ammonia through a series of chemical reactions. Together, the NOx and ammonia pass from the decomposition reactor to the SCR catalyst chamber where they react to form nitrogen and water vapor. The end result is exhaust with near zero emission levels.

DEF is a critical component and without it your emissions system will not work. EPA requires that the vehicle emissions system must be fully operational at all times so certain safeguards are put in place to ensure that you cannot continue to operate the vehicle without DEF. To ensure you know how just much DEF is in your tank, a DEF gauge will be located on the instrument panel. In most cases this is a series of four LED bars built within the diesel fuel gauge. Four green bars will be displayed whenever the DEF tank is fuel. Three green bars indicate ¾ full while two green bars indicated ½ full and one green bar indicates ¼ full. Once the level of DEF in the tank reaches 10% that last green bar will turn to amber. Typically the LCD Information Center on the instrument panel will display a “LOW DEF” warning in addition to displaying a warning icon on the panel. The vehicle will continue to operate normally but if you have allowed the tank to get this low this is the point where you really need to consider how soon you can add more DEF to the tank. If you continue on without adding, the amber LED bar will turn red when the DEF level in the tank reaches 5%. The Information Center will now display “ENGINE PERFORMANCE DERATE EMMINENT”. You now have very little time left to add DEF before your engine derates. Once the level drops to 3% the display changes to “ENGINE PERFORMANCE DERATE ACTIVATED” and your engine will be derated and there will be a 25% reduction on its torque output. You’ll still be able to limp off the road at reduced power but you’ll still be burning DEF as you do so and the next step is the final step.

DEF filter location
The DEF filter is located at the rear of the DEF tank at the very bottom. Access to the DEF filter is gained by unscrewing the removable round filter cover.

Once the DEF tank gets down to 0% the red LED bar remains illuminated and the Information Center now displays “SPEED RESTRICTION ON. DEF REQUIRED”. Engine torque will now be limited to 60% and the vehicle speed will be limited to 5 MPH. That’s enough to pull off to the side of the road but you will need to fill the DEF tank to at least 10% in order to drive to a location where you can top off the tank. Carrying a 2.5 gallon jug or two of DEF as a safety precaution can be a wise choice, even if you normally refill with pump DEF at truck stops.

DEF systems aren’t high maintenance. If you store your coach over a longer periods of time or run the engine for very short and infrequent runs your DEF can get old and beyond its shelf life. In that case the best practice is to drain the DEF from the tank and replace it with fresh every year. There is a DEF filter located near the bottom of the DEF tank. This filter should be replaced every 200,000 miles or two years, whichever comes first. The filter is easily removed with a 1-1/16” 12-point socket and extension. If the cap wasn’t excessively tightened you may be able to remove the DEF filter cap with a channel-lock pliers without damaging the cap. The filter is at the base of the tank so you’ll be looking up at it. Just be sure that you are off to one side when you remove it because a bit of DEF will dribble out of the filter housing. The actual Cummins DEF filter is a small cartridge filter and comes with a small tool to help yank the filter out of the housing. This filter is sometimes forgotten by owners when servicing their chassis but a plugged or restricted filter can lead to a failed DEF pump, which is a more expensive repair, so do not neglect this filter.

Cummins DEF filter and tool
The Cummins DEF filter includes a small removal tool to aide in removing the filter from the housing.

DEF Sensor Issues

The DEF head are inserted into the top of the DEF tank. The head consists of the DEF pickup tube, the engine coolant heater tube and the DEF sensor. The sensor was originally designed to sense the level of DEF in the tank but in 2016 the EPA mandated that new sensor designs were required that also detected the concentration level of DEF to ensure that owners weren’t diluting their DEF with water and rendering the emissions system ineffective. These sensors showed up mainly in the 2017 model year coaches. Unfortunately, while the original 2016 and prior year sensors were trouble-free, these new sensors began failing at a rapid rate. In particular, the sensors used in the Spartan chassis had the most failures. These sensors were made by Shaw and had issues where the electronic circuit board had failed, which was determined to be heat related due to the close proximity of the engine’s exhaust system as well as the fact that hot engine coolant was constantly circulating through the DEF head.

When the sensor failed the engine would shut down, typically displaying one of the following fault codes showing abnormal update rates:

            SPN 3364 FMI 9 (Cummins Fault Code 3868) –DEF quality

            SPN 1761 FMI 9 (Cummins Fault Code 4677) – DEF tank level

            SPN 3031 FMI 9 (Cummins Fault Code 4572) – DEF tank temperature

These codes were designed to detect weak DEF, low DEF level or DEF that had been overheated in the tank. If the sensor chip fails any one of these fault codes may appear, even though there is nothing wrong with the DEF itself. But the faulty sensor will derate and eventually shut down your engine.

The Shaw sensors were revised over time but the newer revisions still had issues and as of generation 6 there were still continuing failures and many RVs were sidelined due to a lack of replacement sensors, mainly due to the electronic chip shortage affecting the automotive industry in general in addition to the time involved in developing a new chip revision. The EPA began working with Cummins to allow an industry-wide software solution to allow vehicles with failed sensors to operate temporarily until replacement parts are available. But the EPA and Cummins haven’t yet determined how soon this software update will be available so a few RV owners simply took the initiative and developed the software to create a DEF sensor simulator.

This simulator was a small electronic piece that is used to communicate with the ECM in place of the faulty DEF sensor. You simply unplug the 4 conductor harness from the DEF sensor and connect it to the simulator. The simulator is designed to provide normal readings to the engine’s ECM so that it can continue to operate without shutting down by transmitting normal readings to the engine’s ECM. The caveat to this simulator is that you no longer have any information as to the level of DEF in the tank so you needed to physically monitor the level of DEF in the tank. However, it allowed the owner to operate the vehicle and complete the trip rather than be sidelined. The system will still meter DEF to the SCR so the pollution abatement portion of your emissions system will still operate as designed.

The simulator is a DIY home build operation if you are handy with electronics. Check out for instructions on how to build this if you are interested. The software is offered free of charge. You just have to buy the components you need, download the free software to its chip and build it yourself. While it is illegal to modify a vehicle’s emissions system in an effort to defeat the DEF system, this simulator does not defeat the emissions operation so is therefore not illegal. It’s strictly a temporary way to operate your vehicle until it can be properly repaired whole still retaining full emissions capability.

Shaw’s latest generation 7 sensors began to arrive around May of 2021 and as of this writing there don’t seem to be failures with these sensors so it’s quite possible that this issue is no longer a concern. However, having a simulator on hand may just be the insurance you need to keep your motorhome operating should a failure occur.

National Indoor RV Centers blogger Mark Quasius profile picture
Mark Quasius is the founder of, the past Midwest editor of RV Magazine, writes for numerous RV-related publications and a regular Contributor to FMCA’s Family RVing Magazine. Mark and his wife Leann travel in their 2016 Entegra Cornerstone.