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While driving from Klamath, California to Cresent City, California in our 2022 Entegra Accolade XL 37L Super C, we got the dreaded dashboard light that no one wants so see: our DEF warning light turned on. I panicked. Kasie remained calm. When we arrived at our campground, we drank a beer. Then we got into task mode and immediately began researching what to do. Here’s what we learned.

Why Does Our RV Have DEF?

Our RV is built on a Freightliner S2RV chassis with a Cummins 6.7L ISB diesel engine. Back in 2010, new emissions regulations came into effect for diesel engines, which required the use of diesel exhaust fluid (DEF) to reduce oxides of nitrogen (NOx) emissions.   Small quantities of DEF are injected into the exhaust upstream of a catalyst, where it vaporizes and decomposes to form ammonia (NH3) and carbon dioxide (CO2).  The ammonia, in conjunction a catalyst, converts the NOx to harmless nitrogen (N2) and water (H2O).

If you want to know more, check out the Cummins Diesel Exhaust Fluid Q&A.

What are the Four Stages of DEF-Related Warnings for a Cummins Diesel Engine?

Cummins created a nice little flyer on DEF-related warnings. Our DEF gauge doesn’t quite look line the one in the flyer. Instead, on our S2RV chassis, we have four little bars that tell us if our DEF tank is 25%, 50%, 75%, and 100% full. There’s also a light that will show up as amber or red, depending on the DEF system status and level. Amber means there is a problem; red means there’s a BIG PROBLEM. Here’s a couple of photos from others showing what the dash might look like on the S2RV chassis.

Did We Really Have an Issue with Our DEF or Did We Just Have Bad Sensor?

The first thing that we wanted to do was isolate our DEF issue.  We have a Scan Gauge D that tells us error codes.  We were getting the 3364 code with the amber DEF warning light. We googled the code and found that this means that the DEF fluid quality was not right.

But did we really have a DEF quality issue? Or did we have a bad sensor?

As background, we typically fill our DEF tank with Super-Tech DEF (blue jugs) from Walmart. We always open the cap to make sure the seal is still in tact before purchasing our DEF to make sure it has not been tampered with. Given that we had driven over 100 miles or since adding DEF and we were camping in mild climates, we felt very confident that our DEF was fine and that we had a bad sensor. After all, why would the DEF suddenly go bad?

We had a couple of options to test our “bad sensor” theory. First, a refractometer can be used to test the DEF fluid concentration, which is supposed to contain 32.5% urea. If that looks good, there probably isn’t a DEF quality issue and instead the issue is a sensor. Second, the existing DEF can be siphoned out of the DEF tank and replaced with fresh DEF. If fault code persists after trying to clear it, then the issue is likely the sensor.

Why is a DEF Fault Code so Scary to RV Owners Like Us?

Simply put, DEF Fault codes cause the RV to derate. Essentially, the engine will lose power over time. We both vaguely recalled people complaining about being derated on the Facebook forums. Given our circumstances, we didn’t know exactly how fast or to what extent we would lose power. I immediately freaked out about the possibilty that we could end up being derated to 5 mph (a/k/a “limp mode”) as we made our way along some highways through the Redwoods. That’s scary.

How Fast will an S2RV Super C Derate with a DEF Issue?

Derating Under the Current EPA Regulations

When the EPA implemented the NOx / DEF regulations, the agency basically left it up to the manufacturers to induce drivers to fix a faulty DEF system. The general idea is to have the vehicle “derate” over time until the problem is fixed. A 2009 EPA Guidance Document stated:

Possible approaches for the manufacturer to degrade performance include a derate of the engine’s maximum available engine torque of a sufficient magnitude for the operator to notice decreased operation (a derate of at least 25% is likely to be needed for such an effect) and progressing to further degradation that could include operation of the engine being disabled or severely restricted, implemented in a manner designed to prevent operation without DEF. EPA recognizes that there may be safety concerns regarding a complete disablement of the engine or severe degradation occurring while the vehicle is moving, and therefore believes that such degradation would best be initiated at the time of refueling, parking or restart.

See CISD–09–04 REVISED.

I contacted Cummins and asked about the derate shedule in our S2RV chassis. When would we start to derate? When would we be in “limp mode” (derated to about 5 mph)? The answer I got back from Cummins was not helpful. Cummins replied: “When a fault code comes up there isn't a certain amount of time before it happens.. you need to take it into an authorized shop as soon as possible.” Geesh….

The “best” information I could find about a possible derate schedule was a 2021 article from the Entegra Owners Association President titled “How to Survive a DEF Sensor Fault.” The article stated:

So when the DEF light turns on indicating a problem, NOTHING happens for the first hour…you just have a warning…but know the clock is ticking. After the first hour, the engine de-rates by 25% for the next two hours. You probably won’t even notice a decrease in performance at 25%. At the fourth hour, however, the Engine Control Module (ECM) will de-rate the engine by 40%, and again, you probably won’t notice that unless you’re traveling in some pretty hefty hills. After the 40% de-rate, the ECM goes into the 5 mph “LIMP MODE.” Now in a 40% de-rate, you can run INDEFINITELY, unless one of these 3 things occur:

1. You idle for more than one hour;
2. You turn off the engine;
3. You fill your diesel WITHOUT also filling your DEF fluid. The ECM Looks for that!

If any of those three conditions occur, your engine will de-rate to the 5mph LIMP MODE. Of course, if you get the SPN#1761 – FMI9 DEF TANK VOLUME, you cannot recover from that and it goes into LIMP MODE. So to continue traveling under these conditions, DON’T LET ONE OF THOSE 3 CONDITIONS OCCUR! Keep going! If you need to fill up, LEAVE THE ENGINE RUNNING….AND…fill your DEF TANK too. Keep driving until you land somewhere safe where you can have the problem addressed safely.

Keep in mind that this information was likely for an Entegra Class A motorhome, not a Super C, like ours. This article was a data point, but it certainly did not give me much comfort.

Derating in the Future (2027 and on)

Driving down a busy highway at significantly reduced speeds is certainly a safety issue. The regulators have finally realized this. As such, in 2022, the Biden Administration implemented new regulations (effective for 2027 models) that, among other things, provide operators more time to fix the DEF issue before derating occurs and implement a 25 mph final derate speed.

See 40 CFR 1036.111. The new rules will also require manufacturers to give way more more detailed information on what exactly the problem is. See 40 CFR 1036.110. Among other things, “[i]f there are additional derate stages, also [the system must] indicate the next speed restriction and the time remaining until starting the next restriction.” Essentially, those with newer rigs won’t be guessing like we are. Lastly, the new regulations will require manufacturers to warrant the DEF system for 10 years (instead of the current 5 years). See 40 CFR 1036.120. That is welcome news given all of the issues with DEF systems.

What Options Did We to Fix our DEF Sensor Issue?

After getting the DEF fault code, we called the nearest Freightliner Service Centor (Medford, Oregon) get their advice on what to do. They assured us that they had a replacement DEF header and could fix the problem if we drove to Medford. However, like Cummins, they didn’t know the derate schedule for our Super C, but told me that “the clock was ticking” and it was possible that we could derate into “limp mode” (maximum speed ~ 5 mph) within a few hours.

Keep in mind that (1) we had already driven 40 minutes (from Klamath to Cresent City) and (2) the closest Freightliner Service Center was another 2.5 hours away. I was fearful that we would increasingly derate on our journey to Medford. Moreover, our route would take us down curvy Highway 199 through the Redwood National park. Where we going to lose significant power and speed on our journey? And if so, when?

We briefly investigated other options.

• Service Tech. Freightliner offered to have a service technician travel to our RV (at a cost of $175/hour + mileage). That travel time wouldn’t be covered by our warranty. We were skeptical that the technician would have the parts and ability to fix the issue on site as well.

• Tow our RV. I researched a few tow companies in the area. However, I doubted that our warranty would cover a tow because we had not yet underwent a significant derate (at least that we could feel). Moreover, neither of us wanted to put BIG LADY through a tow if we could help it.

• The best “solution” we could come up with was to build a DEF Sensor Simulator. If we started to significantly derate, the plan was to use it.

What is a DEF Sensor Simulator?

During the pandemic, DEF system failures were abundant — perhaps because parts (especially working sensors) were hard to come by. As a work-around, a group of really smart folks developed the DEF Sensor Simulator. Bascially, when a DEF sensor failure occurs, the DEF system stimulator allows the operator to temporarily drive without being derated so that the operator can drive to a repair facility safely with full engine power. The DEF system simulator has some computer code that tricks the DEF system into thinking that the DEF tank is 75% full and operating properly. 

We belong to several Facebook groups where other Jayco Seneca and Entegra Accolade owners had built their own DEF Sensor Simulator for less than $100. If they could do it, we figured that we could as well.

Pro Tip: Several people offer homemade DEF Sensor Simulators for sale. If you purchase one of those for our Freightliner S2RV chassis, make sure that it contains the Freightliner connector plug.

How Can Someone Build a DEF System Simulator?

We aren’t going to reinvent the wheel here, folks. This DEF Sensor Simulator website provides detailed instructions on how to build the sensor simulator and how to use it. THANK YOU! THANK YOU! THANK YOU!

(Note: The website now a passive archive of the original DEF sensor simulator site. The original site was privately hosted and has been deactivated. This site is hosted via a free service at wordpress.com, so should live on indefinitely unless WordPress elects to remove it. The content was last updated in 2022 and is no longer monitored or updated, however most of it has been checked for accuracy and is intact. No support or additional instruction can be provided. Any comments present in the included posts are from the original site and are just a part of the archive. No new comments can be accepted. Use at your own risk.)

Most of the parts (links below) are available on Amazon too.

Please note that many of these items can be sourced from alternative suppliers. These suppliers may be able to provide the parts quicker and cheaper than Amazon. For example, we found that we could get many of the parts much faster from Digikey (Raspberry pi) and Mouser Electronics (4-Hole Wire Harness Connector, Male Terminals (TE), Connector Cover). Because time was important to us, this was a key factor in our sourcing. Many of the parts were also cheaper when purchased from a non-Amazon source (at least at the time of our our build).

Our cost for the parts (including about $23 in shipping) was $82.70.

As for equipment, we did purchase soldering gun and upgraded our multimeter as part of the DEF Sensor Simulator build.

Was the DEF Sensor Simulator Hard to Build?

The DEF Sensor Simulator was not hard to build given the detailed instructions on the DEF Sensor Simulator website. Kasie built it less than 2 hours. Keep in mind that she had never soldered anything before (we didn’t even own a soldering gun before this project).

Perhaps the hardest part about the project was figuring out where to mount the DEF Sensor Simulator if we needed to use it. In our S2RV chassis, there’s a small slot where the DEF Sensor Simulator was easily positioned behind an existing bolt head. We had some high-strength magnets on hand that we placed around the DEF Sensor Simulator in order to hold it in place. If we had to use the newly-built DEF Sensor Simulator, that sucker wasn’t going anywhere.

Is Temporarily Installing a DEF Sensor Simulator on an RV to get to a Service Center Legal? 

WARNING! The information contained in this site is provided for informational purposes only, and should not be construed as legal advice on any subject matter.

You should not act or refrain from acting on the basis of any content included in this site without seeking legal or other professional advice. The contents of this site contain general information and may not reflect current legal developments or address your situation. We disclaim all liability for actions you take or fail to take based on any content on this site.

You should consult with your own legal counsel to answer this question.  However, I personally think that there are strong arguments that a DEF Sensor Simulator used in “emergency” situations is not illegal.

Section 40 CFR § 1068.101(b) contains the relevant provisions:

(1) Tampering. You may not remove or render inoperative any device or element of design installed on or in engines/equipment in compliance with the regulations prior to its sale and delivery to the ultimate purchaser. You also may not knowingly remove or render inoperative any such device or element of design after such sale and delivery to the ultimate purchaser….  For a manufacturer or dealer, we may assess a civil penalty up to $44,539 for each engine or piece of equipment in violation. For anyone else, we may assess a civil penalty up to $4,454 for each engine or piece of equipment in violation. This prohibition does not apply in any of the following situations:

(i) You need to repair the engine/equipment and you restore it to proper functioning when the repair is complete.

(ii) You need to modify the engine/equipment to respond to a temporary emergency and you restore it to proper functioning as soon as possible…..

(2) Defeat devices. You may not knowingly manufacture, sell, offer to sell, or install, any component that bypasses, impairs, defeats, or disables the control of emissions of any regulated pollutant, except as explicitly allowed by the standard-setting part. We may assess a civil penalty up to $4,454 for each component in violation.

If an RV owner is on busy highway and being substantially derated, the RV owner could argue that temporarily installing a DEF Sensor Simulator to get to the nearest service center might fit within one of exceptions in Section 1068.101(b)(1).    After all, the RV owner is in the process of trying to “repair” the DEF system.  Certainly, if the RV were significantly derate, that is a safety issue that could be categorized as a “temporary emergency” as well.  

Likewise, the DEF Sensor Simulator itself does not “defeat” the emissions operations.    The DEF sensor simulator tricks the system into thinking that the DEF tank is 75% full and operating properly.  As long as there is DEF in the tank, the DEF system should theoretically continue to operate properly until the driver can get to the nearest service center. (On that note, it’s my understanding that someone should NOT use the DEF sensor simulator if there is not sufficient DEF in the tank. Doing so would damage the DEF system and/or the engine itself!).

Of some interest, in late 2020, the EPA offered some guidance in a non-binding Enforcement Policy:

The EPA typically does not take enforcement action for conduct that might be a violation of section 203(a)(3) of the Clean Air Act if the person engaging in the conduct has a documented “reasonable basis” to conclude that the conduct (or, where the conduct in question is the manufacturing or sale of a part or component, the installation and use of that part or component) does not and will not adversely affect emissions. This Policy Statement does not apply, however, to conduct affecting an OBD system, which may be subject to enforcement regardless of effect on emissions.

See The EPA Enforcement Policy on Vehicle and Engine Tampering and Aftermarket Defeat Devices under the Clean Air Act (Nov 23, 2020). Although this document expressly carved out conduct affecting the OBD system from the Enforcement Policy, the guidance suggests that EPA is largely concerned with prosecuting activities and devices where the intent and actual effects on emissions are much different that those involving the temporary use of a DEF Sensor Simulator to help transport an RV to a service center for repair.

Given that the EPA issued new regulations to provide more time for operators to fix a DEF issue before significantly derating (effective for 2027 models), I also think an argument can be made that the temporary use of a DEF Sensor Simulator by RVers who are being substanitally derated is alligned with the overall goals of the new regulations.

Lastly, I’m not aware of any RV owner being fined for installing a DEF Sensor Simulator under this circumstances. (If you are aware of an RV owner who was fined, please leave a comment or send an email to ClassCBroads@gmail.com and let me know).

Did the DEF Sensor Simulator Work?

As a retired lawyer, I’m very risk adverse. As such, I’m not going to tell you whether we actually used the DEF Sensor Simulator or not on the road. Yes, you heard that right. After all of the trouble of building the DEF Sensor Simulator and mounting it, I’m going to take a hard pass on whether the DEF Sensor Simulator worked. The bottom line is that I don’t want to get into trouble with the EPA on the off-chance that they don’t buy my arguments about the legality of the using the device under our circumstances.

I will tell you that (1) having the DEF Sensor Simulator on hand did give us peace of mind and (2) we drove from Cresent City to the Medford Freightliner Service Centor with no problem.

And, it was a bad sensor.

Stay safe. Stay classy.