DEF Sensor Simulator


DEF systems in modern diesel engines are key to reducing Nitrous Oxide (NOx) emissions  and protecting the environment.   The DEF system is a combination of software, hardware and DEF fluid that enable this important task.  Recently there has been a pandemic of failures affecting one of the elements of the DEF system called the DEF sensor.  The origins of this “DEF pandemic” are not  known for certain but the failures may be related to heat, poor design, or a high failure rate of the internal semiconductor.

The reason doesn’t really matter as the consequences of DEF sensor failure to the driver and passengers are significant.  This failure causes an instant and progressive slowing of the engine function called “derate”, even though the basic function of the emissions systems continues to operate.  Derate strands vehicles and their occupants on the side of the road, or worse forces them to go slower than traffic which is equally dangerous.

Replacement parts are not readily available.  Covid-19 disruptions in the supply chain for semiconductors has exacerbated the availability problem.  As of the fall of 2020, industry analysts are forecasting this chip shortage to last into 2022.  Many diesel equipment users are sidelined for days, weeks or months while waiting for replacement parts. The irony is that despite the danger and the inconvenience, the effected emissions system is not really broken, rather the  DEF sensor stops reporting accurate information, so the ECM (Electronic Control Module) makes an incorrect assumption that the emissions system is broken and begins to derate the user. The classic symptom of the failed sensors are a Check Engine light (and possibly other indicators) with three fault codes followed by derate. Click here for details on the fault codes. Being unnecessarily derated in a diesel powered vehicle is dangerous to its occupants.

What are the solutions:

Currently there are two known repairs:

  1. After being derated, make your way to a dealer and get the faulty sensor replaced (if and when available)  or;
  2. Both the EPA and Cummins have been made aware of the dangers of derate as a result of  DEF Sensor failure and have announced, (but not yet implemented) a software solution.  When available, bring your vehicle to a dealer where the dealer will apply an EPA approved and Cummins supported modification to your ECM software called a “calibration”.

Both of these solutions will put your vehicle back on the road.  Derated users should avail themselves of these repairs as soon as practical.  Both solutions however assume that you can safely bring your vehicle to a repair facility.  But what if that’s not possible?

About the DEF Sensor Simulator (DSS):

The DEF System Simulator (DSS)  is a hardware/software solution that a user can build.  It is made of inexpensive, readily available parts to assist derated users to be able to perform a temporary fix as soon as a DEF sensor failure occurs and safely operate their vehicle until a repair can be obtained. Upon installation, the DSS removes (and following a set of procedures contained in the documentation) the derate condition and will permit its users the ability to reach a repair facility safely with full engine power.   Further, derate progressively decrements engine performance based various mileage limitations, fuel level and start/stop conditions.   These conditions may not be practical for a user as they may be too far from a repair facility, need to fill the fuel tank, stop for he night, etc.  A key feature of the DSS is that it contains no such restrictions.  Once installed, users are free to safely drive their vehicle until a permanent repair can be found.  Assuming there are no other actual component failures within the emissions system, the DSS described here will actually restore normal emissions operation.

What the DSS is NOT:

Modifying emissions systems to defeat their purpose is not legal.  The DSS is NOT a means to defeat the DEF system, a DEF system “Delete” or in any way provide a means to discontinue use of the emissions system on your diesel powered equipment. Your dashboard DEF Gauge will no longer be accurate.   You MUST manually monitor and continue to keep your DEF tank filled with fresh DEF fluid until you get a permanent repair.  It is highly recommended that you keep accurate records and check your DEF level at regular intervals.  For more information see the Quick Build section. It does not, and will not, allow continued use of your vehicle if legitimately failed DEF system components are present.

Build your own DSS:

Before you begin, please click here to read an important compatibility notice regarding the ECM (Electronic Control Module, aka Engine Computer) and info on compatible DEF head connectors.

To get started with building your own DSS as quickly as possible go to the Quick Build page. As there are now several versions of the DSS hardware, Quick Build will guide you to the best and most current option.

For more information about this project, how it works, and various alternative construction methods and add-ons, see the Info & Options section.

20 Comments to DEF Sensor Simulator

  1. Mike Smoker says:

    Just curious, what happens if and when you run out of def? There is no way to check the level of the def in my motorhome. I think this is a great thing you have done, as I have a model that has been prone to def head failure. Thinking of building this just in case of failure. Hopefully, as with most spares, won’t be needed if you have it. Thanks again, Mike

    • Voltdoc says:

      Really, you have no DEF tank level gauge? What is your model of motorhome and engine? If you run out of DEF (with or without the DEF Simulator) the emissions sensors (different than the DEF tank sensors) will detect an out of range condition and take corrective action including a derate – or worse. Also running out of DEF could possibly damage other parts of your expensive emissions system. The DEF Simulator is ONLY capable of temporarily supplementing a failed DEF head and actually restores normal operation of your emissions system, so you must manually monitor the tank level and keep it adequately filled with fresh DEF.

  2. DAVID SHARP says:

    What is the possibility of buying an assembled unit?

  3. Sean Hart says:

    I made one of these DEF sensor simulators for a 4.5 cummins on a hydroseeder. It is still showing active codes for DEF level, and temperature.

    • Archer2 says:

      Sean, I don’t know anything about your engine. The DSS was designed for Motorhomes and some HD trucks so it has mostly been tried on ISL9, ISX11 and ISB6.7 but I would like to try and help. Can you send me more details like the engine ESN number, which ECM it has, what year, and most importantly exactly which codes you are getting (both the SPN and the FMI). Also if you know who made the OEM sensor or the part number along with what kind of connector. Anything you can provide might help. If you would rather send info by email use and I’ll get back to you as soon as I can.

    • Archer2 says:

      Hi Sean. I’ve sent you two emails directly to your email address asking for a little more info to help us try to help you. Please respond directly to the email address

    • Archer2 says:

      Sean, I’m sorry that you were not able to get the DSS you built to work on your equipment. I believe after doing a little research that it should work. I have made several attempts to contact you to try to help but you haven’t responded, so good luck figuring it out and if you do, please let us know what you did to fix it. It might help somebody else.

  4. Mike McClain says:

    Freightliner S2RV chassis; built, tested and put away for a rainy day. Thank you!

  5. Billy Patterson says:

    Will this (CANBED) DEF Sensor Simulator work on a 2018 Tiffin Powerglide Chassis with ISL9? I have all of the SPN Codes for the DEF Head Failure and Tiffin has me on the DEF Head list (Think Organ Donor Transplant List)

    • Archer2 says:

      Hi Billy. Yes the CanBed board with the DT04 connector should work fine. All versions of Arduino boards and programming are interchangeable. The only difference is in the style of connector that your chassis uses.
      Somebody on this board might also have a spare they could sell to you and get it shipped as quickly as you are willing to pay for if you are stranded.

    • Fred Jones says:

      Can I supply 24VDC to the board instead of 12VDC?

  6. Fred Jones says:

    I was going to use a CanBed RP2040 (if I can find one).

    • Vic Chanko says:

      According to the specs the RP2040 is good for up to 28VDC. I think the other small CanBed board (the 32U4) is the same. The Copperhill DUE with extended power option is good up to 36VDC.
      I don’t remember seeing anything on the SEEED CANBUS V2 shield that was used for the UNO and atMega boards that said anything other than 12-ish (automotive nominal voltage). That doesn’t mean it can’t support higher but you would have to identify the DC to DC switching regulator and look at the specific chip specs.

  7. Lonnie Manuel says:

    I have several Cummins QSB 6.7 CM2350 B105(s) in the fleet I maintain. I tested the DSS build on ESN:74524573, with ECM code: ER80254. Although, the instructions recommended not testing until there is an issue, I wanted to be certain it was a reliable solution so I te x Ted on an engine with no codes. As soon as I turned the key on codes 3714, 3868, 4572, 4667 illuminated. I did not complete the high idle procedure. I just plugged DEF header back in to clear the codes. Is it normal for the codes to set when plugged in without the high idle procedure? Thank you.

    • Archer2 says:

      Hi Lonnie. No those codes are not anything I’ve ever seen or heard of and the “high-idle procedure” is only used to force the Cummins on-board diagnostics to run and clear code 3031 if necessary. I searched the J1939 document for the 4 codes you got and they are 3714-related to DPF regen inhibit, 3868-proprietary, user defined AUX I/O error and 4572 & 4667-joystick position errors. In other words nothing to do with the DEF sensor (or each other for that matter), they appear to be random.
      My guess is a wiring error on the DSS. Sorry, but we don’t have any way to verify that the wiring of the connector given in the instructions is correct for your industrial application so you should check your specific wiring diagram to make sure that it’s wired properly. I’ve seen random errors before if the CAN High and CAN Low wires are reversed or the connector is otherwise mis-wired. I don’t even know whether your engine is one that uses the troublesome temp, level & quality combined J1939 DEF Semsor.

      • Lonnie Manuel says:

        Thank you for the reply. Sorry I did not specify that the codes are the Cummins number.

        3714 (SPN 1569 FMI 31) Engine Protection Torque Derate – condition exist.

        3868 (SPN 3364 FMI 9) Aftertreatment 1 Diesel Exhaust Fluid Quality – Abnormal update rate.

        4572 (SPN 3031 FMI 9) Aftertreatment 1 Diesel Exhaust Fluid Tank Temperature – Abnormal update rate.

        4677 (SPN 1761 FMI 9) Aftertreatment 1 Diesel Exhaust Fluid Tank Level- Abnormal update rate.

        I have received above codes with previous bad DEF Headers as well. If my memory is correct, the wiring was exactly as the examples shown on this website. Also, our systems are 24V and we use the all in one Quality, Quantity & Temperature Header. I am currently out of town and will be back at machine Tuesday. My first step will be to confirm wiring, plug in DSS with battery power off, then go through the 3X 5 minute high idle procedure. I will post the results. Thank you again.

        • Archer2 says:

          Hi Lonnie, thanks for the further explanation. Now that makes sense. The codes you are getting are textbook examples of a sensor that is not putting out any data at all.
          I would look at a few things. First would be a wiring error with the +V/-V (AKA ground) or that your connector pinout is not the same as what is in the website instructions. Second would be a programming issue. Third would be that you have a board that has failed.
          Also I think you are somewhat confused about the whole “5 minute idle 3 times” thing. It is not part of the installation procedure. It is just a way to force the ECM diagnostics to clear codes that might be set by a faulty installation, failed sensor or failed DSS. The procedure is mostly for users who do not have access to a scan tool to allow them to reset the codes. But for the procedure to work you have to have a working sensor or a working DSS plugged in so that it will pass the diagnostic. So in your case you would want to re-connect the functioning OEM sensor before doing the 3X idle thing. Or just use an Insite tool to do the same thing if you have one.

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