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.

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

4 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?

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