Fault Codes Explained – DEF Sensor Simulator (2024)

A modern diesel engine is a very complex device. Over the many years that they have been used there have been steady increases in power, reductions in fuel usage and significant reductions in harmful pollutants. Some claim the exhaust of a modern diesel engine can actually be cleaner than the air going into it!

These improvements have required more and more sophisticated control systems. At the heart of the Cummins engines is the Electronic Control Module or ECM. It performs a number of important tasks and among them is continuous component fault monitoring. Whenever the key is turned on the ECM checks on the status of dozens of components and it continues to keep tabs on all of them while the engine is running. If it detects a fault, it evaluates which parameter is not correct and what the nature of the fault is. Some of these faults are minor and the ECM just stores them for future analysis. Some of them, like the emissions controls systems, are important enough to alert the operator to the presence of a problem and if the problem persists, the ECM will force action by reducing performance until the problem is resolved. This is called an Inducement or Deration. Unfortunately the ECM in this case is not able to distinguish a real problem that needs to be resolved from a false alarm caused by the failure of the very component that monitors the DEF supply.

All of these faults are reported in a standard format and use a few acronyms. The description of which parameter needs attention is called a Suspect Parameter Number or SPN Number and they are usually a 3 or 4 digit number. There is also information about the general nature of the fault. These are called Failure Mode Identifiers or FMIs. FMIs are a 1 or 2 digit number and each FMI number has a specific meaning. For instance a problem with SPN 1761 with FMI 9 indicates a problem with the DEF Fluid Level and the nature of the problem is an Abnormal Update Rate.

That’s enough background for you to understand generally what these fault codes are all about. Now for the specifics of what this means for the DEF sensor problem.
In the unlucky event that your coach is having a DEF sensor failure the first indication that you will see is the appearance of 3 very specific faults and the illumination of 1 or more warning lights. The lights that may be activated on the instrument panel are the amber Check Engine Light (CEL), the yellow Malfunction Indicator Light (MIL) and eventually the red Stop Engine Light (SEL). Sorry but engineers do love their acronyms. Where you will see the codes and other information varies and you should consult your Owners Manual or contact your coach’s Customer Service Department for how to display yours.

There are 3 specific SPN numbers along with their Failure Mode Indications that are associated with a DEF sensor failure:

SPN 1761 FMI 9 Aftertreatment 1 Diesel Exhaust Fluid Tank Level-Abnormal Update Rate
SPN 3364 FMI 9 Aftertreatment Diesel Exhaust Fluid Quality-Abnormal Update Rate
SPN 3031 FMI 9 Aftertreatment 1 Diesel Exhaust Fluid Tank Temperature – Abnormal Update Rate

If all three of these fault codes appear more or less at the same time, then it’s virtually certain that the problem is that your DEF sensor has stopped outputting any data at all. The “Abnormal Update Rate” message just means that the ECM has not received any valid DEF data in a certain amount of time. It’s Abnormal because it is not there at all.

After installing the DEF Simulator the ECM diagnostics system will clear most fault codes and warning lights soon after the next key on event. However EPA regulations call for certain codes, notably the SPN 3031 code, to complete 3 consecutive 5 minute long engine operations without error before it will allow the codes to be cleared.

After installing the Simulator, usually you can just start the engine and idle for 5 minutes 3 times in a row and the codes will be gone the 4th time you turn the key on. Some versions of the ECM software will allow an external device like a code reader to clear the codes immediately but some versions don’t. Either way all warnings should clear themselves eventually so don’t let it worry you.