Below is a recap of some issues we were experiencing with the truck going in and out of derate. Even though clearing the codes and performing a regen may get you back on the road, this may be a temporary solution and require attention shortly after everything seems okay.


If you have an active fault code of: P1490 Def System NOx trap low efficiency code, which basically translates to an issue with the Dosing control module not seeing the NOx reduction it believes it should see after activating the dosing module.


Our first step was to check the DEF Fluid (Quality and Quantity)

In our encounter with this issue the driver did inform us that he has not had to add any DEF Fluid to the vehicle in over three weeks and after checking the DEF level and seeing that the DEF tank is full we can immediately deduce that something is going on with the injection system, due to the fact that no DEF fluid is making it into the system at all.


To proceed with the inspection, we can start with the checking pump and the injector first and proceed from there.


DEF Quality was checked, and it tested good so we can move on to inspecting the pump itself. Using our TEXA Diagnostic software we can perform an activation test on the pump to listen for activation and function. This setting can be found in the Engine control module (Diesel Injection) under the Activations Tab. The test is called Diesel Exhaust Fluid (DEF) Supply Pump. Please follow all on screen instructions to ensure no damages occur while performing this test.


In our testing we were able to hear the pump activating when we performed the test, so our next step was to look at the at the injector. We ran an activation of the injector and were able to hear the electrical activation of the injector but do keep in mind that there could still be a mechanical issue even though the injector is receiving the correct electrical voltage.


Next, we will check the mechanical functionality of the injector. We can do this by removing the supply line from the injector and activating the pump to ensure that we are receiving fluid at the injector.


Upon removing the injector supply line, we immediately noticed debris inside the injector and the supply line. Therefore, it is vital to not rely solely on just the activation test. We were able to here the pump and injector activating but had we not removed the supply line we would not have come upon this issue.


After replacing the supply line, we wanted to ensure there was no further contamination in the supply line or DEF tank itself. Using the TEXA Diagnostic software we can perform the Supply Pump Actuation to flush the line and tank.


Once we completed the draining of the system, we were ready to install our new injector and supply line.


Now that the new injector and supply line has been installed, we want to pressurize the system and check for leaks, and we can do this using our TEXA Diagnostic software.


The system passed the pressure test and we are ready to perform the necessary resets required after performing an injector replacement. This can be done using the DEF Injector Data Reset under the settings tab in our TEXA Diagnostic software. The reset was successful and now we can clear the stored memory code that we had initially.


The last thing we will need to do to check proper operation will be to perform a DPF Normal Regeneration. Please be sure to follow all safety advisories to ensure no damages occur during this procedure.



This concludes our overview of the issue we encountered when attempting to diagnose our 2011 Isuzu NQR. The important thing to take away from this is that even though  our TEXA Diagnostic software was informing us that everything was operation properly there was an issue that would not have been discovered had we not followed through on our inspection of the system in question.

WARNING: The makes and distinguishing signs of the vehicle manufacturers included in this document are only provided to inform the reader about the potential suitability of the said TEXA products to be used for the vehicles of the above-mentioned manufacturers. References to makes and models contained in this document are to be considered only as a rough guide.  

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