Dealing with NOx conversion problems in Cummins engines can be a common headache, but with the right approach and tools, like a capable 2017 F750 Scan Tool, diagnosis doesn’t have to be daunting. Often, mechanics encounter vehicles where significant money has been spent on attempted repairs without resolving the underlying issue. Fortunately, these systems aren’t inherently difficult to troubleshoot. Cummins provides valuable data points, including NOx readings at the inlet and outlet of the Selective Catalytic Reduction (SCR) system, as well as NOx adaptation factors for both sensors. By analyzing this data alongside Exhaust Gas Temperatures (EGTs), pinpointing the root cause of NOx conversion inefficiency becomes manageable.
A crucial first step is examining the Freeze Frame data using your 2017 F750 scan tool to check the NOx adaptation values. If the adaptation factor deviates by more than 100 ppm, it signals a potential problem within the NOx reduction system.
Next, perform a Diesel Exhaust Fluid (DEF) injector test using the scan tool. While the precise fluid volume isn’t always critical, focus on the spray pattern. A healthy spray pattern, coupled with good pressure during the test, generally indicates adequate volume delivery.
Following the injector test, visually inspect the decomposition tube. Dried DEF deposits within this tube are a common finding and should be cleaned out.
If DEF is present inside the SCR catalyst, ensure the engine control module (ECM) software is up-to-date. Running the SCR System Test with a 2017 F750 scan tool equipped with the latest software is essential. Newer SCR system tests are designed to reach temperatures high enough to induce crystal sublimation, effectively cleaning the SCR catalyst. Older software versions might only perform a basic SCR Performance Test, lacking this crucial crystal-cleaning function.
Road testing is the final diagnostic step. Monitor NOx conversion efficiency while the system is actively dosing DEF. Simultaneously, observe the temperature increase across the SCR catalyst. Proper function should exhibit good NOx conversion and a corresponding temperature rise.
It’s important to note that while SCR catalyst failures are reportedly rare in Cummins engines, age and mileage can eventually lead to such issues. Another key consideration is whether the system employs an ammonia (NH3) sensor. Systems without an NH3 sensor operate in an open-loop configuration. These systems do not actively adjust DEF injection rates to compensate for suboptimal NOx conversion, making accurate initial diagnosis even more crucial. By methodically working through these steps with your 2017 F750 scan tool, you can effectively diagnose and address NOx conversion problems in Cummins engines.
