Diagnosing commercial vehicle issues can be challenging and stressful, especially when you run into a laundry list of fault codes while plugging in an engine control module (ECM). Deciphering which codes are relevant can become overwhelming in the moment.
When I was first learning how to diagnose, I was taught the KISS (Keep It Simple, Stupid) method. That was tough to grasp in the beginning because diagnosing a sophisticated Class 8 truck is far from simple. During early attempts, my eyes would glaze over, my head would empty, and by the end of the day, my body would feel numb. There were times when I went in the wrong direction, and possibly in a circle a few times. Hours or days later, I would find the answer.
Now, because I learned how to look at problems differently and approach issues more logically, the answers come much easier. Instead of feeling exhausted and numb, I experience a sense of accomplishment after a successful diagnosis and repair.
Finding a good repair path will help you avoid the inevitable “diagnostic headache” you get when exhausting your options one by one. With the right vantage point and approach, any resourceful technician can solve even the most complex problems.
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My method of troubleshooting a complex unit—one that contains many codes within multiple systems—is to first look at the big picture. Then put the pieces together and use the process of elimination to find your root cause of failure. This includes reviewing the driver’s concerns and the vehicle’s repair history and code history. The most important piece is to review the active and inactive codes within the module to separate system concerns. The goal is to organize your systems and use the process of elimination to find your correct diagnostic and repair path.
It is important to treat active fault codes with a higher degree of priority. Inactive fault codes should be part of your diagnostics through your process of elimination, but inactive codes might not meet a threshold in that current moment, such as with key on/engine off tests or if a vehicle has an intermittent concern. Even at times when the engine is running, you may still need to meet thresholds before that system code comes forward.
Once you read all the code descriptions, it’s now time to separate the descriptions by their systems. For example, you may get EGR, DPF, and SCR codes in one scan. I would then group the system codes separately. I find it less overwhelming to look at each system group of codes individually. However, keep in mind that the different system groups may be related through symptoms within the systems and may not show through a code.
For example, if an air compressor intake hose ruptures and goes unnoticed for an extended period, resulting symptoms would eventually lead to EGR system concerns in the form of a plugged mixer duct and EGR cooler. Then the DPF and the diesel oxidation catalyst (DOC), which are downstream components, can be adversely affected. Low intake pressure can cause excessive black smoke, plugging exhaust filters and leading to excessive or incomplete regenerations.
You should note that each OEM does have a fault code ranker in development to help point you in the right direction. However, let’s focus on how to find the best diagnostic path assuming that we lack a current OEM fault code ranker. When we have all our systems separated and our repair history, code history, and driver concerns reviewed, we are ready to find our diagnostic paths.
While this process is still very complex and technical, it can be simplified. The most logical way I form my diagnostic paths is by using failure mode identifiers (FMIs). Rather than attempting to memorize hundreds of DTCs or SPNs (suspect parameter numbers), FMIs have 23 values. These describe how a circuit, component, and/or system has failed. When I do not have the manufacturer’s FCAP or diagnostic tree to follow, I use FMIs. The FMIs are going to send you down the best diagnostic path in order to verify your repair process.
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Under each system, start by grouping direct circuit failures together. These FMIs are 3, 4, 5, 6, 9, 19. This group is based on actual circuit failures and should be diagnosed first. Nevertheless, I have seen FMI 9 and FMI 19 derive from failed sensor modules as well, like NOx and QLS sensors.
You must understand basic electrical theory when diagnosing in this direction. The logic within the module will depend on how the circuit is looked at. You may get FMI 3 (an out-of-range high) with a two-wire sensor or FMI 4 (an out-of-range) low with a three-wire sensor, and possibly have the same cause, a broken wire.
Next, group and diagnose the components. This group puts you toward a monitored system component failure and/or components associated with that system of concern. Those FMIs are 2, 7, 8, 11, 12, 13, and 14. On occasion, component FMIs need to be diagnosed first if you see a 2, 13, or 14. For example, a VGT actuator has an FMI 13, or “out of calibration.”
The cause is most likely the actuator and will possibly require calibration during a new install. Some FMIs, like 11,14, and 31, may be manufacturer-specific.
Lastly, the third group is System FMIs: 0, 1, 10, 15, 16, 17, 18, 20, 21, and 31. These describe the system’s inefficiency and may provide the level of condition. For example, if the unit is in derate and the DPF is plugged, or has SCR system-related failures, you’ll get a red stop engine. You will see active FMI 0, 15, and/or 16 in the system for “Data Valid But Above Normal Operational Range—(15 for least, 16 for moderately, 0 for Most Severe Level).”
If your ECM displays FMI 0, the data your ECM is receiving is valid and now meeting the threshold of most severe level.
If you don’t have the right strategy, discovering this could take much longer and lead to much higher stress levels. That’s not good for you or your customer. But if you approach it with the right technique, diagnostics can truly be fun. Just remember to look at the bigger picture and narrow down the root cause using the process of elimination and a systematic method, and then make the necessary corrections.
Getting those trucks out of the bay as working right will provide you with that amazing feeling of success and allow you to move on to the next challenge.
