If you enjoy doing maintenance on your own vehicles like I do, every now and then you run into some surprises. I’ve always thought no harm can be done doing the simple remedial tasks (oil change, air filter replacement, fuel filter replacement, etc). This is not always the case with some vehicles.
I recently received a call from a shop that was working on a 2007 Mercedes ML 320 CDI (Common Rail Diesel Injection). The shop had sold some additional maintenance work, which usually is a good thing, but this time it turned out to be a nightmare. They replaced the engine air filter and fuel filter; performed a transmission service and an oil change; and then added a liquid for reducing NOx emissions. The whole procedure was quite simple and routine, and they never anticipated having any repercussions from the service.
Sometime after the customer left the shop, the dreaded “Check Engine” light came on. After the recent expense of maintaining his vehicle, the last thing the customer expected to see was a warning light telling him there was an issue with the engine.
When the vehicle was returned to the shop, fault codes DTC 2644 and DTC 2646 were recorded in the CDI engine control unit. Both fault codes referred to “B2/6” (left hot film mass air flow – MAF – sensor) being above limit value. Before calling Identifix, the shop replaced not only the left hot film MAF sensor, but also the right side since they come as a whole unit and are not sold separately. The technician erased the codes and took the vehicle on an extensive test drive. On the return trip, the codes reappeared. Again, fault codes DTC 2644 and DTC 2646 were stored. Now what? This is when he called Identifix.
If you are diagnosing a MAF sensor fault …
1) Check for air induction leaks. With the CDI logic program, you need to check for leaks downstream of the sensors and upstream as well.
2) If no leaks are detected, make sure the engine air filters are not plugged up. Having clogged filters can allow MAF sensor faults to set.
3) If the engine air filters need to be replaced, there are some adaptive values that need to be relearned. With a factory compatible scan tool, access the CDI engine control unit and follow the path to “Control Unit Adaptations.” Both “Reset Air Filter Learned Values” and “Reset Values for HFM Drift Compensation” have to be performed to ensure proper function of airflow volume calculations in the CDI engine control unit.
After following these procedures, and resetting the adaptive values, the tech erased the codes and took the vehicle on a final test drive. The codes did not reappear. He was glad to have this one fixed.Lesson learned: Just because there is a fault code for a specific part, the part is not necessarily defective. It could mean that the control unit monitoring the sensor may only need to be reset.
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