My '09 ML320 Bluetec got a CEL with the code 150200 (The upper limit value of component HFM-SFI has been exceeded) on my iCarsoft scanner. As I can tell, my vehicle has no performance drop at all and minimal change of MPG under normal driving conditions. The code seems to be closely affiliated to the MAF sensors so I decided to remove them, clean with carb cleaner, then reinsert + clear code. Code persists when I attempt to clear codes. This previous post on the forums provided alot of information.
Checked the voltage supply to the MAF sensor using this test procedure on the linked forum thread.

I used an oscilloscope to get my voltage readings in both DC and AC so I can get good resolution compared to digital multimeters. Photos of results are attached

15.2V DC voltage supply on right MAF sensor. 10V/div.

3 Volts peak-to-peak AC on same sensor. 1V/div.
Ignition On, Engine off: 11.8 Volts for both sensors.
Engine On: 15.2 Volts DC. Peak to peak voltage of 3 Volts from the 2 spikes during AC measuring.
Air Flow Rate from MAF Sensor: 18.52 g/s
A mean voltage of 15.2 going into the MAF sensor when the engine is running seems a bit excessive. Not to mention the AC spikes as well.
Does anyone know if the supply voltage for the sensors should be that high? It feels like that this could trigger the CEL and the code. The AC spikes feels like that the AC-DC rectifier for the alternator could be failing too. Dont really want to start tossing cash around unless I really know what is going on.
If anyone has some insight into this issue, your help would be much appreciated. Thanks!

 

Why didn't you check the voltage at the battery and the alternator?

Check the voltage manually at both MAF sensors with a DVOM.

 

Alternator voltage is 14.3V. Battery is fine. As for the procedure you posted. I did exactly that. There is no point for me to use a DVOM when an oscilloscope provides better resolution and sampling rate. My values are what I posted above.

 

Did you ever figure this out?

 

As of last week, I finally have some closure on my issue. Car is working fine now with no CEL.

Turns out in the end, it was truly the MAF sensors. However the defect stemmed from another issue which resulted in my faulty sensors. So for a while I had a slight oil consumption issue which is very likely from the piston rings. At this point, the oil burning isnt too bad but there was enough oil vapor coming from the PCV valve going into the turbo. The vapor was enough to go into the MAF sensors, and give false air flow data. This caused the car to go into a partial limp mode and display the CEL.

The major steps I did are listed below.

1. Install a catch can between PCV valve and the orifice where the exhaust gases will be fed into the turbo. This will catch the oil vapor into the container instead of going into the Y-pipe and then the turbo.
2. I replaced the MAF sensors for some fresh ones. Typically you will need to buy the entire Y-pipe assembly that comes with the MAF sensors. However if you are willing to do some hunting, you are able to find them on places such as Ebay to save some cash. There is a risk involved with that as well however.
3. Made sure my intake air filters were at least OEM. I am the 5th owner of this car with 170k miles. Some previous owner used K&N filters instead of the OEMs. I chatted with a very helpful mechanic which stated that he does not like K&N filters due to having a thin oil film within their filters. That oil can also go into the MAF sensors and ruin the measurements that way. I replaced it with OEM ones. Easy enough.
4. Replaced the air temperature sensor. I did that for safe measure. Old sensor was partially coated with some oil anyways so why not replace it. Easy access and not to expensive. ~20USD.

When I put everything back together, CEL went away. Drove it to work a couple times now and no fits at all. Drives smoother overall now! Also found out that a previous owner did a EGR and swirl flap delete as well from my electrical testing.

I am happy this issue is behind me. Labor wise it is a cake walk but annoying to diagnose properly. Hope this helps.