I have an 03 Mercedes C320 4 door sedan and I had a CEL come on with error code P0172. I have noticed a visible reduction in fuel mileage as well as power. In the last year I had the spark plugs replaced, O2 sensors and also the MAF sensor replaced. How can I diagnose the root cause of this problem ? I have a dashcmd OBD scanner as well. Would like to learn and understand before I take it to a mechanic shop. 


I would do a fuel pressure test. Its very simple to do on the M112 egnine. There is a scharder valve where you connect the gauge if you remove the front part of the engine cover.

Also inspect for any vacumm leaks.

You mentioned that you replaced the MAF. Did you buy an aftermakert one or did you go with stock?

The last thing that I want to ask do you use any air filter that needs lubricating such as K&N?


 1.Did you buy an aftermakert one or did you go with stock?

I bought a new Bosch(OEM) MAF sensor. I am hoping that it works well. Any way to isolate and test this ?

2. Do you use any air filter that needs lubricating such as K&N

   No Lubrication. Just the standard "MAHLE" air filter set which is used Part number 2730940404. Does air filter influence this ? It has been 2 years since I changed the air filter. 





If it has been two years since you change the engine air filter, it is time to change it again. Make sure there are no air leaks and the housing for the air filter closes properly. I would be concered if you used air filters that use lubrification as they can cause oil to coat the MAF sensor and cause problems.

Bosch sensor should be good.

I would recommed that you have mechanic with a professional diagnostic scanner look at the parameters and determine if anything is out of range. A professional scanner will have recommended valvues for the various engine sensros. Basic OBD2 scanners don't usually.

Let us know what you find out.

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