Hello Folks,

I have a '99 ML430 that is displaying the 'Check Engine' light. I took it to a local motorsport shop and had the codes read, po170 and po173. I was told it means a bad air mass sensor. I bought my vehicle from CarMax and the MaxCare extended warrantee gave my local dealer such a run around that they suggested taking it to CarMax for repair. CarMax is telling me that those codes indicate a bad fuel pressure regulator. After searching the forums, the major of what I read would indicate a bad air mass sensor. What are the chances that CarMax is correct in that it's a faulty fuel pressure regulator? Anyone ever heard of this? Thanks for your help. -Norm

The code P0170 and P0173 are adaptation codes. That means the controller has tried to repair a mixture problem and couldn't within the software parameters it was built with. The typical AMM problem causes the part load adaptation to go to 1.32 trying to repair a lack of load signal from the AMM. That mean it is adding 32% more fuel than original calibration because the car is way too lean. The diagnostician will know if the parameter is failing rich or lean by reading the adaptation values.

A fuel pressure regulator can cause the car to run rich or lean due to fuel pressure being out of spec. If spec is 40psi, then 60 will be 50% too much fuel (probably... may not be linear). Fuel pressure regulators usually fail by raising the pressure so one would expect numbers in the range of .68 (32% less fuel adaptation) as the controller tries to fix a rich mixture.

So the situation is simple for a knowledgable tech. Readapt to 1.00 and drive the car, if it relearns one way or the other one can simply read the fuel pressure (as simple as reading tire pressure on that model) and verify first if it is out of spec and second if it is out the same direction as the adaptation.

Deciding and proving the AMM isn't quite as easy as its output is dynamic and there isn't an exact this for that type measurement.

From your standpoint it should be easy. Let them put a fuel prressure regulator on it. They will be wrong and when in a week or two the light is back on you can get them to repair it right.

BTW after the AMM is replaced an exact confirmation is to be had by anyone knowledgable enough to look. The proof is that after ten miles or so of driving the thing is no loner having to adapt. It becomes cut and dried at this point.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Last week I had a customer with a 1998 C230, complaining what they though was a transmission relayed drivability problem, lack of power, bucking and erratic shifting. The car had two codes, P0300 random misfire and code P0170 fuel trim bank 1 malfunction, the adaptation value was at the max. Road testing the car, it felt like a load input problem, the Fuel pressure was right. I change the MAF sensor, problem solved. Get a second opinion.

Hey Steve and C32AMG!

Thanks for your quick and detailed response, I really appreciate your input. Yeah, initially wanted to dispute the CarMax tech and disagree with his diagnosis. But I just casually stated that another tech read the codes to mean a faulty air mass sensor. But then I figured that I'll end up getting a new fuel pressure regulator out of the deal. I really wanted a Mercedes shop to make the repairs, but MaxCare warrantee makes it so difficult for a thirdparty to make a claim. I have learned a valueable, albeit, expensive lesson in car buying. The next time, I'll buy from a Mercedes dealership. Again, thanks for you insight and advice. -Norm

I'm still a little concerned about how long it's taking CarMax to get around to the repairs. It's been about 2.5 weeks that the Check Engine light has come on. It will take another week for CarMax to order the part and schedule an appointment. I suspect that this is the wrong diagnosis anyway and will take yet another week to order the correct part and install it. Is there any danger of damaging any other component(s) due to a faulty mass air flow sensor? Also, how long and how difficult is it to install a fuel pressure regulator? Thanks again for your help. -Norm

Well, CarMax now tells me that it's a bad fuel pump causing these codes. I asked if they were using a code scanner to come to this conclusion. They said yes, that they are using the very same one that Mercedes uses. If that is the case, couldn't they see any abnormalities in fuel pressure coming from the fuel pump? Couldn't they check for pressure at the fuel pump? It's almost like they are just guessing and replacing parts. The service tech was confident when he said it was the fuel pressure regulator. Then he said it's the fuel pump. I would bet that once he replaces the pump, he's going to finally get around to tell me it's the MAF. -Norm

Can you please tell me what fixed the problem eventually? My C320 gives the exact same codes P0170 and P0173. Thanks

i have a 95 c220, with the same problem.i replaced the mas airflow sensor amd it corrected the problem for about three weeks.then that same code reappeared. this leads me to that was not the problem.you can hear the fuel pump cycling on and off. try checking your fuel pressure at the fuel pump.then try checking fuel pump relay....

I have 99 ML430 and just today check engine light came on. I used OBD 2 car reader and found out it has DTC codes P0173 and P0170. any one knows if it is safe to drive this car before getting it fixed? and also if this can be expensive repair or minor work. my first time using this site so excuse me if I seem novice. thanks for quick advice. sid

Here's my take from years ago:

Had the CE light come on. Ignored it. Disconnected the battery to reset the codes.

Occasional hard start issues followed. ignored.

Noticed issues with severely-poor acceleration...ignored.

CE light came on again...diconnected battery and ignored.

Six months later. Took a Memorial Day holiday trip with the ML. Stalled on the highway several times for no apparent reason. Highway was jammed full of cars, so decided to take an adjacent country road. ML stalled and would not restart.

So here we are in a desolate country road with a dead SUV (and very little cell signal strength). Family and tow truck arrives at 11:30PM. ML is towed 80 miles to the nearest MB dealership.

Diagnosis...fuel pump dead! Replaced. MAS was also diagnosed as failing. Replaced. Vacationed for an additional three days due to waiting for parts, etc. Trip required an additional $1100 for that ordeal.

I'm not sure which part started the fight, but I believe one ailing part probably caused the other to fail.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Thank you so much for quick response. Do you remember roughly how much was the cost to fix the problem and which parts were replaced? Appreciate help on this.

Total parts and labor (dealer)...$1100... fuel pump and MAS...that was three years ago. If you havent already, spring for the fuel pump retrofit (i.e.update) as well. I tried to get that done too, but the parts were on backorder and we were already overdue to go back home. So they had to button everything back up after the normal repairs.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car