Trouble Code P0170
 

The check engine light on my '97 C230 w/75k mi has come on, the engine seems to be running lean. My scanner pulled trouble code P0170, "Fuel Trim Malfunction." I have read in other posts that this is usually a result of the MAS. Data from the MAS was in real-time mode so I believe it is OK but wouldn't know otherwise. However, the data from my O2 sensors looks like #2 may be bad. #1 O2 ran at about 0.1 volts. Once the #2 warmed up it ran at about 0.8 volts. What is the nominal operating voltage of the O2 sensors? None of these sensors have ever been replaced. Thanks...

I can only give you a couple of ideas.
O2 sensors are capable of generating between 0.1 ( lean ) to 1.0 ( rich ) volts.
The pre-cat O2 just switches faster ( responds to mixture changes quicker ), than the post-cat sensor.
The post-cat sensors purpose is two-fold. It watches the pre-cat sensor & also " measures " the efficiency of your converter.
If you are running out of fueltrim, your # 1 sensor probably needs replacing.
Gee, I sound like an O2 sensor salesman, which I am not !
:D

P0 170 means the computer isn't controlling the fuel mixture from sensor imput.
Rarely does an oxygen cause that code unless there are OTHER O2 sensor codes. 99/100 the air mass sensor will cause this code.
OF course fuel pressure & engine vacuum leaks can cause that code as well!

M.B.DOC

I am still not very familiar with M-B, so I tend to speak in generic terms when I try to explain the function of pre & post cat O2 sensors.
I'm sure I will eventually catch on to M-B's logic, being german and all. ;)

Pull the vac line off the fuel pressure regulator and see if any gas drips out ... this means a bad regulator and will cause trim code..

Thanks for the info. There was no fuel in the vacuum lines so I ordered an air mass sensor.

For general info, shouldn't both of the O2 sensors have approximately the same reading? Otherwise, it seems they would send contradictory info to the computer. Thanks again for your help...

No, the front O2 sensor monitors the amount of oxygen before the catalyst & then the computer adjusts the fuel mixture.
The rear sensor monitors the oxygen after the catalyst & that info goes to the DME for long-term catalyst info.

Did the new MAS fix your code P0170? I have had it and I thiught I fixed it by fixing a vacuum leak, but now a month later (about forty drives in that time) the same code shows up. I replaced the MAS only a year ago!

Thanks,
-Tom

I had the same code on my w210 97 e320. I checked the vacuum lines for leak (no leak) and ended up changing the MAS. I was told on another board (or maybe it was this one) that after MAS replacement, I need to get the fuel trim reset by a MB computer. I haven't resetted the fuel trim yet and the check engine light came back with the same p0170 code after only a couple of drive cycles. The car drives fine except the annoying CEL.

delta
97 e320 with 57000 mi

It has been only a few days since I installed the new MAS. The check engine light has not reapeared, knock on wood, but it usually takes a while for it to come back on. If it does I will post here again. The car is running much smoother in the low RPM's and is more responsive so I think that did it.

I am not aware of having to reset the fuel trim other than turning the Check Engine Light off by disconnecting the battery or OBDII port.

What is a good way to check for a vacuum leak? Those lines run to multiple places and seem to be controlled/vent thru the electrical gizmo/manifold on the front of the engine (near the water temp sensor) on the C230.

delta,
I read stevebfl's post about resetting the adaptation values. Too late for me to do that now. At least it looks like it will eventually adapt to a good setting.

aTOMic,
If it's not a vacuum leak, you may have dirty air getting to the MAS. Other than the obvious air filter, there is a big o-ring at the end of the MAS. You may want to verifiy that it was reinstalled when the MAS was replaced.

PPatterson,

My fuel trim did not re-adapt after at least 100 drive cycles. I ended up taking it to an independent tech. for the fuel trim reset. Hopefully this ends my check engine light saga.

Delta