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Auto Psychic
Auto Psychic, Mercedes Technician
Category: Mercedes
Satisfied Customers: 1437
Experience:  semiretired mercedes benz master guild technician w 4+ decades of diagnosis and repair experience
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1998 mercedes ml320: two new o2 sensors (bank2..Air pump..pump fuse

Customer Question

1998 mercedes ml320. Reocurring code po161 even following two new o2 sensors (bank2,sensor2). Air pump fuse ok. Car stalled for the first time ever but has not reoccurred.
Submitted: 5 years ago.
Category: Mercedes
Expert:  Auto Psychic replied 5 years ago.

Auto Psychic :

hi, i'll be happy to help you in anyway i can.

Auto Psychic :

sorry to hear of your misfortune :(

Auto Psychic :

my recommendation is replace the other 2 02 sensors as a first step.

Auto Psychic :

standing by,

Auto Psychic :

gary

Customer:

Hi Gary: Here is the history. I have replaced the O2 sensor as follows: check engine light comes on, checked codes with obd 2 and found po156 and po161. Replaced O2 sensor with new Bosch OEM which seemed to be ok for about 10 days. Check engine light comes on again. Checked for codes and find po161 only. Replaced sensor with Bosch OEM 13782 oxygen sensor as before. Check engine light comes on, check code and find 161 again. Time between replacement and new check engine light was 10 days on first and 7 days on second. The replacement location is on the right side car exhaust system and second sensor back. Quick check do not reveal any connector wire damage on the car side of the sensor or any cracks or damage to the cat converter nearby. Tightened sensor without torque specification to about spark plug tightness. Thanks

Customer:

Hi Gary: In all there are 4-02 sensors. Two each side. If the others are bad would not the obd indicate that?

Auto Psychic :

not necessary as your module is comparing current draw and resistance.

Auto Psychic :

a scan by a dedicated mercedes scanner would give you a much more indepth look at whats going on.

Auto Psychic :

the fact that its a rear sensor and its heater circuit rather than the control circuit suggests high resistance somewhere.

Customer:

I can move the the new sensor to the other side and take one from that side to see what happens. Is there value in doing that as a way to isolate the problem before spending more for sensors.

Auto Psychic :

since changing the sensor turns the lite out for 10 days i would look at the connector inside the car that the 02 sensor plugs into. look for water or corrosion.

Auto Psychic :

switching sensors is a good idea and will at least tell you a little more about whats going on.

Auto Psychic :

we can chat as long as you want, so try that and we'll go from there.

Auto Psychic :

gary

Customer:

So, the problem may lie in the car wiring to the sensor wiring? the plug looked clean and in good shape and to make sure I cleaned it with electric cleaner. I will change the sensor and see what happens. However, I wonder whether it will take days again to show up. Thanks. I will follow up when I can report back .

Customer:

I installed the new sensor on the opposite side exhaust system (no code problem) and swapped the sensor from that side to the problem side. since then there has not been a recurrence of the check engine light. While it has been 6 days now, perhaps early, it occurs to me that there might have been something about the four prong plug contacts where the o2 sensor connects to the car's connector. In any event, I appreciate the interaction which led me to swap the unit to rule out manufacture problem. Anything from now on will either identify the unit faulty or car wiring damaged. dave

Auto Psychic and other Mercedes Specialists are ready to help you
Expert:  Auto Psychic replied 5 years ago.
hi, dave,
sounds like your making progress.
this is very reminiscent of an m class i struggled with a few years ago.
turned out to be the ecu i'm afraid, we dont want to go there if we can avoid it, spendy.
if it runs ok, erase the codes every two years to pass smog inspection.
i have a piece of electrician tape over my wifes check engine lite.
but dont tell anybody.
what about a maf sensor?
gary

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