If a vehicle runs fine with a fault code showing a malfunction in one of the O2 sensors after one of the CATs is there any need to replace it? This is not on my Porsche but another car. I figured the OBD2 stuff with the 4 O2 sensors would be the same for all cars, then maybe not.

In any case I don't want to replace an O2 sensor just because a computer told the check engine light to come on unless it's necessary. For all I know the O2 sensor may be barely out of the designated parameters but still work just fine.

Vehicle runs and idles fine; no different than before check engine light came on.

 

OBDII reads and tracks the performance degradation of the O2 sensors in front and back of the CAT.

It will also do end of life predictions of the sensors based on comparative analysis mentioned above.
It knows when one is bad and when it is going bad.

There is a chance you could reset the car through battery removal and watch for the CEL.
You will have to run the car through 5-6 cycles of cold-operating temp- cold to see if the CEL reappears.

If it comes back on, you will need to replace the sensor.

Hope this helps.
Scott

 

Well I disconnected my battery to reset the check engine light. I started my vehicle up and the check engine light is now not illuminated when I’m driving.

I did a little reading and it seems that the parameters the computer requires for the check engine light to stay off for a P0156 fault code are very narrow. So it may come on again soon; I really don’t know.

According to the code the O2 sensor on the drives side of the vehicle behind the CAT is the one that supposedly is malfunctioning. If I’m not mistaken it measures the exhaust emissions after they go thru the CAT. So one of two things is happening. Either the O2 sensor that threw the code is not capable of reading the emissions that are passing thru it as being within the parameters, or theO2 sensor in front of the CAT is not working properly either and therefore the computer is allowing an improper mixture which causes the O2 sensor behind the CAT to detect that the emissions are not within spec.

Since the vehicle is running identical to the way it was running before the check engine light went on it would seem logical that the O2 sensor behind the CAT is having trouble measuring the gases to tell the computer that everything is as it should be. To conclude otherwise one would have to assume that the O2 sensor in front of the CAT is malfunctioning, but the computer is not reading the fault from the O2 sensor before the CAT, which is unlikely.

I don't think the O2 sensor malfunction of the sensor behind the CAT can affect the way a vehicle runs unless the O2 sensor before the CAT is malfunctioning too.

Bottom line: There seems to be there is nothing wrong with the way my vehicle is running.

I am correct or mistaken in my analysis.

 

One very unsavory thing to consider is that the CAT on the driver's side is failing.
One thing I forgot to mention by the differential O2 sensing is they use the pair of inputs
to predict CAT degradation......... If the sensor behind the O2 is showing out of range, then either the sensor is bad or the CAT is failing.

With one O2 sensor of four failing, it is unlikely you will see a performance change. You will, however, fail any emissions testing.

There is a safety factor to consider. If the O2 sensor failed where the feedback signal is telling the computer to run rich, you could be dumping unburned fuel into the exhaust system.
This was a problem with 928s. If you do a search on CAT fires on the 928 board, you should find some interesting reading.

What kind of car is this on and how many miles on it?

FWIW, if the car were mine, I'd replace the sensor...... $50 and some time this weekend should take care of things.

 

The after cat sensors do not control anything in the computer, but only report. Running with a bad after cat sensor will do no harm to the car. Easy and cheap to replace if you are concerned.

 

"According to the code the O2 sensor on the drives side of the vehicle behind the CAT is the one that supposedly is malfunctioning."

You need to use an OBDII scanner to monitor the O2 sensor voltage before and
after the CATs. The O2 sensor after the CAT it only used to determine the
CAT efficiency, as mentioned previously. If the after the CAT O2 sensor voltage
is varying significantly then the CAT is "weak". If the voltage of that O2 sensor
is out of range (too low/high) then the sensor is bad.

Bottomline: Don't guess at the source of the problem, as these repairs can be costly.

 

Scott,


The vehicle is a 1999 ML 320 with 107,900 miles on it.

I live in South Carolina and we do not have emission testing so that is not a concern.

The sensor is a little over $100. It not so much that I don’t want to spend the $100, it’s more like I have a fair amount of disdain for OBD2 and I don’t want to replace a part just because a computer tells a dash light to illuminate when the vehicle runs just fine.


Loren,

Can the O2 sensor be tested when it’s removed from the vehicle or does it have to be done installed with the vehicle hooked up to an OBD2 tester?

 

"Can the O2 sensor be tested when it’s removed from the vehicle or does it have to be done installed with the vehicle hooked up to an OBD2 tester?"

It would be very difficult to test the O2 sensor removed from the engine's exhaust
system because it requires a very high temp for operation. The best approach
would be to borrow an OBDII scanner or go to a "big box" auto parts store and
use their unit. Many allow the use of their scanner at their facility.

Also, with the mileage indicated (108K), the O2 sensors are way beyond their normal
life, i.e. 60-80K. So, you've been very lucky in not having to replace them before.

 

The code was previously read at Advanced Auto; that's where they told me the fault code was P0156. I go back and see if I can get them to check the sensor outputs so I can tell if it's the CAT or the sensor.

 

P0156 per the Porsche OBDII manual:

Signal for Oxygen sensor after three way CAT is implausible (cylinders 4-6).

This implies that the O2 sensor is out of range and needs replacement & not the CAT.

 

Loren,

The P0156 code is for my Mercedes ML 320.

Interestingly, when I started the vehicle this morning the check engine light came on; this is after I cleared it by disconnecting the battery yesterday. Then while driving but before I got over to my buddy's shop the check engine light when out.


I had a buddy hook my vehicle to an Ottoscan OBD2 reader. The Ottoscan shows live data. He compared the graphs for each bank. While I did not understand all the numbers and graphs he said the problem was the sensor, i.e., the one behind the CAT on the drivers side, I think he said it's about worn out and that the voltage variation between the O2 sensor before the CAT and the sensor after the CAT did not vary to such a degree as to indicate that the CAT is failing

He said the problem is not with the CAT, but with the sensor after the CAT as indicated by P0156 code.

He said as other have too, that the sensor behind the CAT only measures emissions and does not affect performance. Since my CAT is functioning fine there is no reason to replace the sensor other than the light going on.

At the present time vehicles do not have telemetric links to big brother telling him or her as the case may be that my vehicle may be emitting exhaust gases not in compliance with whatever the current law mandates. So with that in mind I guess, at least for now I'm safe.

 

"The P0156 code is for my Mercedes ML 320."

Actually, most generic type OBDII codes are very similar for all OBDII cars.
Check out this link ( www.systemsc.com/codes.htm ) for some of the generic ones.