Vehicle is 1999 ML 320 with 107,900 miles on it.

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

Code reader shows P0156 which is Bank 2 Sensor 2 malfunction. I believe that refers to the driver's side O2 sensor after the CAT.

Questions
Should I replace that O2 sensor? And if so why when the vehicle is running fine. Will any harm be done to the vehicle to run it with this fault?

Edited:

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.

Gillyyyy

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Gilly,


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.

Yes the engine control module will be pretty fast to throw a code on any of the sensors, and yes I think you are on the correct sensor. I'd slow down a little to fault the front sensor, more than likely it's the sensor itself, I would tend to think along the lines of a bad cat (not operating efficiently) before I'd fault the front sensor. If the front sensor is junk you'll get a code for it, don't worry about that! I'd plan on the rear sensor sometime. Problem with disconnecting the battery is now you've lost misfire recognition, need to relearn the crank angle sensor adaptaion, don't know if I have the instructions around here for that or not, you can adapt it yourself but won't be able to prove it's adapted properly without an SDS computer or the like.
Gilly

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Now that is a possibility since that is the CAT that had a hairline fracture around it last August. The CAT perimeter was TIG welded and so far has held. The CAT never came apart nor rattled, but maybe it was compromised.

I don't understand what the above quote means, would it be possible to put it in simple terms for me.

Yeah, stop disconnecting your battery to clear check engine lights. The control module gets pissed. Your control module does not presently have a clue if your engine starts misfiring. If the engine starts misfiring and the control module doesn't take appropriate steps, a rear O2 sensor is gonna look like a bargain.

Gilly

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