Hello, I have a 96 850 turbo wagon with 130,000 miles that has always run strong. A few weeks ago the check engine light came on, the gas mileage dropped to 16, and a rich very nasty smell is always present from the exhaust. It still drives ok but has lost a little top end power. The auto parts store ran the code and told me it is the bank one o2 sensor and it's reporting incomplete, or less than optimal, combustion. The car has good plugs, wires, etc... so nothing I can do there. I tried to clean the o2 sensor by soaking it in gas overnight but no change in conditions. The sensor appeared to be covered with black soot which I could not clean off very well with gas. Are there vacuum lines or other things I can easily check before I buy a new sensor? Also, is there a wiring harness link somewhere for the sensor, or do I need to solder wires if I get a new one? Is there any way to know for sure if the sensor is bad or if something else is causing the bad, ultra rich, combustion? This ultra rich, smelly, low mpg condition came on all at once with the check engine light, prior to this the car ran excellent and the engine still idles fine and sounds good from a mechanical standpoint. Thanks for your help. Arlo

 

Get an O2 sensor. I usually pull them from the junk yard but make sure you get an OEM one imo.

 

+1 on what rspi said - don't get the universal ones. I've heard more than a couple people say they did and they still were throwing codes for O2 sensors. This is one place where you wanna get the OEM (bosch) part.

 

+2 on the OEM Bosch sensor...

Ive pulled 2 from the yard in the passed so now when I throw an o2 code I pull the dirty one and swap it out for the clean one.

Then I clean the dirty one, its usually at least a 2-3 day process but I go almost a week soaking it.

 

I've pulled a few from junk yards and had good success. I think most people would get to the steeler and get those fixed. A year later the car craps totally out or it gets into a finder bender and gets totaled. So you have a 1 to 3 year old O2 sensor sitting in a junk yard.

The trick I like is to pull it yourself. If it comes out easy, it probably has not been in the car for 13 years.

 

I'm so excited, I went to the U-pull-it where I have not been in a few years and they had a decent selection of 850s and V70s. I got 2 nice looking Bosch o2 sensors for $7 each and the first one I put in the car seems to fix the problem. I did a short test drive and the smell is gone and the mpg looks back to normal. The light is still on but that may take some time to go away if I don't take it somewhere to get it re-set. How long will it take to go away if I did fix the problem? I will research the light issue in the forum search feature. I love the junk yard again, it used to be I could never find enough 850s to make it worthwhile but now they are stocking them so I can keep this car going forever.

 

If you DO get a new one, get either OEM or a Bosch non- universal one from the parts store.

However, codes don't necessarily mean you need a new sensor. Which code exactly was it? There isn't one that means "incomplete combustion." There's one for lean mixture, there's one for rich mixture, there's one for "slow switching," (this one is almost always a bad sensor) there's one for no signal (usually bad sensor or wiring) and several others. Rich or lean often mean just that- the engine is running rich or lean, but the computer isn't able to do anything about it. So it sets that code- the O2 sensor is reporting "X" condition, and I can't do anything to correct it. Doesn't mean the O2 sensor is bad, just that what it's telling the computer is different from what the computer expects.

On the other hand, O2 sensors DO start to degrade over time. A rule of thumb on OBD-II cars is that the sensors should be replaced every 100,000 miles, and I personally don't advocate installing used ones. O2 sensors are quite fragile and are easily damaged during removal. If you're strapped for cash, try a used one using the "easy to remove" way of determining age as listed above. Make sure to coat the threads lightly with anti- seize lubricant- new ones will have this pre- applied.

 

Hey Carrots,

I had my mileage on my 740 wagon dropping suddenly to 20 mpg from 26, and the codes showing 'lean mix', so I got a brand new Bosch sensor.
Because I need my car all the time I didn't find the time to go to the garage to put the new sensor in. I wonder how hard it is to do this yourself on the 740. Pierce didn't seem to recommend doing it yourself, however.
If I have to get to it from under the car I'd rather visit the garage. Maybe they can do it on the spot, how long do you think replacing it takes?

Greetings, Stephan

 

The garage put my new sensor in. I had gotten the original Bosch part, and it appears that it has fixed my 'Check Engine Light' problem. It also appears my idle is smoother than before. Will have to wait a few days before I can say that consumption is back to where it should be.