I recently replaced the front oxygen sensor on our 1996 850 (gang, meet Arabella; Arabella, meet the Brickboard bunch), and thought I'd share things I learned for the benefit of others.

Bay 13 has a discussion of how to do it (http://www.volvospeed.com/Repair/o2sensor1.php), but I still learned quite a bit. The following notes supplement Bay 13's discussion:

1. The online parts houses, and Bay 13's discussion, both say that you have to replace sections of wiring harness to prevent contamination of the reference wire. On the other hand, a local, large Volvo-only parts house, and the dealer, said this is not necessary on 1996 850's, only on 1997 and newer. I went with the cheaper advice; time will tell if I was right. I wound up paying less by going with the local parts house, from not buying the wiring harnesses.

2. The connectors are at the rear of the engine on the left, on a little bracket bolted to the engine. As Bay 13's discussion indicates, you have to remove the flex duct from the exhaust manifold to the airbox to get to them. They are locked together by the red cap shown in Bay 13's discussion, and lifting it up about 1/4" will allow them to be separated. If you have trouble separating the parts, see if the cap will lift up more, because they should come apart fairly easily.

3. The clips that hold the wires in place as they run from the sensor to the connector can be removed by squeezing the body of the slip with pliers. I first tried getting to the part that clips into the body, but it's almost inaccessible, even with my best needle-nose pliers. The lower clip is in a tricky location, but a little grunting should make it work.

4. Like the O2 sensors on older Volvos, the sensor takes a 7/8" wrench, but not, alas, a box wrench. There's enough room for an open-end wrench (aka spanner for Brits).

5. My big lesson: that red cap that holds the connector together also pulls the parts together. You line up the parts, push them together until you encounter resistance, then push the red cap down. Little fingers running down into the body of the connector have slanted slots that engage tabs on the connector from the O2 sensor. As you push the cap down, the slots pull the two connectors together. Clever, though very elaborate. I found it easiest to assemble them while they were loose from the bracket, but that may be because it was how I understood the function of the red cap.

6. Final lesson: the connector from the O2 sensor has a clip on the side that snaps onto the bracket to hold it all in place. Don't try removing this once you've pushed it on; the little finger that snaps into a slot on the bracket is easily broken - he said with the voice of experience. If you do break it, you'll add five minutes to the work and use up about a pound's worth of, um, frustration threading a zip-tie around the body of the connector to hold it on the bracket.

Thanks for the post - I am getting ready to replace the front O2 sensor on my wife's '95 850 Turbo and your information has answered many of my questions! I was able to get the clip off of the sensor leads down underneath the car without breaking it. I removed the old sensor and reinstalled it; I wanted to see if it was stuck, and if the O2 sensor socket for my '80 245DL would fit (fits 7/8" and 22mm), which it did. I wasn't sure how the red cap worked; now I know, thanks to your post!

I was also wondering about replacing the wiring harness for contamination reasons; FCP Groton specifies on their website that the replacement is necessary for the 1997 and newer 850's only; I feel reasured, though, to have that information confirmed by your research.

I have a few questions:

1. How many more clips for the leads are there and where are they located?

2. Did you use a universal sensor, and if so, how did the splicing go?

Thanks again,
Bill

On my '96, there were two clips, both on the stamping for the motor mount. One is visible from the top, one from the bottom. The bottom one is in a rather awkward spot, as the sway bar is in front of it - visible, but tricky to reach.

I had seen the clips advertised as an available part, and hadn't paid much attention, thinking I could use good old zip-ties if needed, but in fact they wouldn't work very well.

No, I didn't use a universal sensor - I asked about this when speaking with the parts house staff from whom I bought the part, and they said that they hadn't seen them work out well. Apparently, mechanics had difficulty getting the connections to stay over time. From the pics, it also looks like you have to commit to the universal sensor. All in all, the extra money seemed worth it to use the factory's original plan.

I used a universal sensor on a 96 850 and got poor results. The problem wasn't in the "splice" which, if done correctly with the waterproof kit supplied with the sensor, about has to be good and is very difficult to screw up. I checked my splice many times with a ohm meter and it was fine. The vendor even sent me another sensor (very unusal to get a return on an electrical part that has been installed) that worked briefly and quit. Used a "direct fit" sensor and presto, problem solved. A lot of guys here on the BB get great results with universal sensors but I didn't for some reason.

bl

Hmm, that's discouraging news about the universal sensors. Did you use the Bosch universal, or another brand? As far as commiting to universal from here on out, it looks like the splice should be made between the sensor and the original connector; that way, if a return to the original setup is indicated, one just has to unplug the old spliced lead and replace the sensor, lead and connector with an OEM type setup.
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Ten Sleep 1980 245DL 1995 850 Turbo

It was a Bosch universal sensor I used. I've heard to avoid all generic sensors (Walker etc.) but again, I've read a number of posts here on the BB from guys that used generics and have had no problems at all. Beats me. Maybe just my bad luck. On universal sensors with splice kits, the connectors used are the "bullet" type. If you use them correctly it would be very unlikely to have a bad connection. I took the connector off and did my splices in the house so I'm sure they were connected and secure. As I mentioned, I checked them with an ohm meter and the connections were secure and I triple checked my wires to be certainthey were correct. I'd say the odds of getting two bum sensors would be VERY remote but neither worked, When switched to "direct fit" type, all was well

bl