I put the new O2 sensor on my 850 R Saturday. I was getting 22.4 mpg prior to that. I checked it today, and I am getting 25 mpg now, and I ran the heck out of it for a while right after putting the new one in. I got about 200 miles on it since changing so it's not a real good test, but it sure looks like an improvement so far. Other than the fact that I lost that piece of vacuum line, it was not very hard to do. I could not get the new one to set back in place where the electrical hook up is. Is there some trick to getting it fit back where it came from? I sure could not see how it was in there so I just strapped it to the rear sensor.

 

It is a pain in the Butt to do.The one side of the plug slides onto the metal bracket.

 

What vacuum line are you talking about?
Where does it start and end?
O2 alone should not do that kind of change, were you throwing a code?

 

Sorry, misunderstood your post. There is no trick really, depends on what type of O2 sensor you bought. Bosch don't fit right, so you have to cut off you original connector and use that. If you used an OEM it should go easily back inplace, if not they might have given you the wrong O2 sensor, this happened to me.

 

More background info at...
https://volvoforums.com/m_10203/tm.htm

 

I used the OEM O2 sensor. I just could not figure out how to get the electrical connection side back in the metal bracket. I strapped it to the rear sensor so it's going to stay put. Maybe someday when I have nothing to do, I will look at it again and see if I can figure it out based on what Tech said.