97 850 Gas Mileage Dropping
 

I have a 97 850 wagon with almost 273,000 miles. The CEL has been on for last two years throwing code for 02 sensor low voltage (Bank 1, Sensor 1). I haven't noticed anything until the last month or so and being the sensor costs about $150 I have been too cheap to replace since haven't noticed a problem. Recently though my gas mileage has dropped from about 26-27 mpg to 21-22 mpg. I'm assuming the sensor has finally got me. Any thoughts? Could I put a used one in to save some $$$?

Have you noticed a loss in power? I am wondering if your cat is getting plugged. It would show as loss of power, poor gas mileage and throwing O2 codes.

Sounds like the o2 caught up to you... to me... (thats proper english im sure)

You can get a used one to save money, but may be in the same boat in short order... You could get a universal one that you need to splice wires on... but I would just bite the bullet and get a new oem one... =(

I haven't notice any loss in power. It has been hard to start for the past few months, especially if it sits for a day or two. I think part of that is the starter not staying engaged.

Ok, I finally replaced the front O2 sensor. My fuel mileage has gotten better, still not back to where it was 6 to 8 months ago. My check engine light has come on again, I assumed it would be for the rear O2 sensor. But after checking the codes it came up with a code for System too Rich (Bank 1). I don't even know where to begin on this. Any ideas??? Thank you.

When was the last tuneup? Plugs, wires, cap, rotor, filter?

I called FCP groton to see if I could return the O2 sensor wiring harness they say you need for 97 (which I have a 97 and didn't need it). While on the phone I asked about my new code for the System too Rich. He suggested the possibility that my cat is bad. He mentioned that I could possibly replace the cat with a "test pipe" or "down pipe" and use a O2 defowler (not sure on spelling) to replace the rear sensor. He thought this might solve my problems. Any comments or suggestions on this would be greatly appreciated. Thank you.

I did the plugs, cap, and rotor probably last year. I was getting codes for misfire. Changed them and not getting codes for that anymore. The filter (I'm guessing you're talking about air filter) looks ok but I have been planning to replace it soon.

My 1st guess would be the rear 02. I installed a used one about 8 months ago and have not had a problem. I did get a code for the other one a week or so ago.

If there was a problem with the rear O2 sensor, wouldn't it give some kind of code for the O2 sensor circuit?

Rear O2's do nothing other than tell the ECU that the cat is working, thats all they do.
What the guy at FCP Groton was talking about is a "dummy" O2, you put it inplace of the rear O2 and it sends a false signal to the ECU so that it thinks there is still a catalytic converter on the car.
I am surprised the guy told you to do that, technically here in the US it is illegal to remove your cat.
People do it all the time, but I am just saying...

That is what I thought. That's why this code seems to be something telling me there is a problem with the cat. That is what I thought. I have heard of them before but no clue where to get one. Yeah, he said it was something he had done for a car that he races, I think. So would you conclude that I do have a problem with my cat?

Faulty O2 sensors cause cats to fail. I had read before on this board about testing O2 sensors when I replaced mine, here is the link https://volvoforums.com/forum/showth...t=32093&page=2