98 V70 AWD P0172 and O2 Sensor

[rmmagow]

Chronic P0172 code. Vacuum lines looks OK, I still need to tear into the intake path again. I hooked up a streaming OBD meter to watch the O2 sensor voltage. The front one goes up and down as expected, the rear one however sort of stuck at .48V and didn't fluctuate hardly at all, a .47 one in a while but that was about it. Apply gas while watch didn't change the behavior of the rear sensor. I understand the function of the rear sensor but even given it role in emission control, I would have expected it to fluctuate some. Or am I just seeing the results of a well functioning CAT?
I also get the P0455 but generally "pending".

[clarkS70glt]

I too had the P0172 code on my '98 S70 GLT and it took weeks to figure out it was the cat plugging up. I never had a cat efficiency code thrown which was weird. Hopefully, this is not the case for you.

Based on some research that I did for a friend that does have a cat efficiency code, the rear O2 sensor should remain steady around .5V with a small amount of fluctuation (I do not know the amount it should fluctuate). Also, a test we performed for the cat was measuring the temp before and after the cat, I used a non-contact laser temp gauge. It should be "about" 150 degrees F hotter coming out than going in. Your voltage being stuck at .48V seems suspect to me, could be a failing sensor. It should move some when you apply gas, of course not as much as the front one because of fuel trim controls.

[rmmagow]

I had a used Volvo cat welded in about 2 years ago due to the original breaking apart inside. I'm going to try the temp thing next. Replacement cat is somewhere near 900 bucks, a lot yeah, but it would last a long time. My .48V would fluctuate a very very small amount. I will hook up the device and drive the car in logging mode to see if real-world shows more variation. I also have a new front O2 sensor to put in, I'll do the rear too. My mileage is not so great at about 23 mpg (AWD is disconnected).

[850 LPT]

Your gas mileage seems to be about right for some mixed, but mostly highway driving. I get the same (23.5 to be exact) also with AWD disconnected. Keep in mind that these cars were rated only for 25mpg highway, which you will never see.
Interestingly enough, my mileage only went up about 0.5 mpg when I took the driveshaft out.

[rmmagow]

Since gas prices have been dropping I figured I treat the thing to a couple of tanks of 93/94 octane. It has only been popping the P0172 since then. The 455, yeah, hose problem someplace, but the 172 I haven't seen that for a few years. I'm back to mid-grade gas now, 89 octane I think. Mileage didn't move with hi-test either. I drive at 70~75 mph most times and am not a hypermiler by any stretch, so I guess 23 is OK.