Hi,
I have a 1996 wagon 850, no turbo. I have had this code pop up in the last few weeks. The mechanic wanted to jump straight to replacing both 02 sensors, but think it might be simpler (and less expensive). I have had my MAF replaced in the past 5 years and don't see any vaccum lines with obvious leaks.

One interesting thing is when I'm on the highway the engine light turns off, is this due to higher speed=higher air intake= system no longer too rich?

Poking around I found this small hose unconnected to anything other than the air filter box. I cannot find where the other end is supposed to go. Please let me know if you know what it is or where to connect it.

 

It connects to a rubber elbow on the air filter box underneath

 

I agree with your thinking that the p0172 is most likely a small vacuum leak, and your observation about the light going out at speed supports that theory. since you found an odd open tube you can try plugging the end to seal it up (or find what its supposed to connect into) then user an OBD2 reader to clear the code and see if it resets. If that doesn't work, find a shop that can do a smoke test. You may have a vacuum leak or an intake air leak in an obscure place and a smoke test is the best way to suss that out.

PS - Replacing both O2 sensors rarely makes sense anyways and sensors usually fail with their own codes and the two sensors have two different jobs (front drives the fuel trim, rear measures the efficiency of the cat converter). Plus these cars use four wire sensors - two are for the sensor heater element and two are for the measuring circuit. The sensor specific fault codes tell you which sensor, which circuit and what was wrong with the signal (open circuit, short, signal out of range etc).