Subaru Legacy Cruise Control issue

Tou either have an air leak (loose filter cover or detached hose), or a bad front O2 sensor. Check the former; erase the code; if it recurs then you need a front "air/fuel sensor", which is common.

Thank you for this answer! I took a look at the cover and hoses around it. I did find one hose that seemed like it might have not been on as tight as it should be, and erased the code. I then took it out for a long drive, and about 50 miles down the road the light came back on. Is this likely my O2 sensor then like you said? Is there any other way I could test/diagnose for that?

Of course! Just visit any store or gas station that will scan your ECM and provide (and erase, if you'd like) the CEL code (POxyz). Then just look it up online. If it repeats as the air/fuel sensor then buy one and install it. Will take you only a few minutes, butgetting the old one off may take a bit of elbow grease or a long socket, but I remember 2006-2009 2.5i being fairly easy.

Thank you for this! I was finally able to replace my sensor today. I have another question though. As I went to put on the new one on, I realized that the old one has what appears to be something like a large washer around the top of the threads, and the new sensor did not have this. My question is....am I suppose to figure out how to get this thing off and put it on the new sensor? Or did I somehow end up with an incorrect sensor? (Though apart from this it looked exactly like the old one and wire plug in fit where it was supposed to). I assume the answer is not simply that "it doesn't matter"...

One more question for you.... just a couple days ago (before I replaced my sensor), the car was starting to make like a chugging noise when I accelerate. Would this potentially be related to this same issue, or something else?

Same issue; washer unimportant if you got a good tight seal.