'07 Legacy Limited 2.5i, non-turbo, 140K miles.
Since 2011, I have replaced an O2 sensor 5 times. Each time, it's due to CEL and error code P0131. These sensors are supposed to last 100K miles. I drive about 10-12K miles per year. Replacing the sensor does resolve the problem for a year or two (lately more like just a year), but it seems like maybe there's another issue somewhere.
Any ideas? The car is properly maintained otherwise, no other CELs, no known oil/exhaust leaks. The sensors aren't coming out filthy or anything. My mechanic told me to just replace the sensor.