Intermittent P0171 and other issues. 2003 Legacy.

[td16288]

Hi all. I recently bought a 2003 Legacy with 108k miles. Now I am getting this P0171 code about 60% of the time, ie it turns off for about 3 hours of driving and then comes back on for about the same time.

Whenever I have the cel, I get a rough idle up and down but no issues driving. When there’s no cel, the idle is smooth. Mileage is also extremely poor + soot in the exhaust.

I’ve sprayed brake cleaner around and haven’t found any vacuum leaks. My next idea would be the upstream o2 sensor or an exhaust leak.

Any other ideas? Thanks