03 Forester threw P0420 code on occasion.  After changing "lazy" O2 sensor, code went away for over a year. It's back, and exhaust seems to smell rich and tailpipe is a bit sooty; also, gas mileage seems a little off.  Thoughts?

any maintenance need catching-up? sparkplugs?

check plug wire boots for oil - VC gaskets may be leaking. that leads to misfires.

live data could be helpful here, maybe vacuum leak is causing the ecu to dump in more fuel or ????

If I recall correctly the P0420 code on that age Subaru is generally for the rear O2 sensor seeing above 0.8VDC (?) during car operation.  It's just there to monitor cat efficiency.  You could try the spark plug anti-fouler trick or this one

https://www.amazon.ca/CNT-90-Degree-Angled-Extension/dp/B016P42V4W/ref=pd_cp_263_3?pf_rd_m=A3DWYIK6Y9EEQB&pf_rd_p=95448740-5917-4df8-bb6a-086a0d0bb079&pf_rd_r=J6MZWBK5MHS91Y90C53B&pd_rd_wg=oU758&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&pd_rd_w=kFTGe&pf_rd_i=desktop-dp-sims&pd_rd_r=f73843f6-d72d-11e8-8593-9beb2ea54215&pd_rd_i=B016P42V4W&psc=1&refRID=J6MZWBK5MHS91Y90C53B

If its a running issue it could be the front 02 sensor (use OEM brand).  Live data would be helpful to troubleshoot.

 

Where are you long term and short term fuel trims? What's your idle MAF grams/second?

GD