Hello all, newbie here. New to this site and the Subaru Forester family. I recently needed to replace plugs and 2 or the 4 appeared to have melted center electrodes. For the last month or so I have pulled the P0420 Cat sys efficiency below threshold bank 1 code. After resetting, it would usually stay off for a few days. Eventually, in addition to the P0420 code, I got a cyl 3 misfire code, clear that and in a couple of days I got a cyl 1 misfire instead of the cyl 3. So I changed out the four plugs. Plugs for cyl 2 & 4 for, while not great, looked much better and had gaps that were close to correct. Plugs for cyls 1 &3 looked to have melted (rounded and very low) center electrodes with huge gaps. I looked online for possible causes of the plug problems, but didn't see much. After installing new plugs the vehicle ran great, no codes for about 2-3 weeks. Now the P0420 is back, but no misfire codes. I haven't had the opportunity to pull one of the problem plugs to see if there is an ongoing issue. What could have caused the plug problem? And would the plug issue, probably not burning all fuel, cause the P0420 or is it unrelated. Thanks for any and all input.