My engine started to stutter during its first 5 minutes within startup in February. I'd been suspecting that it was due to the oxygen sensor nearing its lifespan. However, the problem was so intermittent that I couldn't possibly read the codes out with my OBD-II.

Fingers crossed, I pulled out my spark plugs that I've been meaning to change for a year and a half. The oxygen sensor in my Subaru is hard to get at according to the mechanics at Pep Boys. I wasn't in the mood to be ripped off on a pointless inspection, only to be offered some other unneeded "fix". Bad plugs were also a natural suspect since it seemed like cylinders were being skipped every shaft rotation.

Used Spark Plugs

The spark plugs were obviously the issue here... They're all burned out and the gaps were f-ed up. I luckily had the right spark plugs at home so I fixed it immediately. The plugs were really hard to get out and also get in, but with some effort and a torque wrench I set those suckers in at 16 lb-in and my engine was humming as if nothing had happened.