What should I check first for a mild cylinder 4 misfire on my Civic?

[TimothyS]

My 2017 Civic’s check engine light came on yesterday, and the only symptom is a very faint, intermittent rough idle when I’m stopped at a light. I scanned it and got a P0304 code for a cylinder 4 misfire, but the car isn’t shaking badly or losing power. I’m just not sure where to even start looking since it’s so mild—could it be something simple like a spark plug, or is this pointing to a more serious issue with a coil or even a fuel injector?

[Olivia80]

P0304 means cylinder 4 is misfiring. With a faint idle like you described, it’s tempting to shrug it off, but it can be as simple as a marginal spark plug. Look at the cylinder 4 plug — is it fouled, oil-coated, or gapped oddly? If the plug looks fine, move on to the coil.

[Violet65]

I’ve done the test where I swap the coil‑on‑plug from cyl 4 with another cylinder. If the rough idle and the misfire code follow the coil, the coil is weak or bad and should be replaced.

[MarkP]

Could also be fuel related a bit. A marginal fuel injector or a tiny vacuum leak on the runner can cause a single‑cylinder misfire without dramatic symptoms. Check for cracked hoses around the intake, and if you can scan live data look at the fuel trim on cylinder 4. A weak fuel pressure could show up there.

[Steven17]

Not sure if this is just a sensor hiccup or a real mechanical fault. Do you have live data access to see misfire counts and short/long term fuel trims?