P0304: OBD-II code P0304 means the PCM has detected misfires on cylinder 4 — an ignition, fuel, or compression problem is stopping that cylinder from firing properly.

Symptoms

• The check engine light is on or flashing, and the engine shakes noticeably at idle or under load.
• Power loss, hesitation, and a rough, uneven idle — often described as running on 'three cylinders' on a four-cylinder engine.
• Fuel economy drops sharply, a raw fuel smell may come from the exhaust, and prolonged misfires will eventually damage the catalytic converter.

Common causes

• A worn or fouled spark plug in cylinder 4 — the cheapest and most common cause, and always the first thing to check.
• A failing ignition coil-on-plug (COP) or coil pack feeding cylinder 4 — swap it with a known-good coil to confirm.
• A clogged, leaking, or dead fuel injector on cylinder 4, or a mechanical issue like a burnt valve or low compression.

What to do?

Pull the spark plug on cylinder 4 and inspect for wear, fouling, or damage — replace all plugs if any are due. Swap the ignition coil with a known-good cylinder and rescan; if the misfire follows the coil, replace it. Test injector resistance and listen for its click. If ignition and fuel test good, run a compression test to check for mechanical issues.