P0304 — Cylinder 4 Misfire Detected

P0304 means cylinder 4 is misfiring. On 4-cylinder engines this is often the rear-most cylinder — start with spark plug and coil diagnosis.

Cost range: $30–$600. Severity: high. Safe to drive? Avoid driving with a flashing CEL.

Most common causes of P0304

1. Bad spark plug on cyl 4 — Replace and clear. ($10–$30)
2. Failing coil on cyl 4 — Common after 80k miles. ($40–$200)
3. Carbon-clogged injector — Try injector cleaner first. ($15–$600)

Symptoms drivers report with P0304

• Idle vibration
• Power loss
• CEL