P0304
Cylinder 4 Misfire Detected
Misfires concentrated on cylinder 4. A cylinder-specific code points away from shared causes (fuel pressure, MAF) toward components dedicated to that cylinder — coil, spark plug, injector, compression. Verify by swapping the suspected coil/plug to a different cylinder; if the code follows, the part is bad.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
What we see in the workshop
ON FORD ECOBOOST
As with P0301 — coil and plug first, then carbon and compression on that cylinder.
Symptoms
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Common causes
Ordered by how often each turns out to be the cause.
- Ignition coil for cylinder 4 failedCommon
- Spark plug for cylinder 4 worn or fouledCommon
- Cylinder 4 injector cloggedPossible
Parts involved
- Ignition Coil
- Spark Plug
- Fuel Injector
P0304 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.