P0304 Cylinder 4 Misfire Detected

P0304 indicates a detected misfire on cylinder 4. Treat it as a cylinder-specific diagnostic lead rather than a direct instruction to replace ignition parts.

Code details

Generic OBD-II · P0

System: Ignition / Misfire
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not overlook mechanical causes such as compression loss or valve sealing when ignition and injector checks pass.

Symptoms of P0304

Rough idle or a distinct shake, often worse at low RPM.
A stumble or dead spot traced to cylinder 4.
A flashing check-engine light when the misfire is active.
Slight power loss and worse fuel economy.

Common causes of P0304

A worn spark plug or failed ignition coil on cylinder 4.
A clogged, leaking, or dead injector on cylinder 4.
A vacuum leak or low compression local to that cylinder.
Carbon-tracked plug boot or damaged coil wiring.

Useful checks before replacing parts

Check whether cylinder 4 misfires at idle, under load, during cold start, or all the time.
Inspect ignition, injector control, injector flow, compression, vacuum leaks, and harness condition at cylinder 4.
Confirm the repair with misfire counters and a drive cycle under the same conditions that set the code.

Related OBD-II codes

These nearby ignition / misfire entries can help compare wording and pin down the fault area.