The most common cause of P0304 is a misfire in cylinder 4, typically due to a faulty spark plug or ignition coil.
What P0304 means
P0304 is a diagnostic trouble code indicating a misfire in cylinder 4. A misfire happens when the air‑fuel mixture in cylinder 4 does not ignite properly or burns incompletely, leading to rough engine operation, reduced power, higher fuel consumption, and increased emissions. The code is detected by the engine control module (ECM) using data from sensors and the crankshaft position sensor, and it is often accompanied by a flashing check engine light if the misfire is severe.
Most common causes
These are the practical culprits technicians examine first when diagnosing P0304, focusing on cylinder 4’s ignition and fuel delivery as well as nearby air leaks.
- Worn or fouled spark plug for cylinder 4
- Defective ignition coil or coil-on-plug for cylinder 4
- Clogged or malfunctioning fuel injector on cylinder 4
- Vacuum leak or intake gasket leak affecting cylinder 4
- Low fuel pressure or general fuel delivery issue impacting cylinder 4
- Compression problems in cylinder 4 (valve seating, piston rings, or head gasket leak)
Understanding these causes helps prioritize the diagnostic path and potential repairs for a P0304 code.
How to diagnose
Technicians follow a methodical approach to confirm a cylinder 4 misfire and identify the root cause. The steps below reflect a typical diagnostic workflow used in many shops.
- Review all stored trouble codes and live data with a scan tool to confirm cylinder 4 is consistently misfiring.
- Inspect the spark plug for cylinder 4 for wear, fouling, or damage, and inspect the ignition coil or coil-on-plug for cracks or faults.
- Perform a swap or swap-test by exchanging the spark plug or coil with another cylinder to see if the misfire follows the component.
- Check the fuel injector operation for cylinder 4 (inspection of injector resistance and injector pulse signal; consider a balance test).
- Inspect for vacuum leaks around the intake manifold and associated hoses near cylinder 4.
- Conduct a compression test or leak-down test on cylinder 4 to assess mechanical integrity of valves, piston rings, and head gasket.
- Verify fuel pressure and fuel system performance to rule out pressure or delivery issues.
These diagnostic steps help separate ignition, fuel, air, and mechanical problems from one another and guide the repair plan.
Common repairs
Based on the identified root cause, repairs can range from simple part replacement to more extensive service. The list below outlines typical fixes for a P0304 misfire.
- Replace the spark plug for cylinder 4
- Replace the ignition coil or coil-on-plug for cylinder 4 if defective
- Clean or replace the fuel injector for cylinder 4
- Repair vacuum leaks or replace intake manifold gaskets near cylinder 4
- Address fuel delivery issues, such as replacing a failing fuel pump, filter, or correcting pressure regulator
- Repair or replace mechanical issues causing loss of compression (valve seating, piston rings, or head gasket repair)
After repairs, clearing the codes and performing a road test or recheck with a scan tool confirms that cylinder 4 is functioning normally and that misfire symptoms have resolved.
Summary
P0304 signals a misfire in cylinder 4. The most common cause is a faulty spark plug or ignition coil, but other factors such as fuel delivery problems, vacuum leaks, or compression issues can also trigger the code. A systematic diagnostic approach—starting with ignition and fuel delivery for cylinder 4, then checking for air leaks and mechanical integrity—helps pinpoint the exact cause and inform the appropriate repair. If in doubt, consult a qualified technician to ensure safe and accurate diagnosis.