Ignition & misfire · Powertrain · generic (SAE-defined)
P0304: Cylinder 4 Misfire Detected
P0304 means the engine computer has recorded misfires in cylinder 4, indicating incomplete combustion in that cylinder.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0304 sets
The powertrain control module (PCM) uses the crankshaft position signal to detect when cylinder 4's power stroke is weaker than expected. If those events add up past a manufacturer-defined limit within a set number of revolutions, P0304 sets.
Common causes of P0304
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Faulty ignition coil or plug wire on cylinder 4
- Fouled or worn spark plug in cylinder 4
- Cylinder 4 injector clogged, leaking, or not firing
- Open or shorted injector or coil wiring
- Vacuum leak affecting cylinder 4
- Valve or head gasket problem lowering compression
P0304 symptoms
- Check engine light on or flashing
- Rough idle
- Stumbling or loss of power
- Engine shake felt in the cabin
How P0304 is diagnosed
- Record freeze frame data and check for other cylinder misfire codes.
- Inspect cylinder 4 coil, plug, and connectors.
- Move the cylinder 4 coil and plug to another cylinder to see whether the misfire follows.
- Test the cylinder 4 injector for proper pulse and flow.
- Check fuel trims and smoke-test for vacuum leaks.
- Perform compression and leak-down testing if needed.
Typical fixes for P0304
- Replace the cylinder 4 coil or plug wire
- Replace spark plugs
- Clean or replace the cylinder 4 injector
- Repair vacuum leaks
- Repair head gasket or valve damage if found
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0304 FAQ
What does code P0304 mean?
P0304 means the engine computer has recorded misfires in cylinder 4, indicating incomplete combustion in that cylinder. The standard definition is "Cylinder 4 Misfire Detected."
Can I drive with a P0304 code?
A steady check engine light with only mild roughness generally allows careful driving to a shop. If the light flashes or the engine shakes badly, pull over and stop driving soon to avoid catalytic converter damage.
Can a head gasket cause P0304?
Yes. A head gasket leaking between cylinders or into a coolant passage can reduce compression in cylinder 4. Coolant loss or white exhaust smoke alongside the code makes this more likely.
Why does P0304 come back after replacing the coil?
If the misfire returns, the cause may be the spark plug, injector, wiring, or compression. Swap testing and compression checks help narrow it down.
Keep researching
Engine guides by brand (known issues, specs)
More ignition & misfire codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.