OBD codes / P0304
P0304
SeriousCylinder 4 Misfire Detected
Treat P0304 with some urgency. A steady check-engine light with a mild misfire is usually drivable short-term, but a FLASHING light means raw fuel is reaching the catalytic converter and can ruin it — pull over and have it towed or diagnosed. Misfires also tend to worsen, so don't put the fix off.
What this code means
A misfire means a cylinder isn't burning its air-fuel mixture properly. P0304 points to one specific cylinder — number 4 — which makes it easier to diagnose than a random misfire. The usual cause is that cylinder's spark plug or ignition coil; a quick test is to swap the coil or plug with a neighboring cylinder and see if the misfire follows. Less commonly it's a clogged or dead fuel injector, or low compression in that cylinder from a valve or ring problem. Because unburned fuel can overheat the catalytic converter, a steady misfire shouldn't be left for long.
Symptoms you might notice
- Rough idle or shaking, often felt at a stop
- Loss of power or hesitation under acceleration
- Flashing check-engine light during an active misfire
- Reduced fuel economy
- Engine may stumble or be hard to start
What it costs to fix
Typical range: $80–$600 · about ~0.3–3 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Spark plug (single) | $4–$30 | ~0.3–1.5 hrs |
| Ignition coil (single) | $30–$150 | ~0.3–1 hrs |
| Fuel injector (single) | $100–$400 | ~1–3 hrs |
A single plug or coil is the cheap, common fix; an injector is pricier; low compression (a valve or ring issue) is a major repair that varies too much to quote — get it diagnosed if plug, coil, and injector all check out.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0304 for my exact vehicle →
How to fix P0304
Easy — basic tools, most owners can do this
- Confirm which cylinder is number 4
Look up the firing order and cylinder layout for your specific engine — cylinder 4 is the last one on an inline four but lands in different spots on V configurations. A service diagram or owner's forum for your model settles it in a minute.
- Move coil 4 and see if the misfire travels
Swap the cylinder 4 coil with one from a smooth-running cylinder, clear the code, and drive. If the misfire code follows the coil to its new home, buy one coil; if cylinder 4 keeps misfiring, keep diagnosing.
- Fit a fresh plug in cylinder 4
Inspect the old plug for cracks, oil, carbon, or electrode wear — any of these explains the misfire on that hole. Even if it looks marginal, a new plug is the cheapest part in this whole diagnosis.
- Test injector 4
Confirm injector 4 clicks at idle with a stethoscope, or trade it with a neighboring injector and recheck codes. A misfire that follows the injector means the injector is clogged or failing.
- Rule out mechanical trouble
If spark and fuel both check out, run a compression test on cylinder 4 and compare it to the others. Low compression means the problem is inside the engine — burned valve, worn rings, or a head gasket — and that's a shop diagnosis.
DIY or shop? The coil swap, plug change, and injector click test are all beginner-friendly on most four-cylinder engines, where cylinder 4 is usually easy to reach. Hand it off to a shop when compression is low or when the misfire only appears under load and you can't reproduce it in the driveway.
Tools for the DIY steps: OBD-II scanner · Spark plug socket and ratchet · Torque wrench · Mechanic's stethoscope (optional) · Compression tester (optional)
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0304.
P0304 on specific vehicles
The likely causes shift by model. We’ve written up the patterns we see most:
Frequently asked
Can I drive with P0304?
With a steady (not flashing) light and only a mild misfire, short trips are usually fine — but get it looked at soon. If the light is flashing, the active misfire is dumping raw fuel into the catalytic converter and can destroy it; stop driving and have the car towed.
How do I know if it's the coil or the spark plug?
Swap cylinder 4's coil (or plug) with a neighboring cylinder and clear the code. If the misfire moves to the other cylinder, the part you moved is the culprit — a cheap way to avoid replacing the wrong thing.
How much does it cost to fix P0304?
Often modest — a single spark plug or coil is the common, inexpensive fix. It rises if the cause is a fuel injector, and a compression problem (valve or ring) is a major repair that varies widely. Confirm the cause before spending.
Should I replace all the coils or just cylinder 4?
If only cylinder 4 is misfiring, replacing just its coil/plug is reasonable. Some owners replace all of them at once if the parts are old and high-mileage, to avoid chasing the next one — but it isn't required to fix P0304.
Seeing P0304 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0304 for my vehicleSymptoms that trigger P0304
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0304 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.