P0304 code: Cylinder 4 Misfire Detected
P0304 means cylinder 4 Misfire Detected. The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0304 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.
P0304 symptoms
- 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 P0304
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.
Start a case with P0304 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0304?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0304 on specific vehicles
The likely causes shift by model. We’ve written up the patterns we see most:
P0304: common questions
Can I drive with P0304?
There's no way to tell that from the code alone. With a steady (not flashing) light, 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.
Symptoms that trigger P0304
Drivers who get this code often notice one of these. Each has its own 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 guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0304 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.