Diagnostic code
P0304 Code: Cylinder 4 Misfire Detected
P0304 means cylinder 4 is not firing reliably. The code narrows the search to one cylinder, but the final cause still has to be separated between spark, fuel, air, compression, and wiring faults.
Educational reference only, not professional repair advice. A fault code is a starting point, not a diagnosis. Vehicle symptoms, live data, service history, and manufacturer-specific procedures change the repair path. If the check engine light is flashing, the vehicle is overheating, brakes or steering feel unsafe, fuel odor or smoke appears, or the engine is running poorly, stop driving when safe and get professional help.
Sources
Safety limits for this page
Use this page as a reference only. Do not perform hazardous repairs from a web guide. Stop when safe and contact a qualified mechanic if any warning condition below applies.
- Opening fuel lines, fuel tanks, or pressurized fuel components
- Flashing check engine light
- Overheating, smoke, or strong fuel smell
- Loss of power, severe shaking, unsafe braking, or unsafe steering
- Fuel odor, visible leak, smoke, or hard starting after refueling
- Burning smell, exhaust leak sound, or visible exhaust damage
Can I drive?
If the check engine light is flashing or the engine shakes, stop driving when safe. With a steady light and smooth engine, short local driving to a shop may be possible, but avoid highway speeds and hard acceleration until diagnosed.
Repair range
$100-$1,400
Scanner note
Basic OBD-II scanner with live data
Plain-English Meaning
P0304 means cylinder 4 is not firing reliably. The code narrows the search to one cylinder, but the final cause still has to be separated between spark, fuel, air, compression, and wiring faults.
Common Causes
Common possibilities (most common first):
- Bad spark plug in cylinder 4
- Ignition coil or plug wire failure
- Injector circuit or flow problem
- Low cylinder compression
- Intake gasket leak near cylinder 4
Order varies by make, model, mileage, and operating conditions. Do not replace parts based only on this list — verify with a scan tool and qualified mechanic.
Symptoms
- Rough idle
- Power loss
- Vibration when accelerating
- Gasoline smell from exhaust
Diagnostic Steps
- 1Confirm cylinder 4 location
- 2Inspect and test ignition components
- 3Check injector connector and pulse
- 4Look for intake leaks near the runner
- 5Have a qualified shop run compression testing if the fault remains.
Confusable And Related Codes
P0300
Compare →P0300 means the engine is misfiring randomly or across multiple cylinders instead of one clearly identified cylinder. Ignition, fuel, air leaks, compression problems, or timing issues can all interrupt combustion and trigger this code.
P0301
Compare →P0301 means cylinder 1 is not burning its air-fuel mixture consistently. The problem may be isolated to that cylinder, such as a spark plug, coil, injector, compression issue, or local air leak near the intake runner.
P0302
Compare →P0302 means cylinder 2 is misfiring enough for the engine computer to identify it. The cause may be electrical, fuel-related, mechanical, or an air leak that affects that cylinder more than the others.
P0303
Compare →P0303 means cylinder 3 has a repeat misfire pattern. A technician will usually confirm the cylinder location, inspect ignition parts, check injector behavior, and rule out compression or valve problems before replacing parts.
FAQ
Can I drive with P0304?
Only minimally if the engine is smooth and the light is steady. Do not drive with a flashing light.
Is P0304 always a spark plug?
No. Spark plugs are common, but coils, injectors, leaks, and compression can also cause it.
Can P0304 damage the catalytic converter?
Yes. Unburned fuel from a misfire can overheat the converter.
This page is educational and is not a substitute for hands-on vehicle diagnosis.