What does P0304 mean?
The computer watches crankshaft speed. When a cylinder fails to fire properly, the crank slows down for a moment and the computer counts it as a misfire. This code points at cylinder 4.
Codes are a starting point. They tell you which test failed, not which part to replace. Check for other stored codes and fix those in a sensible order.
Symptoms
- Shaking or rough idle
- Flashing check engine light when misfires are severe
- Loss of power
- Hesitation or stumble when accelerating
- Fuel smell from the exhaust
Common causes
- Worn spark plug
- Failed ignition coil or plug wire
- Clogged or failed fuel injector
- Vacuum leak near that cylinder
- Low compression (valve or head gasket problem)
- Wiring fault to the coil or injector
How to diagnose P0304
- Check how many misfires live data reports per cylinder.
- Swap the coil from the misfiring cylinder with a neighbor. If the misfire moves, replace the coil.
- Pull and inspect the spark plug: look for wear, oil fouling, cracks or a wide gap.
- If coil and plug are fine, test the injector (swap or noid light) and run a compression test.
How to fix it
- Replace worn spark plugs (usually as a set)
- Replace the failed ignition coil
- Replace or clean the fuel injector
- Fix vacuum leaks
- Repair mechanical issues found in the compression test
Estimated repair cost
Likely repairs for P0304 run from about $100 to $600, depending on the cause.
| Repair | Typical range (parts + labor) |
|---|---|
| Diagnostic fee (scan + testing) | $100 - $200 |
| Spark plug replacement (4-8 cyl) | $120 - $450 |
| Ignition coil replacement (one coil) | $150 - $400 |
| Fuel injector replacement (gasoline, one) | $200 - $600 |
| Compression / leak-down test | $100 - $250 |
These are rough US estimates for an independent shop in 2026, based on typical parts prices and standard labor times. Dealers, big trucks, luxury brands and high-cost areas often run higher. They are not quotes. Use the repair cost estimator to adjust for your vehicle.
How serious is P0304?
High severity
If the check engine light is flashing, slow down and stop driving as soon as you can. Unburned fuel can overheat and destroy the catalytic converter.
P0304 FAQ
What is the quickest way to diagnose P0304?
Swap the ignition coil from cylinder 4 with a neighboring cylinder, clear the code and drive. If the misfire moves to the other cylinder, the coil is bad. If it stays, check the spark plug, then the injector, then compression.
Where is cylinder 4?
Cylinder numbering depends on the engine. Check a service manual or a diagram for your exact engine; on many V engines, the odd and even cylinders are on opposite sides, but not on all of them (Ford V8s number 1-4 on one bank and 5-8 on the other).
What does P0304 mean?
P0304 stands for "Cylinder 4 Misfire Detected". The computer watches crankshaft speed.
Can I drive with a P0304 code?
If the check engine light is flashing, slow down and stop driving as soon as you can. Unburned fuel can overheat and destroy the catalytic converter.
How much does it cost to fix P0304?
It depends on the cause. The likely repairs for P0304 range from about $100 to $600 at an independent shop in the US, plus a diagnostic fee of roughly $100-$200. These are rough estimates; dealer prices and some vehicles run higher.