Engine codes
P0304: cylinder 4 misfire detected
On a four-cylinder engine, cylinder 4 is usually at the end opposite the timing drive, and P0304 is one of the most common single-cylinder misfire codes. After the coil and plug, the injector is the main suspect.
P0304: can I drive, and what will the labor cost?
Estimated labor, replace one ignition coil
$22 to $88
| Labor time (estimate) | 0.2 to 0.8 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| High: limit driving, fix within days Limit driving; stop if the light flashes. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0304 is the generic OBD-II code for "cylinder 4 misfire detected". As with P0301, P0302 and P0303, the computer has counted repeated combustion failures, here on cylinder 4. After the ignition coil and the spark plug, which the swap test checks, the fuel injector is the next most frequent cause. An injector can clog, so that it delivers too little fuel, leak, or stop working because of a broken wire or a failed coil inside it. A shop tests it by measuring its electrical resistance, by listening to it click with a stethoscope while the engine runs, by watching its signal on a scope, or by swapping it with a neighbor as for coils. Low compression comes last. On some engines cylinder 4 is near the PCV or brake booster connection, so a leaking hose at that end makes it lean first. Limit driving and stop if the check engine light flashes.
Coil swap test: does the misfire follow the coil?
Result
Coil
| What it means: The misfire moved with the coil: replace that ignition coil. |
Swap only with the engine off and the coil connectors clipped back. Read the per-cylinder misfire counters, or wait for the new P030x code, before concluding.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0304 at a glance
| Code | P0304 |
| Standard description | Cylinder 4 Misfire Detected |
| System | Ignition and misfire |
| Severity | High: limit driving, fix within days |
| Common symptoms | rough idle, stumble under load |
Checking an injector
- Resistance: unplug the injector and measure across its two pins. Compare with the other injectors; one that reads open or very different is faulty.
- Sound: with the engine running, a screwdriver or stethoscope on the injector body should pick up a steady clicking. Silence means it is not opening.
- Noid light: a small test lamp in the connector flashes if the computer is sending the signal, which separates an injector fault from a wiring fault.
- Swap: on port-injected engines, moving injectors between cylinders is a matter of a fuel rail; on direct-injection engines it is a longer job with new seals.
Firing order and neighbors
The firing order of most inline four-cylinder engines is 1-3-4-2. Cylinders do not fire in their numbered order, and a coil wire or plug lead swapped between two cylinders produces misfires on both. If the misfire began right after work on the ignition, check that every connector went back where it came from.
Vacuum lines at the end of the engine
Hoses from the intake to the brake booster, the PCV valve or the purge valve often connect near one end of the manifold. A crack there can lean out the closest cylinder at idle. A hiss, a hard brake pedal or an idle that changes when the hose is pinched are clues. Use the coil swap tool on this page first, then compare with P0300 if more cylinders join in.
Likely causes of P0304, in order
| Cause | How often |
|---|---|
| Ignition coil on cylinder 4 | most frequent |
| Spark plug | most frequent |
| Injector on cylinder 4 | frequent |
| Low compression | less frequent |
P0304 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace one ignition coil | 0.2 to 0.8 h | $22 to $88 |
| Replace spark plugs (inline 4 cylinder) | 0.5 to 1.5 h | $55 to $165 |
| Replace one fuel injector | 1 to 3 h | $110 to $330 |