P0304 — Cylinder 4 Misfire Detected

Does P0304 prove that cylinder 4 itself is damaged?

No. P0304 identifies the cylinder where the misfire was detected, but the underlying fault may be a plug, coil, injector, vacuum leak, wiring problem, or low compression.

Is it safe to drive a vehicle with P0304?

Limited driving may be possible if the misfire is mild and the engine runs smoothly, but heavy load should be avoided. Severe shaking, power loss, or a strong fuel odor warrants stopping and having the vehicle inspected.

What should be checked first when diagnosing P0304?

Start by reading stored and pending codes with an OBD2 scanner, then inspect the cylinder-4 plug, coil, connectors, and wiring before testing the injector, compression, and nearby leaks.

Why might P0304 return after replacing a spark plug?

A replacement plug only corrects the fault if the plug was the cause. A weak coil, leaking injector, vacuum leak, low compression, or intermittent wiring fault can allow P0304 to return.

When your scan tool displays P0304, the engine control module is reporting that cylinder 4 is missing beats during the combustion process. This falls squarely in the powertrain category of OBD2 trouble codes, and it means the monitoring system has observed cylinder 4 failing to fire properly over a sustained period. The ECU tracks each cylinder's performance through the misfire detection system, comparing actual combustion events against expected patterns based on crankshaft acceleration data. When the variance exceeds acceptable thresholds, P0304 gets set.

The location of cylinder 4 depends on your engine design. On inline-four or inline-six engines, manufacturers typically count from the front of the block—the end where the accessory drive belt lives. V-type engines vary by make, so you'll need the firing-order diagram for your specific vehicle to identify which cylinder carries the #4 label. This matters practically because some cylinders sit deeper in the engine bay, requiring intake manifold removal to access them, which directly affects labor time and your repair bill.

The ECU doesn't set P0304 after a single bad firing cycle. It watches the pattern across multiple drive cycles under varying conditions before committing the fault to memory. When it does store P0304, it also captures freeze frame data—a snapshot of engine RPM, load percentage, coolant temperature, vehicle speed, and other sensor readings at the exact moment the misfire was detected. That snapshot is your most valuable starting point for diagnosis.

Common Symptoms

• The Check Engine Light illuminates steadily, and it may flash if the misfire is severe enough to risk exhaust damage
• The engine idles roughly with noticeable vibration transmitted through the steering wheel and seat
• Fuel economy drops noticeably compared to normal driving history
• The vehicle bucks, stumbles, or hesitates when you accelerate under load
• Raw fuel smell from the exhaust pipe, especially noticeable at startup or deceleration

Likely Causes

• A clogged or leaking fuel injector serving cylinder 4 prevents proper fuel delivery
• Faulty crankshaft or camshaft position sensor signal disrupts ignition timing coordination
• Insufficient fuel pressure from a weak pump or regulator starves the injector
• A failed ignition coil, damaged spark plug, or deteriorated coil wiring fails to ignite the air-fuel mixture
• Low compression in cylinder 4 from worn rings, failed head gasket, or bent valves

How to Diagnose

1. Plug in an OBD2 scanner, verify P0304 is present in the stored codes, and record the freeze frame data before clearing anything—this shows exactly what conditions triggered the fault.
2. Pull any related trouble codes. P0300 (random misfire) alongside P0304 suggests a system-wide problem, while isolated cylinder codes often point to component-specific failures. Search for technical service bulletins under your VIN on the manufacturer's site or NHTSA database.
3. Physically examine the wiring harness and connectors feeding the cylinder 4 injector and ignition coil. Look for chafed insulation, corrosion at the terminals, and make sure all connections are fully seated.
4. Access live data on your scan tool and monitor the per-cylinder misfire counters while the engine runs. This tells you whether cylinder 4 is the sole offender or one of several.
5. Perform a component swap test: move the ignition coil from cylinder 4 to another cylinder and the suspected-good coil into cylinder 4. If the misfire migrates with the coil, that part is faulty. Apply the same logic to fuel injectors if accessible.
6. After repairs, clear the code and complete a drive cycle that replicates the freeze frame conditions (similar RPM, load, and temperature). The code must stay cleared for the ECU to consider the problem resolved.

Repair Options and Costs

Prices below reflect typical parts and labor at independent shops in the United States.

• Clean or replace a faulty fuel injector for cylinder 4: $200–$600
• Install new spark plugs: $90–$250
• Address low compression through engine mechanical repair: $800–$2,500
• Replace a failed ignition coil: $150–$380

Total out-of-pocket depends entirely on which component is responsible. The range spans from simple plug replacement on the low end to full mechanical teardown at the high end. A systematic diagnosis before purchasing parts is the single most effective way to keep costs down—replacing a perfectly good catalytic converter because you ignored a misfire for weeks is an expensive mistake.

Can You Drive with P0304?

Use caution and minimize driving. A persistent misfire dumps unburned fuel directly into the exhaust stream, and that raw fuel can destroy the catalytic converter within miles if the problem goes unaddressed. When the Check Engine Light is flashing rather than steady, the ECU is signaling an emissions-system-damaging condition that requires immediate attention—stop driving and diagnose now.

Related OBD2 Codes

• P0300: Random/Multiple Cylinder Misfire Detected
• P0301: Cylinder 1 Misfire Detected
• P0302: Cylinder 2 Misfire Detected
• P0303: Cylinder 3 Misfire Detected
• P0305: Cylinder 5 Misfire Detected
• P0306: Cylinder 6 Misfire Detected