Can I drive with a P0304 code?

You can drive short distances at low speeds, but a flashing check engine light means stop immediately. Continued driving with a misfire can damage the catalytic converter and increase repair costs. Address the code promptly.

Will P0304 clear itself?

The code may clear after a certain number of drive cycles if the misfire does not recur, but the underlying cause remains. Clearing the code without repair means it will likely return. Fix the root problem first.

How much does it cost to fix P0304?

Repair cost depends on the cause. A spark plug replacement may cost under $50 in parts, while an ignition coil runs $50–$150. Mechanical issues like low compression can exceed $1,000. Diagnosis narrows the expense.

Is P0304 the same as P0301?

Both are cylinder-specific misfire codes with the same diagnostic logic, but P0304 affects cylinder 4 and P0301 affects cylinder 1. The causes and repair steps are identical, applied to the respective cylinder.

Can a bad catalytic converter cause P0304?

A severely clogged catalytic converter can create enough backpressure to cause misfires, but it typically triggers multiple cylinders and sets P0300. P0304 alone usually points to an ignition, fuel, or mechanical issue in cylinder 4.

P0304 Code Meaning: Cylinder 4 Misfire Detected

What P0304 Means

P0304 is a diagnostic trouble code indicating that the engine control module (ECM) has detected a misfire in cylinder 4. According to SAE J2012, this code is set when the ECM measures incomplete or absent combustion events in the fourth cylinder over a defined number of crankshaft revolutions. The misfire detection system monitors crankshaft acceleration; when cylinder 4 fails to contribute its expected power pulse, the ECM logs the event and illuminates the check engine light.

A cylinder-specific misfire code like P0304 points to a problem isolated to cylinder 4, unlike P0300 random misfire, which indicates misfires across multiple cylinders without a consistent pattern. This distinction narrows the diagnostic focus to components serving cylinder 4: its spark plug, ignition coil, fuel injector, and the mechanical integrity of that cylinder.

Symptoms of a Cylinder 4 Misfire

The most immediate symptom is the check engine light. If the misfire is severe or continuous, the light may flash, signaling a condition that can damage the catalytic converter. A flashing check engine light warrants immediate attention.

Drivers typically notice a rough idle and vibration felt through the steering wheel or cabin. The engine may shake noticeably at a stoplight. Acceleration suffers because one cylinder is not producing power, and the vehicle may hesitate or stumble under load.

Fuel economy drops because unburned fuel passes through the exhaust. You may smell raw gasoline from the tailpipe, and in some cases, the exhaust note changes to an uneven rhythm. Left unaddressed, a persistent misfire can overheat and damage the catalytic converter, leading to more expensive repairs.

Common Causes of P0304 (Ranked by Likelihood)

Ignition coil failure is the most common cause. The coil on cylinder 4 may have failed or developed high resistance, preventing adequate spark energy. Coils degrade over time due to heat cycling and electrical stress.

Worn or fouled spark plug ranks second. Carbon buildup, oil fouling, or electrode wear can prevent the plug from firing reliably. Spark plugs have a service life, and exceeding it increases misfire risk.

Fuel injector problems include clogging, electrical failure, or mechanical sticking. If the injector on cylinder 4 delivers insufficient fuel or none at all, combustion cannot occur.

Vacuum leaks affecting cylinder 4—such as a cracked intake manifold runner or a leaking gasket—can lean out the air-fuel mixture enough to cause a misfire. This is less common than ignition or fuel issues but still a frequent culprit.

Low compression from worn piston rings, burned valves, or a blown head gasket prevents the cylinder from building sufficient pressure for combustion. Compression issues are mechanical and typically require more extensive repair.

How to Diagnose P0304

Start by retrieving freeze-frame data and misfire counters using a scan tool. Freeze-frame captures engine conditions when the code set—RPM, load, coolant temperature, and fuel trim—which can reveal patterns. Misfire counters show how many misfires occurred in cylinder 4 versus other cylinders, confirming the code is not intermittent noise.

Visually inspect the spark plug and ignition coil on cylinder 4. Remove the spark plug and examine the electrode for wear, carbon deposits, or oil fouling. Check the ignition coil connector for corrosion and the coil itself for cracks or carbon tracking.

The most effective next step is component swapping. Move the cylinder 4 ignition coil to another cylinder—say, cylinder 2—and clear the codes. If the misfire follows the coil and you now see P0302, the coil is faulty. Repeat the process with the spark plug. This method isolates the problem without guessing.

Test the fuel injector on cylinder 4 by measuring its resistance with a multimeter and comparing it to specification. Listen for a clicking sound with a mechanic's stethoscope while the engine runs; a silent injector may be stuck or electrically dead. Some technicians use a noid light or injector pulse tester to confirm signal and power.

If ignition and fuel components check out, perform a compression test on cylinder 4. Low compression compared to other cylinders points to a mechanical issue. A leak-down test can further pinpoint whether the leak is past the rings, valves, or head gasket. Mechanical diagnosis and repair of low compression typically warrant a qualified mechanic.

An OBD2 scanner that analyzes live data can display real-time misfire counts, fuel trim, and ignition timing, helping confirm the repair and rule out related issues.

Repair and Next Steps

Once you identify the faulty component, replace it. If the ignition coil failed, replace the coil on cylinder 4; many technicians also replace the spark plug at the same time to ensure a clean baseline. If the spark plug was fouled or worn, install a new plug to the manufacturer's torque specification.

For a clogged or failed fuel injector, replacement or professional cleaning may be required. Vacuum leaks are repaired by replacing the gasket or sealing the crack. Low compression usually requires valve work, piston ring replacement, or head gasket repair—tasks best handled by a qualified mechanic.

After the repair, clear the trouble codes and take the vehicle on a test drive that includes varied speeds and loads. Monitor the misfire counters; they should remain at zero for cylinder 4. If the code returns or misfire counts climb, further diagnosis is needed.

Diagnostic output is a decision-support estimate, not a substitute for professional inspection.