P0304 Code: Cylinder 4 Misfire Causes and How to Fix It

What Does the P0304 Code Mean?

P0304 is a generic OBD-II code identifying a misfire associated with cylinder 4. A misfire occurs when combustion is absent or too weak to deliver the expected power stroke. You may notice rough idle, shaking, hesitation, reduced power, or increased fuel consumption.

The engine computer generally detects misfires by monitoring tiny changes in crankshaft speed through the crankshaft position sensor. Each successful power stroke should accelerate the crankshaft. When cylinder 4 repeatedly contributes less than expected and the calibrated misfire threshold is exceeded, the computer records P0304. Detection thresholds depend on engine speed, load, and vehicle programming.

The repair follows one of four paths: restore spark, correct fuel delivery, repair an air leak affecting that cylinder, or restore compression and valve operation.

Can You Drive With a P0304 Code?

Avoid normal driving until the cause is checked. The immediate risks are unpredictable power loss and unburned fuel overheating the catalytic converter.

• Flashing light, heavy shaking, stalling, overheating, or strong raw-fuel odor: stop safely, shut the engine off, and arrange towing.
• Steady light with smooth operation and no other warning signs: a short, gentle trip for diagnosis may be possible. Avoid highways, towing, high RPM, and hard acceleration; stop if symptoms appear.

A steady light is not a guarantee that continued driving is harmless. The misfire may return under load even if the engine seems normal at idle.

Where Is Cylinder 4 on Your Engine?

Use the cylinder-numbering diagram for the exact engine. Cylinder 4 means the manufacturer’s numbered cylinder, not the fourth cylinder in the firing order.

An inline-four has one row of cylinders, but its numbering direction must still be confirmed. V6 and V8 engines split cylinders between banks, and manufacturers use different numbering patterns. Do not assume cylinder 4 is on a particular side or bank. Check service information using the year, model, and engine code before moving any components.

How Do You Diagnose and Fix a Cylinder 4 Misfire?

1. Save the Codes and Identify When It Misfires

Record all stored and pending codes, freeze-frame data, and cylinder misfire counts if supported. Note whether P0304 occurs on a cold start, at idle, or under load. Injector-circuit, ignition-circuit, or lean-mixture codes help prioritize testing. With several cylinders affected, investigate shared fuel, intake, or electrical faults alongside cylinder 4.

2. Inspect Cylinder 4’s Plug Well and Connections

With the ignition off, check for a loose coil or injector connector, damaged wiring, moisture, and oil around the plug. Oil in the plug well can undermine ignition insulation. Repair a confirmed cover or tube-seal leak with the appropriate valve cover gasket and plug-tube seal set, where separately serviceable, and inspect contaminated ignition parts.

For vehicle-specific recurring faults, check applicable service bulletins. The same P0304 code can have different causes on different engines.

3. Test the Spark Plug and Ignition Coil

Inspect the spark plug for a cracked insulator, electrode wear, deposits, and the specified gap. Fuel or oil fouling also calls for finding its source.

On an engine with interchangeable coil-on-plug units, move cylinder 4’s ignition coil to another cylinder, such as cylinder 2, following the service procedure. Keep the plugs in place so you change only one variable. Recheck briefly under safe conditions using misfire counts or a new code: if the misfire moves to cylinder 2, that implicates the moved coil or boot. If it stays on cylinder 4, continue testing.

If appropriate, test the plug separately using the specified inspection or substitution method. On wire-equipped engines, inspect spark plug wires and their connections instead of assuming an individual coil can be moved. Stop a running test if the light flashes or shaking becomes severe.

4. Check Injector Operation and Fuel Delivery

If ignition checks pass, test cylinder 4’s fuel injector, connector, and control circuit. A technician can compare injector flow, leakage, and cylinder contribution. An audible click confirms movement, not correct fuel quantity. Restricted flow or leakage requires the appropriate injector service; a damaged circuit requires wiring repair.

Check system fuel pressure when test results or multiple-cylinder symptoms indicate a shared supply problem. Direct-injection components may require special seals, tools, or coding, so avoid a casual DIY injector swap.

5. Check for Intake Leaks and Compression Loss

A smoke test can locate a leak near cylinder 4’s intake runner or seal. Repair the leaking hose, gasket, or damaged manifold identified by the test.

If the misfire remains on cylinder 4 after ignition and fuel checks, compare compression with the other cylinders and the engine specification. Leak-down and valvetrain checks can distinguish valve-sealing, piston/ring, head-gasket, or valve-operation problems. Those findings determine the mechanical repair; new ignition parts will not restore lost compression.

Which A-Premium Parts Can Help Fix P0304?

From a single failed coil to a confirmed fuel or intake fault, A-Premium offers repair options beyond a basic tune-up. Match the product to the test result:

• Failed ignition component: choose compatible ignition coils, spark plugs, or plug-wire sets for the engine’s ignition design. Compare ignition coil and spark plug kits. Check the actual included quantities and plug specifications.
• Confirmed injector failure: find replacement fuel injectors matched to the injection system, connector, and OE number.
• Oil leaking into plug wells: compare applicable valve cover gasket sets and verify whether the required tube seals are included.
• Confirmed manifold damage: browse replacement intake manifolds; a gasket-only leak may need only seal replacement.

One failed coil can often be replaced individually. The guide to replacing one ignition coil versus the full set helps weigh age, repeated failures, and access labor.

A-Premium combines quality-focused manufacturing, OE interchange and vehicle-fitment information, and competitive online pricing. Confirm the engine, production notes, and package contents before ordering. Product-specific warranty coverage, 24/7 support, and an eligible 90-day return window support the purchase.

How Do You Confirm That P0304 Is Fixed?

After the repair, confirm connectors and intake parts are secure. Clear ordinary stored codes after saving the diagnostic information, then check misfire counts and operation under the original conditions when safe. Recheck pending codes after the drive. A smooth idle alone is insufficient if the original misfire occurred during acceleration or cold starting.

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