P0304
UniversalCylinder 4 Misfire Detected
The engine computer detected that cylinder 4 is not contributing consistently to engine rotation. It generally identifies a misfire by monitoring changes in crankshaft speed; the code indicates a detected condition, not proof that a particular part has failed. A misfire can cause rough running, reduced power, or increased emissions, and a severe or continuing misfire can damage the catalytic converter.
⚠️ Before You Begin (Safety First)
If the check-engine light is flashing, the engine is shaking badly, or power drops substantially, reduce load and stop in a safe place. Continued driving with a severe misfire can damage the catalytic converter. If the light is steady and the engine runs normally, arrange diagnosis soon and avoid hard acceleration.
Common Symptoms
- Rough idle or engine vibration if cylinder 4 is misfiring at idle.
- Hesitation, reduced power, or uneven acceleration if the misfire occurs under load.
- A flashing or steady check-engine light may appear; a flashing light can indicate a misfire severe enough to risk catalytic-converter damage.
- Fuel economy may worsen if combustion is incomplete.
- There may be no noticeable symptoms if the misfire is intermittent or mild.
Common Causes
- A worn, fouled, or damaged spark plug may not ignite the mixture consistently.
- An ignition coil, ignition lead, or electrical connection may fail intermittently or provide weak spark, depending on the vehicle's design.
- A fuel injector may be restricted, malfunctioning, or have an electrical or connection problem.
- An air leak affecting cylinder 4 or its intake path may disrupt the air-fuel mixture.
- Low compression or another mechanical problem may prevent cylinder 4 from contributing normally.
- A wiring, connector, or control problem may interrupt ignition or fuel delivery.
- A misfire may be intermittent, so the code alone does not establish which cause is present.
Diagnostic Steps
- Note whether the check-engine light is flashing and whether the engine is shaking or losing power. Stop driving if symptoms are severe.
- With the engine off and cool, check for visibly loose or damaged ignition and engine wiring connections that are safely accessible. Do not pull on wiring or probe connectors without the correct information.
- Use a compatible scan tool, if available, to record other codes and freeze-frame information before clearing anything. Related codes may help narrow the diagnosis.
- If you are comfortable and have the correct vehicle-specific instructions, inspect the cylinder 4 spark plug and accessible ignition components for obvious damage or contamination. Do not guess the cylinder location; it varies by engine.
- Avoid replacing parts based on the code alone. If the checks do not reveal an obvious issue, have a technician test ignition, fuel delivery, intake sealing, and mechanical condition.
When to Call a Professional
A technician can review other codes and freeze-frame data, check cylinder-specific misfire counts, inspect ignition components and connections, and determine whether the misfire follows a component during a suitable swap test. If ignition checks do not identify the cause, fuel delivery, intake leaks, and mechanical condition may need testing, including injector checks and compression or leak-down testing. Use service information for the exact model, year, and engine.
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Commonly Asked Questions
Does P0304 mean the cylinder 4 spark plug or coil must be replaced?
No. It reports a detected misfire assigned to cylinder 4, but the cause could involve ignition, fuel delivery, an air leak, mechanical condition, or wiring. Testing is needed before choosing a repair.
Can I keep driving with P0304?
A mild, steady-light condition may allow cautious travel to a repair location, but prompt diagnosis is advisable. If the light flashes, the engine runs very roughly, or power drops substantially, stop safely because continued driving can damage the catalytic converter.
How is a cylinder-specific misfire detected?
The engine computer can monitor changes in crankshaft rotation and use them to identify when a cylinder is not contributing consistently. The code identifies the detected cylinder, not the failed part.
Could P0304 appear even if the engine seems to run normally?
Yes. An intermittent or mild misfire may not be obvious to the driver. Scan data and vehicle-specific testing can help determine whether the condition is present and what is causing it.
Is cylinder 4 in the same physical location on every vehicle?
No. Cylinder numbering depends on the engine design. Check service information for the exact model, year, and engine before inspecting or replacing cylinder-specific components.
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