How Do I Fix Code P0304? | Cylinder 4 Misfire Detected

A P0304 trouble code means “cylinder 4 misfire detected.” That means that the engine computer has detected that the 4th cylinder is misfiring (i.e. not combusting properly) repeatedly.

Here’s how you can diagnose and fix it.

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🧰 Symptoms You Might See

When P0304 is active, you might experience:

• Rough idle / shaking

• Engine hesitation, jerking under acceleration

• Loss of power

• Drop in fuel efficiency

• Check engine light (“Service Engine Soon”)

• Possibly a flashing CEL if misfire is severe — which indicates you should stop driving soon to avoid damage

Also Read : Fixed : Skate You Have Been Kicked from the Server

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🔍 Common Causes

Here are the most common culprits for a cylinder‑4 misfire:

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🛠 Diagnostic Steps & How to Fix

Here’s a step‑by‑step process you can follow (or have a mechanic follow). Always work safely (engine off, ignition off, proper tools, etc.).

1. Read & record all error codes (use OBD‑II scanner)

   • Sometimes multiple misfire or related codes appear. That gives you context.

2. Clear the code & test

   • Reset the codes, then run the engine / drive. Sometimes it was a temporary glitch. If P0304 comes back, proceed.

3. Inspect spark plug #4

   • Remove it and look for signs of wear, fouling (oil, carbon), cracks.

   • Check the plug gap is correct.

   • If it’s old or in questionable shape, replace it (often replaced by OEM recommended plug).

4. Check the ignition coil / coil pack for #4

   • Swap coil #4 with a coil from another cylinder (say coil #3).

   • Clear codes and re-run. If the misfire “moves” to cylinder 3, the coil was the issue. If it stays on 4, the coil is likely not the issue.

   • If coil is bad, replace it.

5. Inspect wiring, connectors, harnesses

   • Look for loose connectors, corrosion, broken wires around coil, plug, injector.

   • Repair or replace any faulty wires or connectors.

6. Fuel injector test / check injector #4

   • Use a noid light or injector tester to see if the injector is getting the signal.

   • You can also swap injectors (if same type) with another cylinder to see if misfire moves.

   • Clean or replace the injector if it’s clogged or failing.

7. Check fuel pressure / fuel delivery

   • Use a fuel pressure gauge.

   • Replace or clean fuel filter if clogged.

   • Ensure fuel pump is working correctly.

8. Check for vacuum leaks / intake manifold leaks

   • Inspect vacuum hoses near cylinder 4.

   • Use a smoke machine or listen for hissing to find leaks.

   • Repair or replace leaking hoses / gaskets.

9. Compression / leakdown test

   • Measure compression in cylinder 4 vs other cylinders. If it’s significantly lower, that suggests mechanical issues.

   • A leakdown test can isolate valve or sealing issues.

10. Repair mechanical problems if found

    • If valves, rings, head gasket, etc. are faulty, those need more in‑depth engine work or replacement.

11. After repair, clear codes and test drive

    • Ensure the misfire code doesn’t return.

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💡 Tips & Warnings

• Don’t delay — misfire codes, if left untreated, may damage the catalytic converter (unburned fuel passing through) and worsen engine performance.

• Start with the cheapest & simplest fixes first (spark plug, coil, wiring) before moving to more expensive repairs.

• Keep track of which parts you changed so you don’t replace everything blindly.

• If multiple cylinders misfire (P0300 etc.), the problem may be more systemic (fuel system, sensors, etc.).

• Use OEM or good quality replacement parts (coils, injectors, plugs) because inferior parts can cause recurring issues.