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.
🧰 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
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🔍 Common Causes
Here are the most common culprits for a cylinder‑4 misfire:
| Component / System | How it causes misfire in cylinder 4 |
|---|---|
| Spark Plug #4 | Worn, fouled, damaged, improper gap, or oil/soot contamination → can’t ignite the air/fuel mix properly |
| Ignition Coil #4 (or coil pack, or ignition module) | If the coil fails, the spark is weak or absent |
| Wiring / connectors | Broken, corroded, loose, shorted wires to coil, spark plug, or injector |
| Fuel injector #4 | If clogged, leaking, or electrically failing, it may not supply fuel properly |
| Fuel pressure / fuel system issue | If fuel pressure is too low (or inconsistent), that cylinder may be starved |
| Vacuum / intake leaks | Extra air entering the combustion chamber upsets the air/fuel ratio, causing misfires |
| Low compression / mechanical problems | Worn piston rings, leaking valves, head gasket issue, etc. — when compression is low, proper combustion can’t occur |
| Other sensors / modules | Rarely, problems in camshaft position sensor, crank sensor, or engine control module may play a role in misreads |
🛠 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.).
Read & record all error codes (use OBD‑II scanner)
Sometimes multiple misfire or related codes appear. That gives you context.
Clear the code & test
Reset the codes, then run the engine / drive. Sometimes it was a temporary glitch. If P0304 comes back, proceed.
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).
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.
Inspect wiring, connectors, harnesses
Look for loose connectors, corrosion, broken wires around coil, plug, injector.
Repair or replace any faulty wires or connectors.
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.
Check fuel pressure / fuel delivery
Use a fuel pressure gauge.
Replace or clean fuel filter if clogged.
Ensure fuel pump is working correctly.
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.
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.
Repair mechanical problems if found
If valves, rings, head gasket, etc. are faulty, those need more in‑depth engine work or replacement.
After repair, clear codes and test drive
Ensure the misfire code doesn’t return.
💡 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.
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