OBD-II Code P0304 — What It Means & How to Fix It

What Does This Error Mean?

P0304 means your engine's cylinder number 4 is misfiring. A misfire happens when the fuel in a cylinder doesn't ignite properly or at all. You'll likely feel the engine shaking, vibrating, or running rough. The car may hesitate during acceleration and you might notice a flashing check engine light. Don't ignore this — continued misfires can overheat and destroy your catalytic converter.

Affected Models

All vehicles 1996+
Common in Honda Civic
Common in Toyota Camry
Common in Ford Focus
Common in Hyundai Elantra

Common Causes

Worn or fouled spark plug in cylinder 4 that can't create a proper spark
Failed ignition coil for cylinder 4 not delivering high voltage to the spark plug
Clogged or leaking fuel injector on cylinder 4
Vacuum leak near cylinder 4 causing a lean misfire
Low compression in cylinder 4 from worn piston rings or a leaking valve

How to Fix It

Remove and inspect the spark plug in cylinder 4. Look for heavy carbon deposits, oil fouling, a worn electrode, or cracked porcelain. Replace it with the correct type from your owner's manual.

Spark plugs cost $3-$15 each. If cylinder 4's plug looks much worse than the others, something else may be wrong with that cylinder.
Swap the ignition coil from cylinder 4 with one from another cylinder. Clear the code and drive. If the misfire moves to the other cylinder, the coil is bad and needs replacement.

This coil swap test is the easiest way to confirm a bad coil without any special tools.
Check the fuel injector on cylinder 4. Listen for a clicking sound with a mechanic's stethoscope or a long screwdriver pressed against it. No clicking means it may not be opening.

You can also swap injectors between cylinders the same way you tested the coil.
Inspect for vacuum leaks near cylinder 4. Check the intake manifold gasket area and any vacuum hoses connected near that cylinder. A leak causes a lean condition that makes misfires worse.

Spray carb cleaner around the intake near cylinder 4 while idling. If the RPM changes, there's a leak.
If all ignition and fuel components check out, have a compression test done on cylinder 4. Low compression means an internal mechanical problem that needs professional repair.

Normal compression is typically 125-180 PSI. If cylinder 4 is more than 15% lower than the others, that's the problem.

When to Call a Professional

If new spark plugs and ignition coils don't fix the misfire, see a mechanic. A compression test can reveal internal engine problems like worn rings or a bad valve. Injector problems can also be tricky to diagnose without specialized equipment. Expect to pay $100-$200 for diagnosis. Repairs range from $50 for a spark plug to $1,500+ for internal engine work.

Frequently Asked Questions

Where is cylinder 4 on my engine?

On most 4-cylinder engines, cylinder 4 is the last cylinder on the right. Cylinders are numbered from the front of the engine to the back. On V6 engines, cylinder numbering varies by manufacturer. Check your owner's manual or the top of the engine for a diagram. Some engines have numbers stamped right on the intake manifold.

Can I drive with a P0304 misfire?

You can drive short distances if the check engine light is steady. But if the light is flashing, stop driving and get a tow. A flashing light means severe misfires that are actively damaging the catalytic converter. Even with a steady light, don't put off the repair. Continued misfires waste fuel and can cause expensive secondary damage.

Why does only one cylinder misfire?

Each cylinder has its own spark plug, ignition coil, and fuel injector. When one of these parts fails, only that specific cylinder is affected. That's why the code points to cylinder 4 specifically. If you had a problem affecting all cylinders, you'd see P0300 instead. The specific cylinder number helps you target your repair to the right spot.