P0304 Code: Cylinder 4 Misfire Detected

🟠 Moderate severity · drivable

Short drives with a steady light are acceptable, but fix it soon — and stop immediately if the check engine light flashes.

What P0304 means

Cylinder 4 is misfiring — failing to fire and contribute power on schedule. A single-cylinder code like P0304 is far easier to chase than P0300 (random misfire): the fault is almost certainly isolated to cylinder 4's own spark plug, ignition coil, or fuel injector, so diagnosis starts right at that cylinder.

Symptoms

Check engine light on (flashing when severe)
Engine vibration or rough idle
Power loss and hesitation
Poor fuel economy

Common causes

most common: Worn or fouled spark plug on cylinder 4
common: Failing ignition coil or plug wire for cylinder 4
common: Clogged or leaking fuel injector on cylinder 4
less common: Low compression in cylinder 4 from internal wear

How to fix it

Fix	DIY?	Typical cost
Swap cylinder 4's spark plug and coil with a neighboring cylinder and re-scan to see if the misfire moves	Yes	$0–20
Replace the cylinder 4 spark plug	Yes	$10–50
Replace the cylinder 4 ignition coil	Yes	$50–200
Have a compression test done on cylinder 4	Mechanic	$100–200

Frequently asked questions

Is P0304 always cylinder 4?

Yes — the code is specific to cylinder 4, so the plug, coil, or injector on that cylinder is the first place to look. A misfire that jumps between cylinders would show as P0300.

What's the quickest way to find a P0304 fault?

Swap the cylinder 4 coil and plug into another cylinder. If the misfire relocates, replace the moved part. If it stays put, move on to the injector or a compression check.

How urgent is a P0304?

Moderately urgent. A steady light means you can drive briefly, but raw fuel entering the exhaust can damage the catalytic converter, and a flashing light means stop and get a tow.