P0304 on a Hyundai: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A steady light usually means you can drive a short distance, but a flashing light means the misfire is severe — pull over and get the Hyundai towed.

What P0304 means on a Hyundai

On a Hyundai, P0304 means the engine computer found that cylinder 4 is misfiring — it is not burning fuel on schedule. Because the code names one cylinder, the fault is almost always cylinder 4's own spark plug, ignition coil, or fuel injector. Hyundai engines use coil-on-plug ignition, so each cylinder has its own coil.

Is P0304 common on Hyundais?

P0304 shows up on Hyundais like the Elantra, Sonata, and Tucson, often at higher mileage. Worn spark plugs are the most common cause, and ignition coils fail with age. A weak fuel injector is less common but possible on direct-injection engines.

Commonly reported on: Elantra, Sonata, Tucson, Santa Fe.

Symptoms

Check engine light on, flashing if the misfire is severe
Rough idle or shaking while stopped
Loss of power or hesitation when accelerating
Worse fuel economy

Common causes

most common: Worn or fouled spark plug in cylinder 4
common: Failing ignition coil for cylinder 4
less common: Clogged or leaking fuel injector on cylinder 4
less common: Vacuum or intake air leak near cylinder 4

How to fix it

Fix	DIY?	Typical cost
Replace the spark plugs	Yes	$60–180
Replace the cylinder 4 ignition coil	Yes	$60–200
Swap the coil with another cylinder to confirm the fault first	Yes	$10–30
Replace the fuel injector on cylinder 4	Mechanic	$250–600