P0304 on a Mazda: 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 Mazda towed.

What P0304 means on a Mazda

On a Mazda, P0304 means the engine computer found that cylinder 4 is misfiring — it is not burning fuel on schedule. Mazda's SkyActiv engines run high compression and monitor each cylinder closely, so the computer can pinpoint the exact cylinder. The fault is almost always in cylinder 4's own spark plug, ignition coil, or fuel injector.

Is P0304 common on Mazdas?

P0304 shows up on Mazdas with coil-on-plug ignition, especially the Mazda3, Mazda6, and CX-5. Worn spark plugs and failing ignition coils are the usual causes; on SkyActiv engines spark plugs are typically due around 75,000 miles. A faulty fuel injector is less common but possible on direct-injection models.

Commonly reported on: Mazda3, Mazda6, CX-5, CX-9.

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: Low compression in cylinder 4 from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace the spark plugs	Yes	$50–150
Replace the cylinder 4 ignition coil	Yes	$60–180
Swap the coil with another cylinder to confirm the fault before buying a new one	Yes	$10–30
Replace the fuel injector on cylinder 4	Mechanic	$250–600