P0304 on a Subaru: 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 Subaru towed.

What P0304 means on a Subaru

On a Subaru, 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. Subaru's boxer engine uses coil-on-plug ignition, so each cylinder has its own coil.

Is P0304 common on Subarus?

P0304 shows up on Subarus like the Outback, Forester, and Impreza, often at higher mileage. Worn spark plugs and failing ignition coils are the usual causes. Because the boxer engine tucks the plugs into the sides of the block, access is tight and labor makes the repair pricier than on most cars.

Commonly reported on: Outback, Forester, Impreza, Legacy.

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 cylinder 4 ignition coil	Yes	$70–200
Replace the spark plugs (tighter access on the boxer engine)	Mechanic	$250–500
Swap the coil with another cylinder to confirm the fault first	Yes	$10–30
Replace the fuel injector on cylinder 4	Mechanic	$250–600

Frequently asked questions

Can I drive my Subaru with a P0304 code?

Only if the light is steady and the engine runs okay. A flashing light means a severe misfire, so stop driving to protect the catalytic converter.

What usually causes P0304 on a Subaru?

A worn spark plug or a failing ignition coil on cylinder 4. These are the most common causes on high-mileage Subarus.

Why is fixing P0304 more expensive on a Subaru?

The boxer engine buries the spark plugs at the sides of the block, so replacing them takes more labor than on most cars. A coil is easier and cheaper to change.