P0300 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

If the check engine light is flashing, stop driving — a severe misfire can damage the catalytic converter. A steady light is usually safe for a short drive to a shop.

What P0300 means on a Subaru

On a Subaru, P0300 means the engine computer sees misfires jumping between cylinders instead of on one fixed cylinder. A misfire means a cylinder is not burning its fuel properly, which wastes gas and can damage the catalytic converter if left alone.

Is P0300 common on Subarus?

P0300 is common on Subaru boxer engines in the Outback, Forester, and Impreza, especially at higher mileage. Spark plugs sit sideways in a boxer engine, so they often get changed late, and ignition coils and injectors wear too. Vacuum leaks on the intake are another frequent cause.

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

Symptoms

Check engine light on (flashing if severe)
Rough idle or shaking engine
Loss of power or hesitation
Poor fuel economy

Common causes

most common: Worn or fouled spark plugs
common: Failing ignition coils
common: Vacuum leak
less common: Clogged fuel injectors
less common: Low compression from engine wear

How to fix it

Fix	DIY?	Typical cost
Replace worn spark plugs	Yes	$80–250
Replace failing ignition coils	Yes	$100–300
Find and repair vacuum leaks	Yes	$10–100
Run a compression test to rule out internal engine wear	Mechanic	$100–200