P0300 on a Subaru Outback

What P0300 usually means on a Outback

A P0300 on the Outback flags misfires the PCM could not tie to one cylinder. On the boxer engines the everyday causes lead, aging spark plugs and ignition coils, but the flat layout puts the plugs low on each side of the engine where they are exposed to heat and grime. On EJ25 cars a persistent misfire can also hint at a head-gasket problem letting coolant into a cylinder. Rough idle, a stumble under load, and a check-engine light that flashes on acceleration are the usual signs.

Most likely causes on this model

1. Worn spark plugs or coils

   The leading cause; the boxer's side-mounted plugs run hot and are easy to neglect past their interval.

2. Vacuum or intake leak

   A cracked hose or leaking gasket leans out multiple cylinders and reads as random misfire.

3. EJ25 head-gasket coolant intrusion

   On the older 2.5, coolant reaching a cylinder can cause misfires along with the classic head-gasket symptoms.

4. Weak fuel delivery

   A tired pump or dirty injectors starves several cylinders, especially under load.

What it costs on a Outback

Plugs and coils are the cheapest fix, but boxer plug access adds labor because the plugs sit low on the sides of the engine, so a job that is trivial on an inline four costs more here, often a few hundred dollars. If diagnosis points to an EJ25 head gasket, that is a major repair in the four-figure range because the engine largely has to come apart. Confirm the cause before assuming the worst.

Fixing it

Start with correct-spec plugs and coils; if misfires persist on the EJ25, have the cooling system pressure-tested and the combustion gases checked before condemning the head gaskets.

For the full step-by-step diagnosis order (any vehicle), see the main P0300 guide.

Other codes we cover on the Subaru Outback

Related symptoms

Subaru Outback problems & recalls by year