P0300 Subaru Outback
Random/Multiple Cylinder Misfire Detected · Powertrain Code · High
What is P0300?
The ECM has detected misfires occurring in multiple or random cylinders. Common causes include worn spark plugs, faulty ignition coils, vacuum leaks, low fuel pressure, or a failing catalytic converter.
When the Subaru Outback triggers code P0300 (random/multiple cylinder misfire detected), it typically points to an issue in the powertrain system. This is a high severity code, so address it promptly to avoid further damage or safety concerns. Below you'll find the most common causes, symptoms, and repair options specific to the Subaru Outback.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Subaru Outback.
Common Causes on Subaru Outback
- Ignition coil failure on horizontally-opposed boxer engine (Outback/Forester/Crosstrek/Impreza/Ascent)
- Head gasket external leak allowing coolant into combustion chamber on 2.5L EJ25 (pre-2020)
- Spark plug fouling from oil consumption past valve stem seals on FB25 engine
- Carbon buildup on intake valves from direct injection on FA24 turbo engine (2022+ WRX/Ascent)
- Ignition coil connector corrosion from water pooling in boxer engine valley
Symptoms
- Rough idle with noticeable engine vibration
- Engine shaking or stumbling under load
- Loss of power during acceleration
- Check engine light flashing intermittently
- Poor fuel economy
- Hesitation or stumble on acceleration
Notes for Subaru Outback
Boxer engine spark plug and coil access requires more labor than inline engines due to horizontal layout.