P0300 on a 2016 Subaru Outback
P0300 is a generic OBD2 powertrain code for random or multiple cylinder misfire. Unlike P0301 through P0308, it does not point to one specific cylinder, so the cause can be ignition, fuel, air, sensor, or mechanical.
Not your vehicle? See general P0300 info.
Most common causes on the 2016 Subaru Outback
Ranked from 639 NHTSA owner complaints filed for this vehicle.
- 1
Cam carrier oil leak fouling the engine bay near ignition/exhaust components
Medium severityThis engine has a well-documented cam-carrier sealant leak in the complaint set that fills the engine bay with oil smoke; while primarily an external leak, oil contamination near spark/exhaust components can contribute to rough running if severe.
Reported in 2 NHTSA complaints - 2
Cracked intake manifold causing a vacuum leak
Medium severityOne owner's vehicle was diagnosed with a fractured intake manifold alongside abnormal engine knocking; a manifold crack is a direct vacuum-leak path that produces a lean, rough-running misfire.
Reported in 1 NHTSA complaintParts:intake manifold - 3
Worn or fouled spark plugs
Low severityThe baseline general misfire cause on this flat-four engine; check plug condition and gap, especially on higher-mileage cars.
Common causeParts:spark plugs - 4
Failing ignition coil(s)
Medium severityA weak coil produces a repeatable single-cylinder misfire; swap coils between cylinders to confirm.
Common causeParts:ignition coil - 5
Clogged or leaking fuel injector(s)
Medium severityA restricted or dribbling injector starves or floods a cylinder, producing a repeatable misfire.
Common causeParts:fuel injector - 6
Vacuum/intake air leak from cracked hoses or gaskets
Medium severityUnmetered air causes a lean misfire, typically worse at idle; smoke-test the intake tract and PCV system.
Common cause - 7
Low cylinder compression (worn rings/valves)
High severityReduced compression from wear or oil consumption can present as an intermittent misfire, especially under load.
Common cause
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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0300 on your car still needs shop confirmation before any repair.
Common symptoms
- Flashing or steady check engine light
- Rough idle
- Shaking under acceleration
- Loss of power
- Fuel smell from exhaust
What to check before replacing parts
Scan for cylinder-specific misfire codes
Look for: A companion P0301–P0308 pins the misfire to one cylinder — start there. P0300 with no cylinder code points at something hitting several at once: fuel pressure, a large vacuum leak, or timing.
Inspect the spark plugs and coils
Look for: Pull the plugs: black/sooty = rich, oily = oil intrusion, white/blistered = lean or overheating, cracked porcelain or an out-of-spec gap = replace. Then swap a coil to another cylinder — if a cylinder-specific code follows the coil, the coil is bad.
Check fuel trims for a vacuum leak
Look for: High positive short- and long-term trims (worst at idle, improving as RPM rises) mean unmetered air is getting in. Smoke-test the intake and listen for hissing.
Verify fuel pressure
Look for: Compare running/cranking pressure to the vehicle spec. Low or dropping pressure starves multiple cylinders and reads as a random misfire — suspect the pump, filter, or regulator.
Compression / leak-down test if ignition and fuel check out
Look for: A cylinder 15%+ below the others, or leak-down hissing from the intake (intake valve), exhaust (exhaust valve), or oil filler (rings), means the misfire is mechanical — not a swap-in part.
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