P0300 on a 2011 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 2011 Subaru Outback
Ranked from 649 NHTSA owner complaints filed for this vehicle.
- 1
Rough-idle/near-stall misfire on deceleration (Subaru TSB 16-90-13R idle-control condition)
High severityAn unusually large number of owners describe the engine shuddering, misfiring, or dying specifically when decelerating to a stop — a well-documented pattern on this generation Outback addressed by Subaru TSB 16-90-13R for idle-speed/fuel control calibration.
Reported in 11 NHTSA complaints - 2
Head gasket failure allowing coolant intrusion into a cylinder
High severityMultiple owners report repeated or eventual head gasket failure (some after overheating events) on this model — coolant leaking into a cylinder fouls the plug and disrupts combustion, producing a misfire.
Reported in 5 NHTSA complaintsParts:head gasket - 3
Worn or fouled spark plugs
Low severityStandard first check for random misfire; inspect gap and electrode condition against the service interval.
Common causeParts:spark plugs - 4
Failing ignition coil(s)
Medium severityAn intermittently failing coil drops a cylinder under load; swap-testing coils will isolate it.
Common causeParts:ignition coil - 5
Clogged or leaking fuel injector(s)
Medium severityAn injector delivering too little or dribbling fuel disrupts combustion on that cylinder.
Common causeParts:fuel injector - 6
Vacuum or intake leak
Medium severityUnmetered air entering after the MAF leans out individual cylinders enough to misfire, especially at idle.
Common cause - 7
Low compression from worn piston rings or valves
High severityAge/mileage-related mechanical wear reduces cylinder sealing enough that combustion becomes inconsistent on that cylinder; a compression or leak-down test will confirm.
Common cause - 8
EGR valve/passage fault
Low severityA stuck-open EGR dilutes the intake charge at idle/light load, a classic random-misfire trigger.
Common causeParts:EGR valve
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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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