P0300 on a 2015 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 2015 Subaru Outback
Ranked from 505 NHTSA owner complaints filed for this vehicle.
- 1
Cracked Exhaust Pipe / Exhaust Manifold Leak
Medium severityA cracked exhaust pipe or manifold (often at the bellows) causes exhaust leaks, noise, and fumes in the cabin, which can disrupt O2 sensor readings and cause misfires. Look for exhaust smell in the cabin, noise from the front, or visible cracks in the exhaust manifold/bellows.
Reported in 6 NHTSA complaints - 2
Engine Oil Consumption / Internal Oil Leaks
High severityExcessive oil consumption or internal leaks (like cam carrier or head gasket leaks) can foul spark plugs and cause misfires. Check for low oil levels, oil leaks around the engine, or a history of frequent oil top-offs.
Reported in 6 NHTSA complaints - 3
Camshaft/Crankshaft Position Sensor Issues
Medium severityFaulty cam or crank sensors disrupt ignition timing, leading to misfires or a no-start condition. Check for related codes (e.g., P0340) and verify sensor signals.
Reported in 1 NHTSA complaint - 4
Faulty Spark Plugs
Low severityWorn or fouled spark plugs are a primary cause of random misfires. Inspect spark plugs for wear, carbon buildup, or oil fouling.
Common cause - 5
Ignition Coil Failure
Medium severityA failing ignition coil will not provide sufficient spark, leading to misfires. Test coils for proper resistance and check for physical damage or arcing.
Common cause - 6
Vacuum Leak
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires. Inspect vacuum hoses and intake gaskets for leaks using a smoke machine.
Common cause - 7
Fuel Delivery Issues (Injectors/Pump)
Medium severityClogged fuel injectors or low fuel pressure can cause a lean condition and misfires. Check fuel pressure and perform an injector balance test.
Common cause - 8
Mass Air Flow (MAF) Sensor Failure
Low severityA dirty or failing MAF sensor incorrectly measures incoming air, throwing off the air/fuel mixture and causing misfires. Clean or test the MAF sensor.
Common cause
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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