P0300 on a 2017 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 2017 Subaru Outback
Ranked from 917 NHTSA owner complaints filed for this vehicle.
- 1
Cam carrier and valve cover gasket leaks
High severityOil leaks from the cam carrier and valve cover onto hot exhaust components, causing fumes in the cabin and potential fire hazards.
Reported in 4 NHTSA complaints - 2
Defective Data Control Module (DCM) causing excessive parasitic draw
High severityThe DCM stays active when the vehicle is off, draining the battery below the threshold required for engine operation and causing stalling or no-start conditions.
Reported in 1 NHTSA complaint - 3
Excessive engine oil consumption
Medium severityThe engine burns oil at an abnormally high rate, requiring frequent top-offs between oil changes to prevent engine damage.
Reported in 1 NHTSA complaint - 4
Corroded or failed catalytic converter manifold
Medium severityThe exhaust pipe rusts through at the accordion section, causing exhaust leaks and fumes in the cabin.
Reported in 1 NHTSA complaint - 5
Faulty ignition switch or key cylinder
Medium severityThe key gets stuck in the ignition and the engine fails to shut off when shifted into park.
Reported in 1 NHTSA complaint - 6
Defective electronic throttle body or pedal position sensor
High severityThe throttle fails to respond when pressing the gas pedal, causing delayed acceleration or stalling.
Reported in 1 NHTSA complaint - 7
Faulty spark plugs or ignition coils
Medium severityWorn spark plugs or failing ignition coils are the most common generic causes for a P0300 random misfire code.
Common cause - 8
Vacuum leak or intake manifold gasket failure
Medium severityUnmetered air entering the engine causes a lean condition, leading to random misfires across multiple cylinders.
Common cause - 9
Low fuel pressure or failing fuel pump
Medium severityInadequate fuel delivery causes lean misfires, especially under load or acceleration.
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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