P0300 on a 2019 Subaru Forester
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 2019 Subaru Forester
Ranked from 831 NHTSA owner complaints filed for this vehicle.
- 1
Faulty Thermo Control Valve (TCV) / Engine Coolant Bypass Valve
High severityA known issue on this model year where the thermo control valve fails, often throwing codes like P26A6, P26A3, or P2682, causing the engine to stall, misfire, or enter limp mode, which can trigger a P0300 multiple misfire code.
Reported in 9 NHTSA complaints - 2
Auto Start/Stop System Malfunction / Low Battery
Medium severityThe auto start/stop system fails to restart the engine at stops, causing the vehicle to stall and potentially triggering misfire or stall-related codes. In some cases, this is linked to a weak battery failing to support the restart.
Reported in 7 NHTSA complaints - 3
Failed PCV Valve (Positive Crankcase Ventilation)
High severityThe PCV valve can separate or fail, leading to catastrophic engine damage, oil consumption, and white smoke from the exhaust, which can cause severe misfires.
Reported in 2 NHTSA complaints - 4
Blown Head Gasket
High severityCoolant leaking and mixing with oil can lead to engine overheating and misfires due to loss of compression or coolant entering the combustion chamber.
Reported in 1 NHTSA complaint - 5
Faulty Spark Plugs
Low severityWorn or fouled spark plugs fail to ignite the air/fuel mixture properly, leading to random or multiple cylinder misfires.
Common cause - 6
Faulty Ignition Coils
Medium severityDegraded or failing ignition coils cannot provide sufficient voltage to the spark plugs, resulting in weak or no spark and causing misfires.
Common cause - 7
Vacuum Leaks
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires across multiple cylinders.
Common cause - 8
Low Fuel Pressure / Failing Fuel Pump
Medium severityInadequate fuel delivery prevents the engine from maintaining the proper air/fuel ratio, leading to lean misfires.
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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