P0300 on a 2020 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 2020 Subaru Forester

Ranked from 593 NHTSA owner complaints filed for this vehicle.

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1
Auto Stop-Start restart misfire/stall
High severity
A large cluster of complaints describe the engine violently shaking, stalling, or failing to restart specifically when resuming from the auto idle-stop system, consistent with a restart-cycle misfire; some owners tie this to the TCV defect above.
Reported in 14 NHTSA complaints
2
Thermostatically-controlled coolant/bypass valve (TCV) failure
High severity
The dominant pattern in this batch: a widely reported Subaru thermo control valve (TCV) defect (subject of a TSB/extended-warranty campaign) that disrupts engine coolant temperature signaling and causes stalling; several complaints cite the exact fault DTCs (P2682/P26A/P26A3).
Reported in 11 NHTSA complaints
Parts:thermostatic control valve engine coolant bypass valve
3
Low compression / head gasket damage following TCV failure
High severity
Two complaints describe head gasket replacement or catastrophic engine failure shortly after a TCV repair, suggesting the underlying coolant-control fault had already damaged the engine internally.
Reported in 2 NHTSA complaints
Parts:cylinder head gasket
4
Worn or fouled spark plugs
Low severity
Aging plugs produce a weak spark and intermittent misfire, especially noticeable at idle or on cold/wet starts.
Common cause
Parts:spark plug
5
Failing ignition coil(s)
Medium severity
A coil breaking down under heat/vibration misfires a specific cylinder.
Common cause
Parts:ignition coil
6
Vacuum or intake air leak
Low severity
Unmetered air entering the intake leans out one or more cylinders enough to misfire, particularly at idle.
Common cause
7
EGR valve fault
Low severity
A stuck or leaking EGR valve disrupts combustion stability and can trigger a random misfire code.
Common cause
Parts: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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Matching symptoms

Car Runs Rough Flashing Check Engine Light