P0300 on a 2020 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 2020 Subaru Outback

Ranked from 893 NHTSA owner complaints filed for this vehicle.

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1
Valve-cover/cam-carrier oil leak fouling spark plugs
High severity
Failed cam carrier seals let engine oil seep onto the spark plugs, killing spark and causing misfire; a persistent oil smell/leak near the plugs and an oil-soaked plug well confirm it.
Reported in 1 NHTSA complaint
Parts:valve cover gasket spark plugs
2
Carbon buildup with fouled plugs / failing injectors
High severity
Direct-injection carbon buildup plus worn plugs and marginal injectors produced repeated cylinder-specific misfires (documented 30-45 misfire events/3hr) in this batch; borescope the intake valves and check injector flow.
Reported in 1 NHTSA complaint
Parts:spark plugs fuel injector
3
Worn spark plugs
Low severity
Plugs worn beyond gap spec or fouled produce a weak/inconsistent spark; pull and inspect gap, wear, and deposits.
Common cause
Parts:spark plugs
4
Failing ignition coil(s)
Medium severity
A degrading coil drops spark energy under load; check coil primary/secondary resistance and per-cylinder misfire counters.
Common cause
Parts:ignition coil
5
Vacuum/intake leaks
Medium severity
Unmetered air entering after the MAF leans out one or more cylinders enough to misfire; smoke-test intake boots and gaskets.
Common cause
6
Low cylinder compression (worn rings/valves)
High severity
Mechanical wear reduces cylinder sealing so combustion is incomplete; run a compression or leak-down test to confirm.
Common cause
7
EGR valve fault (stuck open)
Medium severity
Excess exhaust gas recirculating at idle/low load dilutes the charge and stalls combustion; check EGR flow and codes.
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