P0300 on a 2019 Subaru Ascent

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 Ascent

Ranked from 433 NHTSA owner complaints filed for this vehicle.

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1
Turbo/valve-seal oil consumption fouling spark plugs (chronic startup smoke)
Medium severity
A large cluster of owners report thick blue/white, chemical-smelling smoke at every cold start — oil being drawn past worn turbo or valve seals — which fouls plugs enough to misfire.
Reported in 6 NHTSA complaints
Parts:spark plugs turbocharger oil seal PCV valve
2
Low compression from an internal bearing/rod failure under load
High severity
Two related reports describe sudden knocking and a thrown rod while towing/climbing a grade, a catastrophic low-compression event that can misfire immediately before failing outright.
Reported in 2 NHTSA complaints
3
Recalled fuel pump (or its replacement) causing rough idle/misfire
Medium severity
One owner's vehicle began idling and running rough shortly after a recall fuel-pump replacement, suggesting the pump itself is implicated in misfire-like symptoms on this model.
Reported in 1 NHTSA complaint
Parts:fuel pump
4
Worn spark plugs
Medium severity
Standard high-mileage wear item and one of the most common misfire causes on any engine.
Common cause
Parts:spark plugs
5
Failing ignition coil(s)
Medium severity
A coil breaking down under heat/load drops spark energy to a cylinder, causing a misfire.
Common cause
Parts:ignition coil
6
Vacuum/intake leak
Low severity
Unmetered air leans a cylinder enough to misfire, especially at idle.
Common cause
7
Carbon buildup on intake valves (direct-injection turbo engine)
Low severity
Direct-injection engines lack fuel washing the intake valves, letting carbon build up and disrupt airflow/combustion.
Common cause
8
EGR valve fault
Low severity
A stuck-open EGR valve dilutes the intake charge excessively, causing a misfire particularly at idle.
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