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P0300

P0300 on a 2018 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 2018 Subaru Forester

Ranked from 489 NHTSA owner complaints filed for this vehicle.

Sort by:
  1. 1

    Weak or failing fuel pump

    High severity

    Owners report sudden freeway power loss "as if the fuel pump went out," an intermittent no-power/dies condition eventually traced to a fuel-delivery fault, and a fuel pump directly diagnosed as needing replacement.

    Reported in 3 NHTSA complaints
    Parts:fuel pump
  2. 2

    Worn or fouled spark plugs

    Medium severity

    One owner's vehicle was diagnosed with spark plugs needing replacement after a shaking/stalling/smoke event.

    Reported in 1 NHTSA complaint
    Parts:spark plugs
  3. 3

    Failed catalytic converter causing severe back-pressure

    High severity

    An owner had Subaru diagnose and replace a bad catalytic converter (then a second one) after the engine suddenly lost power on the freeway — enough exhaust restriction can produce rough running/stalling.

    Reported in 1 NHTSA complaint
    Parts:catalytic converter
  4. 4

    Failing AVCS (variable valve timing) solenoid

    Medium severity

    An owner reports a cold-start ticking noise with reduced acceleration/torque attributed to AVCS solenoids beginning to fail, a wear pattern that can disrupt valve timing enough to cause rough running.

    Reported in 1 NHTSA complaint
    Parts:AVCS solenoid
  5. 5

    Failing ignition coil(s)

    Medium severity

    A cracked or weak coil drops spark energy to one cylinder, producing a repeatable single- or multi-cylinder misfire.

    Common cause
    Parts:ignition coil
  6. 6

    Vacuum or intake leak

    Medium severity

    Unmetered air leaning out one or more cylinders causes an intermittent misfire, often worse at idle.

    Common cause
  7. 7

    Clogged or leaking fuel injector

    Medium severity

    A restricted or dribbling injector starves or floods a cylinder, producing a repeatable misfire on that cylinder.

    Common cause
    Parts:fuel injector
  8. 8

    Low compression from worn piston rings or valves

    Medium severity

    Mechanical wear that drops compression in one cylinder will produce a persistent misfire on that cylinder regardless of ignition/fuel condition.

    Common cause

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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

1

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.

2

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.

3

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.

4

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.

5

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 RoughFlashing Check Engine Light