P0300 on a 2017 Ford Focus Rs

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 2017 Ford Focus Rs

Ranked from 842 NHTSA owner complaints filed for this vehicle.

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1
Oil-pump timing belt failure ("belt-in-oil" recall 23V905000)
High severity
A large, well-documented pattern in this population: the EcoBoost's in-oil timing/oil-pump belt sheds material or slips, causing low oil pressure, valve-timing disruption, and catastrophic engine damage — misfiring and stalling are common early symptoms before full failure. Many owners report an unavailable recall part and repeated stalls/low-oil-pressure warnings.
Reported in 12 NHTSA complaints
Parts:oil pump timing belt oil pump
2
Head gasket failure / cracked cylinder causing compression loss
High severity
One owner's engine required a full long-block replacement after a failed head gasket repair and a confirmed crack in cylinder 3; low or uneven compression on a cylinder will produce a persistent misfire.
Reported in 1 NHTSA complaint
Parts:head gasket
3
Worn or fouled spark plugs
Medium severity
Worn electrodes increase the firing voltage needed and can misfire under load; check gap/condition against the service interval.
Common cause
Parts:spark plugs
4
Failing ignition coil(s)/coil pack
Medium severity
A weak or intermittently failing coil drops spark energy to one cylinder; swap coils cylinder-to-cylinder to confirm.
Common cause
Parts:ignition coil
5
Clogged or leaking fuel injector(s)
Medium severity
A restricted or dribbling injector unbalances the air-fuel ratio on that cylinder; check injector balance/flow rates.
Common cause
Parts:fuel injector
6
Carbon-fouled intake valves
Medium severity
Direct-injection EcoBoost engines are prone to carbon buildup on the backs of the intake valves, disrupting airflow/atomization enough to misfire; inspect valves with a borescope.
Common cause
7
Vacuum or intake/charge-air leak
Medium severity
Unmetered air entering after the throttle or a leaking boost hose leans out a cylinder enough to misfire under boost; smoke or pressure test the intake.
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

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