P0300 on a 2016 Ford Focus

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 2016 Ford Focus

Ranked from 1,187 NHTSA owner complaints filed for this vehicle.

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1
Canister purge valve (CPV) stuck open
Medium severity
A stuck-open CPV constantly bleeds fuel vapor/air into the intake, creating a lean, rough idle and stall condition; this is a well-documented issue on this exact vehicle, covered by Ford recalls/campaigns (19S22, 24N07) and reported here as rough idle, hesitation, and stalling with a P1450 code.
Reported in 3 NHTSA complaints
Parts:canister purge valve
2
Oil pump drive (wet) belt failure — 1.0L EcoBoost
High severity
The 1.0L EcoBoost's internal wet timing belt driving the oil pump is a documented recall issue (NHTSA 23V905); belt/tensioner failure starves the engine of oil, causing knocking, rough running, and eventual catastrophic misfire/failure, as reported repeatedly for this vehicle.
Reported in 3 NHTSA complaints
Parts:oil pump belt oil pump belt tensioner
3
Worn or fouled spark plugs
Low severity
The most common overall misfire cause; look for wide gaps, carbon fouling, or oil fouling on removal, and always gap/inspect at the mileage interval.
Common cause
Parts:spark plugs
4
Failing ignition coil(s)
Medium severity
A cracked or weak coil boot/pack drops spark energy on one or more cylinders; swap-test a suspect coil to another cylinder and watch the misfire follow it.
Common cause
Parts:ignition coil
5
Clogged or leaking fuel injector
Medium severity
A partially clogged or dribbling injector unbalances the air/fuel mixture on that cylinder; injector balance/flow testing isolates the culprit.
Common cause
Parts:fuel injector
6
Vacuum leak (intake, PCV, or EVAP hose)
Medium severity
Unmetered air leaning out a cylinder or the whole bank causes random misfire, especially at idle; smoke-test the intake and vacuum lines.
Common cause
Parts:vacuum hose
7
Carbon-fouled intake valves (direct-injection buildup)
Low severity
Direct-injected engines like this one accumulate carbon on the backs of the intake valves over time, disrupting airflow/atomization enough to cause misfire at idle or low load.
Common cause
8
Low compression from worn rings/valves
High severity
Mechanical wear reduces cylinder sealing enough to prevent a clean burn; a compression or leak-down test isolates a mechanical misfire from an ignition/fuel one.
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