P0300 on a 2021 Ford Bronco Sport

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 2021 Ford Bronco Sport

Ranked from 665 NHTSA owner complaints filed for this vehicle.

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1
Cracked/leaking fuel injector (NHTSA recall 25V467)
High severity
A dominant, recall-documented defect on this vehicle: cracked high-flow fuel injectors leak fuel onto the engine, causing stumbling/hesitation on acceleration, stalling, and in several cases underhood fires — all consistent with an uneven-fueling misfire.
Reported in 14 NHTSA complaints
Parts:fuel injector
2
Head gasket/cracked engine block allowing coolant into the cylinders
High severity
This 1.5L EcoBoost has a documented cracked-block/head-gasket defect letting combustion gases into the coolant (and coolant into cylinders), which fouls combustion and can cause misfires ahead of full engine failure.
Reported in 2 NHTSA complaints
3
PCV/oil separator failure (known EcoBoost issue, recall 22S21)
Medium severity
One owner traced a persistent rough idle to a failed oil separator later confirmed to be under recall 22S21 — a stuck/failed PCV path causes an unstable idle and can trigger misfire counts.
Reported in 1 NHTSA complaint
Parts:PCV valve oil separator
4
Worn or fouled spark plugs
Low severity
Standard high-mileage misfire cause; check plug gap and condition on any cylinder(s) misfiring.
Common cause
Parts:spark plugs
5
Failing ignition coil(s)
Medium severity
A weak or cracked coil-on-plug unit drops spark energy on that cylinder, producing a random or single-cylinder misfire.
Common cause
Parts:ignition coil
6
Vacuum/intake leaks
Medium severity
Unmetered air entering past a cracked hose or gasket leans out one or more cylinders enough to misfire.
Common cause
7
Carbon-fouled intake valves
Medium severity
Direct-injection engines like this 3-cylinder EcoBoost don't wash the intake valves with fuel, so carbon buildup is common and disrupts airflow/combustion enough to misfire.
Common cause
8
Low cylinder compression (worn rings/valves)
High severity
Mechanical wear reduces a cylinder's ability to build compression, causing a persistent misfire that won't clear with tune-up parts.
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