P0300 on a 2017 Ford Fusion
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 Fusion
Ranked from 1,096 NHTSA owner complaints filed for this vehicle.
- 1
Coolant Intrusion into Cylinders (Design Flaw/Cracked Block)
High severityCoolant leaks into the cylinders (often cylinder #2 or #3) due to a known block design flaw, causing misfires, rough running, and overheating. Confirmed by dropping coolant levels, white exhaust smoke, and borescope inspection showing coolant in the cylinder.
Reported in 26 NHTSA complaints - 2
Faulty Spark Plugs
Low severityWorn or fouled spark plugs fail to ignite the air/fuel mixture properly, leading to random misfires. Confirmed by inspecting the spark plug electrodes for excessive wear, carbon buildup, or fouling.
Common cause - 3
Faulty Ignition Coils
Medium severityIgnition coils degrade over time and fail to provide sufficient voltage to the spark plugs. Confirmed by testing coil resistance or swapping coils between cylinders to see if the misfire moves.
Common cause - 4
Vacuum Leaks
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires across multiple cylinders. Confirmed by performing a smoke test or listening for hissing noises around the intake.
Common cause - 5
Low Fuel Pressure / Faulty Fuel Injectors
Medium severityInadequate fuel delivery prevents proper combustion, leading to misfires. Confirmed by testing fuel rail pressure or performing an injector balance test.
Common cause - 6
Mass Air Flow (MAF) Sensor Failure
Medium severityA dirty or failing MAF sensor incorrectly measures incoming air, causing the PCM to command an improper air/fuel ratio. Confirmed by checking MAF sensor data on a scan tool or cleaning the sensor.
Common cause
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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