P0300 on a 2017 Ford Fusion Hev

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 Hev

Ranked from 1,081 NHTSA owner complaints filed for this vehicle.

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1
Coolant intrusion into cylinders from a cracked engine block / failed head gasket
High severity
The dominant, extensively-documented failure pattern in these complaints: coolant leaking into cylinders (often cylinder #2 or #3) causes rough idle, white/sweet-smelling exhaust smoke, and multi-cylinder misfire codes; subject of Ford TSB 19-2346 and Customer Satisfaction Program 21N12 on this generation.
Reported in 28 NHTSA complaints
Parts:engine long block head gasket
2
Fouled spark plugs / failed ignition coils (often misdiagnosed before coolant intrusion found)
Medium severity
Multiple owners had coils, plug wires, or spark plugs replaced first for a specific-cylinder misfire code before coolant intrusion was ultimately identified as the root cause.
Reported in 2 NHTSA complaints
Parts:ignition coil spark plugs
3
Worn spark plugs (routine wear, unrelated to coolant intrusion)
Low severity
Standard cause of random/multi-cylinder misfire on any high-mileage engine — inspect gap and electrode wear.
Common cause
Parts:spark plugs
4
Clogged or leaking fuel injector(s)
Medium severity
A stuck-open or clogged injector unbalances individual-cylinder fueling and sets a misfire code.
Common cause
Parts:fuel injector
5
Vacuum/intake leak
Medium severity
Unmetered air entering after the MAF causes lean misfire, typically worse at idle; check intake boot and manifold gasket for cracks.
Common cause
6
Carbon-fouled intake valves
Low severity
Direct-injection engines are prone to carbon buildup on intake valves, disrupting airflow/fuel atomization and causing misfire.
Common cause
7
Low compression from worn piston rings/valves
High severity
Loss of cylinder sealing (sometimes accelerated by coolant washing oil off cylinder walls) reduces combustion efficiency enough to misfire; confirm with a compression or leak-down test.
Common cause
8
EGR valve fault
Low severity
A stuck-open EGR valve dilutes the intake charge excessively at idle, producing a misfire.
Common cause
Parts:EGR valve

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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