P0300 on a 2018 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 2018 Ford Fusion

Ranked from 429 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant intrusion into cylinders from cracked engine block / failed head gasket (1.5L & 2.0L EcoBoost known defect)
High severity
Extremely well documented on this generation of EcoBoost engine: coolant leaks internally into one or more cylinders (no external leak) causing rough running, white exhaust smoke, and DTCs P0300-P0304; confirmed by dealers via cylinder borescope/pressure test on many of these complaints.
Reported in 26 NHTSA complaints
2
Spark plug fouling/cracking from coolant intrusion
Medium severity
Coolant reaching the combustion chamber fouls or cracks spark plugs, which several owners had replaced (one found a cracked plug on the misfiring cylinder) as part of chasing the misfire.
Reported in 2 NHTSA complaints
Parts:spark plugs
3
Worn or fouled spark plugs (normal wear)
Low severity
Worn, gapped-out, or oil/carbon-fouled plugs are a top general cause of random/multi-cylinder misfire.
Common cause
Parts:spark plugs
4
Failing ignition coil(s)
Medium severity
A cracked or weak coil-on-plug unit causes an intermittent or cylinder-specific misfire, especially under load.
Common cause
Parts:ignition coil
5
Vacuum leak (intake, PCV, or turbo hose)
Medium severity
Unmetered air entering after the MAF causes lean misfire, particularly at idle; common on turbocharged engines with multiple boost/vacuum hoses.
Common cause
6
Clogged or leaking fuel injector
Medium severity
A partially clogged or dribbling injector unbalances air/fuel ratio in one cylinder, producing a repeatable misfire on that cylinder.
Common cause
Parts:fuel injector
7
Carbon-fouled intake valves / low compression
Low severity
Direct-injection engines are prone to intake valve carbon buildup, which combined with worn rings/valves lowers cylinder compression enough to misfire.
Common cause
8
EGR valve fault
Low severity
A stuck-open EGR valve dilutes the intake charge excessively at idle/low load, causing a misfire.
Common cause
Parts:EGR valve

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Car Runs Rough Flashing Check Engine Light