P0300 on a 2016 Mazda Cx 5

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 Mazda Cx 5

Ranked from 507 NHTSA owner complaints filed for this vehicle.

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1
Spark plug fouling from the Skyactiv-2.5 oil-consumption defect
High severity
Two complaints describe this exact vehicle burning oil into the combustion chamber by ~70k miles, requiring a combustion-chamber cleaning and new iridium spark plugs — fouled plugs from oil intrusion are a direct misfire cause.
Reported in 2 NHTSA complaints
Parts:spark plugs PCV valve intake manifold gasket
2
Failing high- or low-pressure fuel pump causing low fuel rail pressure
High severity
Owners report Denso fuel pump failures throwing low-fuel-rail-pressure code P0192 alongside stalling and rough running, which can present as a misfire when the injectors are starved.
Reported in 2 NHTSA complaints
Parts:fuel pump
3
Rodent damage to engine wiring harness (ignition/injector circuits)
Medium severity
One owner's dealer diagnosed rats nesting in the engine bay and chewing through wiring insulation, which can interrupt ignition coil or injector signals and produce misfire-like symptoms.
Reported in 1 NHTSA complaint
Parts:engine wiring harness
4
Coolant intrusion into a cylinder from a cracked cylinder head
High severity
This engine has reported cylinder-head cracking that lets coolant into the combustion chamber, which fouls that cylinder and causes a misfire before the underlying failure is diagnosed.
Reported in 1 NHTSA complaint
Parts:cylinder head
5
Worn or degraded spark plugs (general wear)
Medium severity
Standard wear item; worn electrodes widen the spark gap and misfire under load as they age.
Common cause
Parts:spark plugs
6
Failing ignition coil
Medium severity
A weak or intermittent coil produces a misfire on the affected cylinder, a standard cause on any coil-per-cylinder engine.
Common cause
Parts:ignition coil
7
Carbon buildup on direct-injection intake valves
Medium severity
Skyactiv direct-injection engines are prone to intake-valve carbon deposits that disrupt airflow and combustion, a known contributor to misfire on GDI engines.
Common cause
8
Vacuum or intake air leak
Medium severity
An unmetered air leak upsets the air/fuel ratio at idle, a standard misfire cause independent of ignition components.
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