P0300 on a 2016 Kia Optima

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 Kia Optima

Ranked from 748 NHTSA owner complaints filed for this vehicle.

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1
Rod bearing failure / spun bearing (Theta II engine defect)
High severity
Multiple owners report audible knocking together with active misfire codes, consistent with the well-documented Theta II connecting-rod bearing failure that causes cylinder-specific misfires before seizure.
Reported in 8 NHTSA complaints
2
Low compression from worn piston rings / oil consumption defect
High severity
Chronic oil consumption (topping off every 1-3 weeks with no external leak) starves cylinders of oil control, dropping compression and producing rough-running/misfire symptoms.
Reported in 7 NHTSA complaints
3
Oil-fouled or worn spark plugs
Low severity
A dealer specifically found fouled spark plugs and recommended replacement plus injector cleaning after misfire codes appeared; oil ingestion into the combustion chamber accelerates plug fouling.
Reported in 1 NHTSA complaint
Parts:spark plug
4
Failing ignition coil(s)
Medium severity
A weak or failed coil-on-plug unit drops spark energy to one cylinder; check for a stored cylinder-specific misfire code and swap coils to confirm.
Common cause
Parts:ignition coil
5
Clogged or leaking fuel injector
Medium severity
A dirty or dripping injector unbalances air/fuel ratio in one cylinder; look for uneven injector balance/fuel trims on a scan tool.
Common cause
Parts:fuel injector
6
Vacuum or intake leak
Medium severity
Unmetered air entering after the MAF causes lean misfire, especially at idle; smoke-test the intake boots, PCV hose, and brake booster line.
Common cause
7
Carbon-fouled intake valves (GDI deposit buildup)
Low severity
Direct-injection engines like this 2.4L GDI have no fuel washing the intake valves, so carbon buildup restricts airflow/atomization and triggers misfire over time.
Common cause
8
EGR valve fault
Low severity
A stuck-open EGR valve dilutes the intake charge excessively at idle/low RPM, producing a rough idle and misfire codes.
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