P0300 on a 2021 Kia Seltos

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 2021 Kia Seltos

Ranked from 346 NHTSA owner complaints filed for this vehicle.

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1
Low compression from worn/failed piston rings and chronic oil consumption
High severity
This batch is dominated by the documented Kia Theta/Smartstream piston-oil-ring defect (recall SC336/25V099): rapid oil consumption, rod knock, and catastrophic engine failure, which drives low-compression misfire.
Reported in 16 NHTSA complaints
2
Ignition coil / spark plug fouling
Medium severity
One complaint documents spark plugs and an ignition coil being replaced during a misfire/power-loss diagnosis.
Reported in 1 NHTSA complaint
Parts:spark plug ignition coil
3
Worn spark plugs
Medium severity
Worn or improperly gapped plugs cause an intermittent spark that shows up as random misfire; inspect gap and electrode wear.
Common cause
Parts:spark plug
4
Failing ignition coil(s)
Medium severity
A cracked or heat-degraded coil drops spark energy under load; swapping coils between cylinders and watching for the misfire to follow confirms it.
Common cause
Parts:ignition coil
5
Clogged or leaking fuel injector(s)
Medium severity
A partially clogged or dribbling injector leans out or floods a single cylinder, producing a misfire on that cylinder.
Common cause
Parts:fuel injector
6
Vacuum or intake air leak
Low severity
Unmetered air entering after the throttle body leans the mixture enough to cause a misfire, especially at idle.
Common cause
7
Carbon-fouled intake valves
Low severity
Direct-injected engines like the Seltos's build carbon on the valve backs over time, disrupting airflow/mixture and contributing to misfire.
Common cause
8
EGR valve or passage fault
Low severity
A stuck-open EGR valve or clogged passage dilutes the intake charge unevenly, which can present as 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