P0300 on a 2018 Hyundai Sonata
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 Hyundai Sonata
Ranked from 366 NHTSA owner complaints filed for this vehicle.
- 1
Oil-fouled spark plugs from the Theta II engine oil-consumption defect
High severityThis exact vehicle/engine (2.4L Theta II) has a well-documented internal oil consumption defect; multiple owners report the burned/leaked oil fouling spark plugs and directly causing misfire, rough idle, and near-stall events.
Reported in 7 NHTSA complaints - 2
Worn piston rings / low compression from Theta II oil consumption
High severitySeveral owners had dealers document worn rings, sludge buildup, or internal engine failure tied to the same oil-consumption defect, which drops cylinder compression enough to cause a persistent misfire.
Reported in 3 NHTSA complaints - 3
Worn spark plugs (normal wear, unrelated to oil fouling)
Low severityPlugs past their service interval lose spark energy and misfire under load; check gap and condition even if oil consumption isn't present.
Common causeParts:spark plug - 4
Failing ignition coil(s)
Medium severityA breaking-down coil causes an intermittent single-cylinder misfire that worsens with heat; swap-test to confirm.
Common causeParts:ignition coil - 5
Clogged or failing fuel injector
Medium severityA partially clogged injector under-fuels a cylinder enough to misfire, particularly at idle or light load.
Common causeParts:fuel injector - 6
Vacuum leak
Medium severityUnmetered air from a cracked hose or gasket leans a cylinder enough to misfire, usually most noticeable at idle.
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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