P0300 on a 2019 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 2019 Hyundai Sonata
Ranked from 154 NHTSA owner complaints filed for this vehicle.
- 1
Excessive Oil Consumption / Piston Ring Failure
High severityExcessive oil consumption leading to low oil levels, which can cause misfires (P0300) due to oil entering the combustion chamber and fouling spark plugs, or internal engine damage. Look for low oil levels, fouled spark plugs, and failed oil consumption tests.
Reported in 16 NHTSA complaints - 2
Internal Engine Failure / Connecting Rod Bearing Damage
High severityKnown Theta II engine issues where metal shavings in oil or bearing clearance issues lead to catastrophic engine failure or seizure, causing misfires and stalling. Check for knocking sounds, metal in oil, and failed bearing clearance tests.
Reported in 5 NHTSA complaints - 3
Faulty Spark Plugs
Low severityWorn or oil-fouled spark plugs fail to ignite the air/fuel mixture properly, leading to random misfires. Inspect spark plugs for wear, carbon buildup, or oil fouling.
Common cause - 4
Faulty Ignition Coils
Medium severityIgnition coils that are failing or have degraded boots can cause weak or no spark, resulting in misfires. Test coil resistance and check for arcing or physical damage.
Common cause - 5
Vacuum Leaks
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires across multiple cylinders. Check intake manifold gaskets, vacuum hoses, and PCV system for leaks.
Common cause - 6
Clogged or Failing Fuel Injectors
Medium severityRestricted fuel flow or improper spray patterns from dirty or failing injectors can cause lean misfires. Perform a fuel injector balance test and check fuel pressure.
Common cause
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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