P0300 on a 2017 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 2017 Hyundai Sonata
Ranked from 758 NHTSA owner complaints filed for this vehicle.
- 1
Connecting Rod Bearing Failure / Severe Engine Wear
High severityKnown Theta II engine defect where rod bearings fail, causing metallic knocking, excessive oil consumption, and triggering limp mode or misfires. Confirmed by bearing clearance tests, metal debris in oil, or code P1326.
Reported in 8 NHTSA complaints - 2
Spark Plug Fouling / Failure
Medium severityExcessive oil consumption leads to oil entering the combustion chamber, fouling the spark plugs and causing misfires. Look for oil-soaked or carbon-fouled spark plugs.
Reported in 3 NHTSA complaints - 3
Ignition Coil Failure
Medium severityIgnition coils can fail or short out, often exacerbated by oil fouling or general wear, leading to a lack of spark and subsequent misfires. Check for damaged coils or electrical shorts.
Reported in 2 NHTSA complaints - 4
Internal Engine Failure / Piston Ring Deterioration
High severityDeteriorated piston rings allow oil to bypass into the combustion chamber, causing severe oil consumption, loss of compression, and misfires. A compression or leak-down test, along with excessive oil consumption, confirms this.
Reported in 1 NHTSA complaint - 5
Burnt Valve
High severityA burnt or damaged valve causes a loss of compression in the affected cylinder, leading to a misfire. A cylinder leak-down test will confirm exhaust or intake valve leakage.
Reported in 1 NHTSA complaint - 6
Vacuum Leak / Intake Manifold Malfunction
Medium severityUnmetered air entering the engine alters the air/fuel ratio, causing a lean condition and random misfires. Look for cracked hoses, intake manifold issues, or abnormal operating stress on intake components.
Reported in 1 NHTSA complaint - 7
Fuel Injector Failure
Medium severityA clogged or failed fuel injector prevents the proper amount of fuel from entering the cylinder, causing a lean misfire. Confirmed by injector flow testing or checking injector resistance.
Common cause - 8
Low Fuel Pressure
Medium severityA failing fuel pump or clogged fuel filter can cause inadequate fuel delivery to multiple cylinders, resulting in random misfires. Confirmed by testing fuel pressure with a mechanical gauge.
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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