P0300 on a 2015 Toyota Corolla
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 2015 Toyota Corolla
Ranked from 218 NHTSA owner complaints filed for this vehicle.
- 1
Vacuum Leaks
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires. Check hoses, intake gaskets, and listen for whistling sounds.
Reported in 2 NHTSA complaints - 2
Low Fuel Pressure / Failing Fuel Pump
High severityInsufficient fuel delivery causes a lean condition and misfires across multiple cylinders. Test fuel pressure at the rail to confirm.
Reported in 1 NHTSA complaint - 3
Excessive Oil Consumption / Low Oil Level
High severitySevere oil burning or low oil levels can affect variable valve timing (VVT) operation or foul spark plugs, leading to misfires and loss of power.
Reported in 1 NHTSA complaint - 4
EVAP System Leak / Cracked Charcoal Canister
Medium severityA cracked EVAP canister or leak can introduce unmetered air or fuel vapors into the intake, disrupting the air/fuel ratio and causing misfires.
Reported in 1 NHTSA complaint - 5
Faulty Ignition Coils
Medium severityIgnition coils are a primary cause of random misfires (P0300). Look for cracked coil boots, carbon tracking, or test with a spark tester/multimeter.
Common cause - 6
Worn or Fouled Spark Plugs
Medium severityWorn electrodes or fouled spark plugs prevent proper ignition, leading to misfires. Inspect plugs for wear, carbon buildup, or incorrect gap.
Common cause - 7
Clogged or Faulty Fuel Injectors
Medium severityRestricted fuel injectors prevent proper atomization and fuel delivery, causing lean misfires. Perform an injector balance test or check for clogged nozzles.
Common cause - 8
Mass Air Flow (MAF) Sensor Issues
Medium severityA dirty or failing MAF sensor incorrectly measures incoming air, causing the ECM to command the wrong fuel amount, leading to random misfires.
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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