P0300 on a 2019 Toyota Highlander

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 Toyota Highlander

Ranked from 467 NHTSA owner complaints filed for this vehicle.

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1
Worn spark plugs or failing ignition coil(s)
Medium severity
This vehicle had a confirmed multi-cylinder misfire (cylinders 1, 3, 5, 6) on scan tool, and a mechanic replaced all spark plugs during a related repair — ignition components are the most common root cause of exactly this random-cylinder pattern.
Reported in 3 NHTSA complaints
Parts:spark plugs ignition coil
2
Cylinder wall scoring / internal engine damage causing loss of compression
High severity
One owner's Highlander began blowing white smoke and misfiring; the dealer found cylinder-4 wall scoring, metal particles in the oil, and the cylinder burning both oil and coolant, requiring an engine replacement/rebuild.
Reported in 1 NHTSA complaint
3
Head gasket coolant leak (known 3.5L V6 issue)
Medium severity
Owner research cites a recurring head-gasket/coolant-port corrosion issue on this V6 engine family; while typically an external leak, prolonged coolant loss and overheating can progress to internal leakage and misfire.
Reported in 1 NHTSA complaint
Parts:head gasket
4
Low compression from worn piston rings
High severity
General wear reduces cylinder sealing enough to cause a weak, repeatable misfire, especially at higher mileage.
Common cause
5
Clogged or leaking fuel injector
Medium severity
A fouled or leaking injector disrupts the spray pattern to one cylinder, producing a repeatable misfire.
Common cause
Parts:fuel injector
6
Vacuum or intake air leak
Medium severity
Unmetered air entering after the MAF leans the mixture enough to cause rough idle and random misfire.
Common cause
7
Carbon-fouled intake valves
Low severity
Direct-injection engines don't wash the intake valves with fuel, letting carbon build up and disrupt airflow until cleaned.
Common cause
8
EGR valve stuck open or clogged
Low severity
Excess or blocked exhaust gas recirculation upsets the air-fuel charge at idle and low load.
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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Matching symptoms

Car Runs Rough Flashing Check Engine Light