P0300 on a 2011 Toyota Prius
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 2011 Toyota Prius
Ranked from 516 NHTSA owner complaints filed for this vehicle.
- 1
Head gasket leak / coolant intrusion into a cylinder
High severityA well-documented Gen-3 Prius 1.8L defect - owners describe rattling/shaking traced to a leaking head gasket with heavy antifreeze use and white exhaust smoke, and one case explicitly diagnosed the resulting rough-running as a misfire from a leaking head gasket.
Reported in 3 NHTSA complaintsParts:cylinder head gasket - 2
Excessive oil consumption / worn piston rings (low compression)
High severityMultiple owners reported burning roughly a quart of oil every 1,000 miles with no external leak, indicating internal wear that lowers compression and promotes misfire.
Reported in 3 NHTSA complaints - 3
Worn spark plugs or failing ignition coils
Medium severityOne owner's shaking/knocking/smoking episode began right after spark plug and coil service on cylinders 1 and 2, pointing to ignition-side wear as a contributor.
Reported in 1 NHTSA complaint - 4
Sticky or faulty EGR valve
Medium severityAn owner's check-engine/stall complaint specifically called out a sticky EGR valve needing service, which can upset the mixture enough to misfire.
Reported in 1 NHTSA complaintParts:EGR valve - 5
Vacuum leak / faulty intake vacuum control valve
Medium severityOne owner's stall was resolved by replacing a vacuum valve assembly and reprogramming the PCM, consistent with a vacuum leak disrupting the mixture.
Reported in 1 NHTSA complaintParts:vacuum control valve - 6
Clogged or leaking fuel injectors
Medium severityUneven fuel delivery to a cylinder is a classic cause of random/multiple misfire.
Common causeParts:fuel injector - 7
Carbon-fouled valves
Medium severityCarbon buildup on the intake valves disrupts sealing/airflow enough to cause an intermittent misfire, especially at higher mileage.
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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