P0300 on a 2018 Honda Accord
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 2018 Honda Accord
Ranked from 1,812 NHTSA owner complaints filed for this vehicle.
- 1
Head gasket failure / coolant intrusion into cylinders (1.5L/2.0L turbo engine defect)
High severityOverwhelmingly the dominant pattern in this vehicle's complaints: the head gasket fails and lets coolant into one or more cylinders, producing cold-start rough running and misfire codes (P0300-P0303); confirmed repeatedly by dealers via borescope/pressure test and the subject of active class-action litigation (Bissell v. American Honda Motor Co.).
Reported in 24 NHTSA complaints - 2
Fuel injector fouling or failure
Medium severityMultiple owners report fuel injectors failing or needing replacement at the same time as the head gasket, likely from coolant/combustion byproducts fouling the injector tips; check injector spray pattern and balance.
Reported in 7 NHTSA complaintsParts:fuel injector - 3
Worn spark plugs
Low severityFouled or worn-out plugs fail to reliably ignite the mixture; inspect gap/electrode condition (one owner was initially, incorrectly, told this was the cause before a proper diagnosis found the head gasket).
Common causeParts:spark plugs - 4
Failing ignition coil(s)
Medium severityA weak coil-on-plug drops one cylinder consistently or intermittently under load; swap/test coils cylinder-to-cylinder to isolate.
Common causeParts:ignition coil - 5
Vacuum leak (intake gasket or hose)
Medium severityUnmetered air causes a lean misfire, usually worse at idle; smoke-test the intake tract to find the leak point.
Common cause - 6
Carbon-fouled intake valves
Low severityThis direct-injection turbo engine has no fuel washing the intake valves, so carbon buildup can disrupt airflow/combustion; a borescope check confirms it.
Common cause - 7
EGR valve fault
Low severityA stuck-open EGR valve over-dilutes the intake charge at idle/low load and can trigger a misfire; check for carbon sticking.
Common causeParts:EGR valve
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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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