P0300 on a 2021 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 2021 Honda Accord
Ranked from 217 NHTSA owner complaints filed for this vehicle.
- 1
Head gasket failure allowing coolant into a cylinder (10th-gen Accord 1.5T/2.0T design defect)
High severityA narrow coolant passage in the cylinder head is a known weak point on this generation; coolant seeping into a cylinder washes out compression and fouls plugs, producing white exhaust smoke, coolant loss, and a random/specific-cylinder misfire. This is by far the most-cited failure in this batch.
Reported in 20 NHTSA complaintsParts:head gasket - 2
Spark plug/ignition coil fouling from coolant intrusion (often misdiagnosed before the head gasket is found)
High severityCoolant reaching the combustion chamber fouls the plug and can hydro-lock a coil; several owners had plugs and coils replaced first (temporary fix) before the head gasket was ultimately diagnosed.
Reported in 4 NHTSA complaints - 3
Turbocharger underboost or fuel injector fault (1.5T/2.0T turbo fuel-delivery issues)
Medium severityOwners report combined turbo/fuel-injector failures and a P0299 underboost code alongside misfire, and dealers diagnosing faulty injectors on hard-idle/misfire complaints on this platform.
Reported in 4 NHTSA complaints - 4
Stretched timing chain (1.5T) causing cam/crank timing drift
Medium severityA known wear point on the 1.5T; as the chain stretches, valve timing drifts out of spec and can produce a misfire alongside rattling noise on cold start.
Reported in 1 NHTSA complaintParts:timing chain kit - 5
Vacuum or intake leak
Low severityUnmetered air entering after the MAF leans one or more cylinders, producing a misfire especially at idle; check intake boots and gaskets with smoke.
Common cause - 6
Carbon-fouled intake valves
Low severityDirect-injection engines build up carbon on the backs of the intake valves, disrupting airflow and triggering misfire over time; a walnut-blast cleaning restores flow.
Common cause - 7
EGR valve fault
Low severityA stuck-open EGR valve dilutes the intake charge excessively at idle/light load, causing a rough idle and misfire.
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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