P0300 on a 2018 Honda Civic

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 Civic

Ranked from 598 NHTSA owner complaints filed for this vehicle.

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1
Ignition coil failure from water intrusion
High severity
Two owners traced misfires directly to water leaking past the ignition coil seals (from the hood scoop/cowl drain area) into the spark plug tubes, rusting the coils -- a well-documented issue on this 1.5T Civic generation.
Reported in 2 NHTSA complaints
Parts:ignition coil spark plug
2
Leaking/faulty fuel injector causing fuel dilution and misfire
High severity
An owner's mechanic diagnosed misfire codes P0302/P0303 and confirmed fuel-diluted oil; replacing the fuel injectors resolved the misfire and restored fuel economy, consistent with the 1.5T's known injector/oil-dilution issue.
Reported in 1 NHTSA complaint
Parts:fuel injector
3
Low compression / internal engine damage
High severity
One owner's Civic needed a full engine replacement after a shop diagnosed a cylinder misfire, consistent with the 1.5T's known bottom-end wear (rod bearing) issues tied to fuel-diluted oil.
Reported in 1 NHTSA complaint
4
Worn spark plugs
Low severity
Worn or improperly-gapped plugs are the most common misfire cause; inspect for wear, fouling, or incorrect gap.
Common cause
Parts:spark plug
5
Vacuum or intake leak
Medium severity
A cracked hose or gasket admits unmetered air, leaning out and misfiring one or more cylinders.
Common cause
6
Carbon-fouled intake valves
Low severity
Direct-injection engines like this 1.5T are prone to carbon buildup on the intake valve backs, disrupting airflow/atomization enough to misfire.
Common cause
7
EGR valve fault
Low severity
A stuck-open EGR valve dilutes the intake charge excessively at idle/low load, causing a rough-running misfire.
Common cause
Parts: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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Matching symptoms

Car Runs Rough Flashing Check Engine Light