P0300 on a 2018 Honda Civic Si
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 Si
Ranked from 598 NHTSA owner complaints filed for this vehicle.
- 1
Fuel pump module failure (NHTSA recall 23V-858)
High severityThe plastic impeller in the low-pressure fuel pump can swell and restrict flow, causing fuel starvation, stalling, and misfire-type symptoms — heavily reported on this vehicle via an open safety recall.
Reported in 5 NHTSA complaintsParts:fuel pump - 2
Ignition coil pack water intrusion
High severityA failed coil-pack seal lets water from the hood-scoop area reach the plug well (confirmed on cylinder #4 with DTC P0304 and visible rust), shorting the coil and causing a misfire.
Reported in 2 NHTSA complaints - 3
Engine oil dilution from unburned fuel (1.5T known issue)
Medium severityRaw fuel washing past the rings into the crankcase — a documented issue on this 1.5L turbo, worse on short/cold trips — thins the oil and fuel-fouls spark plugs, producing a rough-running misfire.
Reported in 2 NHTSA complaintsParts:spark plug - 4
Leaking or faulty fuel injector
Medium severityAn owner with confirmed misfire codes (P0302/P0303) also reported a strong gasoline smell, consistent with an injector not sealing correctly.
Reported in 1 NHTSA complaintParts:fuel injector - 5
Worn spark plugs
Low severityPlugs worn past gap spec or fouled by oil/fuel produce a weak spark and a random or cylinder-specific misfire; inspect gap and electrode condition.
Common causeParts:spark plug - 6
Vacuum or intake leak
Medium severityUnmetered air leaning out a cylinder's mixture can present as a misfire; smoke-test the intake and vacuum lines.
Common causeParts:intake gasket - 7
Low compression from carbon buildup on rings/valves
Medium severityDirect-injection engines are prone to intake-valve and ring carbon deposits that reduce sealing; confirm with a compression test.
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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