P0300 on a 2019 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 2019 Honda Civic Si
Ranked from 356 NHTSA owner complaints filed for this vehicle.
- 1
Water intrusion through the hood scoop onto a cylinder's coil/spark plug (commonly cylinder 4)
High severityA recurring, owner-documented design issue on this platform: rainwater enters via the hood scoop and drips onto a cylinder, rusting the ignition coil and spark plug and setting a misfire/check-engine light.
Reported in 2 NHTSA complaints - 2
Head gasket failure allowing coolant into a cylinder
High severityOne owner reported classic head-gasket symptoms at ~67,000 miles, and another documented cylinders "flooded with coolant" alongside a non-working heater; coolant fouling a cylinder causes a severe misfire and low compression.
Reported in 2 NHTSA complaintsParts:head gasket - 3
Worn or fouled spark plugs
Low severityThe most common overall cause of a random misfire code; check gap, wear, and fouling.
Common causeParts:spark plugs - 4
Failing ignition coil(s), unrelated to water intrusion
Medium severityCoils fail from age/heat cycling on their own as well; swap coils between cylinders to confirm a cylinder-specific misfire follows the coil.
Common causeParts:ignition coil - 5
Clogged or leaking fuel injector(s)
Medium severityA dirty or leaking injector disrupts the air/fuel ratio on that cylinder.
Common causeParts:fuel injector - 6
Vacuum leak
Medium severityUnmetered air entering after the MAF sensor causes a lean misfire, especially noticeable at idle.
Common cause - 7
Low compression from worn piston rings or valves
High severityGeneral mechanical wear independent of the head-gasket cases above will also present as a misfire; a compression/leak-down test differentiates the two.
Common cause - 8
EGR valve/passage fault
Low severityA stuck-open EGR valve or clogged passage disrupts combustion at idle and light load.
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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