P0300 on a 2018 Ford Focus
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 Ford Focus
Ranked from 823 NHTSA owner complaints filed for this vehicle.
- 1
Coolant intrusion into the cylinders (post-recall contamination or head gasket failure) fouling spark plugs
High severityComplaints describe coolant found on the spark plugs and an engine that seized after an oil-pump-belt recall repair, plus a separate report of misfiring, white smoke, and suspected head gasket failure with low coolant.
Reported in 2 NHTSA complaints - 2
Worn spark plugs
Medium severityA complaint documents misfire codes P0301/P0304 traced to completely worn spark plugs at high mileage, resolved by replacement.
Reported in 1 NHTSA complaintParts:spark plugs - 3
Failing ignition coil
Medium severityA weak or intermittently arcing coil produces a repeatable single- or multi-cylinder misfire; swap-test to a known-good cylinder.
Common causeParts:ignition coil - 4
Vacuum leaks
Medium severityUnmetered air causes lean misfire, especially at idle; smoke-test intake boots and PCV hoses.
Common cause - 5
Clogged or leaking fuel injectors
Medium severityA partially clogged or dribbling injector produces a lean or wet-fouled cylinder; check injector balance/flow rates.
Common causeParts:fuel injector - 6
Low compression from worn rings or valves
High severityConfirm with a compression or leak-down test on any consistently misfiring cylinder.
Common cause - 7
Carbon-fouled valves
Low severityCarbon buildup on intake valves disrupts airflow/combustion on affected cylinders; inspect with a borescope.
Common cause - 8
EGR faults
Low severityA stuck-open EGR valve dilutes the intake charge and can trigger a rough idle/misfire.
Common cause
Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.
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.
Rather have a pro handle it? Find a vetted local mechanic.