P0300 on a 2018 Ford Focus Rs
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 Rs
Ranked from 743 NHTSA owner complaints filed for this vehicle.
- 1
Oil pump drive belt failure causing oil starvation and internal engine damage (recall 23V905000/23S64)
High severityA cluster of owners had the oil pump drive belt snap, starving the engine of oil pressure; the resulting bearing and valvetrain damage produces rough running and misfire-type symptoms, with one owner explicitly describing the engine 'misfiring significantly' before stalling.
Reported in 6 NHTSA complaints - 2
Coolant intrusion into cylinders from head gasket/cylinder head failure
High severityIndependent mechanics and dealers diagnosed head gasket failure and a cracked cylinder head letting coolant leak directly into the cylinders; coolant fouling a cylinder is a direct, well-documented misfire cause.
Reported in 3 NHTSA complaints - 3
Worn spark plugs
Low severityWorn or fouled plugs produce a weak spark that fails to reliably ignite the mixture, especially under load.
Common causeParts:spark plugs - 4
Failing ignition coils
Medium severityA weak or intermittently-failing coil drops spark energy to one cylinder, producing a repeatable single-cylinder misfire.
Common causeParts:ignition coil - 5
Clogged or leaking fuel injectors
Medium severityAn injector delivering too little or too much fuel to a cylinder disrupts the air-fuel ratio enough to cause a misfire on that cylinder.
Common causeParts:fuel injectors - 6
Vacuum leaks
Medium severityUnmetered air entering through a cracked hose or gasket leans out the mixture enough to cause an intermittent misfire.
Common cause - 7
Low cylinder compression from worn rings or valves
High severityReduced compression on a cylinder prevents proper combustion and registers as a misfire on that cylinder.
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.