P0300 on a 2018 Ram 1500
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 Ram 1500
Ranked from 644 NHTSA owner complaints filed for this vehicle.
- 1
Connecting rod bearing failure / spun rod bearing (3.0L EcoDiesel bottom-end defect)
High severityNumerous owners report sudden knocking followed by zero oil pressure and catastrophic bottom-end failure (thrown rod, spun bearing) on this EcoDiesel; as a bearing degrades the resulting mechanical roughness and eventual cylinder damage reads as a misfire before full failure.
Reported in 6 NHTSA complaintsParts:connecting rod bearing - 2
EGR cooler failure allowing coolant intrusion into the intake/cylinders (diesel)
High severityRepeat EGR cooler failures -- including after the recall repair (19V757000) -- let coolant leak into the intake manifold and cylinders, fouling combustion and in several cases igniting under-hood fires.
Reported in 4 NHTSA complaintsParts:EGR cooler - 3
Crankshaft position sensor "tone wheel" delamination (EcoDiesel)
High severityA well-documented EcoDiesel defect where the crank tone wheel delaminates, corrupting the crank position signal the PCM needs for cylinder timing; owners report stalling, no-restart, and recall campaigns (20V475000/23V411000) tied to this exact part.
Reported in 3 NHTSA complaints - 4
High-pressure fuel pump/fuel rail failure contaminating injectors (recall Z46)
Medium severityOwners report high-pressure fuel pump/rail failures (NHTSA recall Z46) contaminating the fuel system and damaging injectors and the PCM, producing persistent misfire/rough-running complaints even after the recall repair.
Reported in 2 NHTSA complaints - 5
MDS (Multi-Displacement System) lifter/camshaft roller failure (5.7L Hemi)
High severityOne owner's cam roller failed on the Hemi's variable-displacement cylinder bank, dropping a valve into the cylinder and destroying the piston; MDS lifter/cam-lobe wear is a widely reported Hemi misfire and rod-knock cause.
Reported in 1 NHTSA complaint - 6
Worn spark plugs (gas Hemi variants)
Low severityWorn or fouled plugs on 5.7L Hemi-equipped trucks produce a weak spark that fails to reliably ignite the mixture.
Common causeParts:spark plugs - 7
Vacuum or intake/turbo boost leaks
Medium severityUnmetered air entering through a cracked hose or gasket leans out the mixture enough to cause an intermittent misfire.
Common cause - 8
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
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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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