P0300 on a 2018 Jeep Grand Cherokee
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 Jeep Grand Cherokee
Ranked from 841 NHTSA owner complaints filed for this vehicle.
- 1
Camshaft lobe wear / collapsed lifters and rocker arms (3.6L Pentastar valvetrain failure)
High severityThis dataset shows a strong, repeated pattern of worn camshaft lobes and failed lifters/rocker arms causing progressive multi-cylinder misfires on this exact engine; a dealer confirmed it as a 'typical' failure mode for the platform.
Reported in 11 NHTSA complaints - 2
Ignition coil failure
Medium severityMultiple owners had coils replaced (often alongside plugs) to chase a misfire, though several report the misfire returning, pointing to it as a contributing rather than sole cause.
Reported in 7 NHTSA complaintsParts:ignition coil - 3
Worn or fouled spark plugs
Low severitySpark plugs were replaced alongside coils in most misfire repair attempts on this vehicle; check gap and electrode wear against spec.
Reported in 7 NHTSA complaintsParts:spark plugs - 4
Head gasket failure / warped cylinder head letting coolant into a cylinder
High severitySeveral complaints describe a specific-cylinder misfire paired with white exhaust smoke or coolant loss, confirmed by a dealer as a blown head gasket or warped head.
Reported in 5 NHTSA complaints - 5
Failing or clogged fuel injector
Medium severityOne owner replaced injectors while chasing a persistent misfire, consistent with an injector delivering uneven fuel to a cylinder.
Reported in 1 NHTSA complaintParts:fuel injector - 6
Vacuum or intake air leak
Medium severityAn unmetered air leak causes lean misfire, especially at idle; check intake boots, PCV hoses, and manifold gaskets with smoke.
Common cause - 7
Carbon-fouled intake valves (direct-injection carbon buildup)
Low severityGDI engines lack fuel washing the intake valves, so carbon buildup can disrupt airflow/sealing and cause a rough-running misfire.
Common cause - 8
EGR valve/passage fault
Low severityExcess or uncontrolled EGR flow can cause a lean misfire at idle or light load; check EGR valve operation and passages for carbon blockage.
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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