P0300 on a 2018 Jeep Grand Cherokee Srt
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 Srt
Ranked from 685 NHTSA owner complaints filed for this vehicle.
- 1
Head gasket failure allowing coolant into a cylinder
High severityMultiple owners describe blown/leaking head gaskets around 80-100k miles that let coolant burn in a cylinder, producing a misfire and low-compression complaint pattern widely reported on this engine.
Reported in 7 NHTSA complaintsParts:head gasket - 2
Worn spark plugs and/or failing ignition coils
Medium severitySeveral owners had plugs and coils replaced for a confirmed misfire/rough-idle condition, in some cases with the problem persisting until the coil was addressed.
Reported in 4 NHTSA complaints - 3
Carbon buildup / EGR system fault fouling intake or valves
Medium severityEGR valve and intake manifold components were found caked with carbon deposits and failed outright on two vehicles, which can foul cylinders enough to misfire.
Reported in 2 NHTSA complaintsParts:EGR valve - 4
Worn camshaft/valvetrain components
Medium severityOne owner reported worn rockers and camshaft with an audible tick at under 90k miles, which can disrupt valve timing/lift enough to cause a misfire.
Reported in 1 NHTSA complaint - 5
Vacuum leak (intake gasket, PCV hose, etc.)
Medium severityUnmetered air entering after the MAF leans out individual cylinders enough to misfire, especially at idle.
Common cause - 6
Clogged or leaking fuel injector(s)
Medium severityA restricted or dribbling injector delivers an uneven air/fuel mixture to that cylinder, causing a repeated misfire.
Common causeParts:fuel injector - 7
Low cylinder compression (worn rings/valves)
High severityOnce compression drops enough in a cylinder it can no longer support combustion, showing up as a misfire that plug/coil swaps won't fix.
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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