P0300 on a 2021 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 2021 Jeep Grand Cherokee
Ranked from 397 NHTSA owner complaints filed for this vehicle.
- 1
Oil cooler adapter/seal failure causing coolant-oil intermixing (Pentastar 3.6L V6 known issue)
High severityThis batch has direct evidence: one complaint reports antifreeze and oil intermixing that triggered a literal single-cylinder misfire code and recommends replacing the oil cooler filter adapter (odiNumber 11575153); two related complaints describe recurring engine misfires and overheating consistent with known manufacturer TSBs on this engine, where ignition coils and spark plugs were already replaced without resolving it (odiNumbers 11727024, 11728882).
Reported in 3 NHTSA complaints - 2
Failing ignition coil(s)
Medium severityThis batch documents ignition coil replacement attempted (unsuccessfully, pointing to the underlying coolant-intrusion cause) on a misfiring vehicle in this fleet (odiNumber 11727024).
Reported in 1 NHTSA complaintParts:ignition coil - 3
Worn or fouled spark plugs
Low severityAlso replaced in the same unresolved repair attempt referenced above; fouled plugs are a standard misfire cause even when the root cause is coolant contamination.
Reported in 1 NHTSA complaintParts:spark plugs - 4
Vacuum or intake manifold leak
Medium severityUnmetered air leans the mixture and causes rough idle/misfire; the Pentastar V6 intake gasket is a separately known weak point.
Common cause - 5
Clogged or leaking fuel injector
Medium severityUneven fuel delivery between cylinders produces a random-cylinder misfire pattern.
Common causeParts:fuel injector - 6
Low compression from worn rings/valves
Medium severityReduced cylinder sealing produces a chronic misfire on the affected cylinder(s).
Common cause - 7
EGR valve stuck open
Low severityExcess exhaust gas dumped into the intake at idle can cause rough running and misfire.
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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