P0300 on a 2010 Chevrolet Cobalt
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 2010 Chevrolet Cobalt
Ranked from 582 NHTSA owner complaints filed for this vehicle.
- 1
Fuel delivery fault from corroded/leaking fuel lines
High severityFour owners describe rusted or leaking fuel lines near the rear wheel well causing a fuel odor, dripping gas, and hard/failed starts and stalling -- a documented pattern on this generation Cobalt that can produce lean, misfire-prone running as pressure drops.
Reported in 4 NHTSA complaints - 2
Worn spark plugs
Low severityOne owner's rough, skipping idle was addressed alongside a spark plug replacement, consistent with worn plugs being a contributing cause of the misfire.
Reported in 1 NHTSA complaintParts:spark plugs - 3
Timing chain wear/stretch
Medium severityOne owner's shaking, noisy engine was diagnosed and repaired as a failed timing chain; a stretched or worn chain skews valve timing and can present as a misfire.
Reported in 1 NHTSA complaintParts:timing chain - 4
Failing ignition coil
Medium severityA weak or intermittently-failing coil module produces a repeatable misfire on the affected cylinder(s).
Common causeParts:ignition coil - 5
Clogged or leaking fuel injector
Medium severityAn injector delivering an incorrect fuel charge to a cylinder produces a repeatable misfire on that cylinder.
Common causeParts:fuel injector - 6
Vacuum leak
Medium severityUnmetered air entering the intake leans out one or more cylinders enough to misfire, most noticeable at idle.
Common cause - 7
Low compression from worn rings or valves
High severityHigh-mileage cylinder wear reduces sealing enough that a cylinder can't reliably combust, producing a persistent 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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