P0300 on a 2015 GMC Yukon Denali
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 2015 GMC Yukon Denali
Ranked from 606 NHTSA owner complaints filed for this vehicle.
- 1
Active Fuel Management (AFM) lifter collapse
High severityA large, consistent cluster of owners on this exact truck report rough idle, shaking, ticking, and confirmed misfires traced by dealers/mechanics to collapsed AFM lifters (one explicitly to lifter #7), a well-known failure mode on GM's 5.3L/6.2L AFM V8s.
Reported in 8 NHTSA complaints - 2
Worn or fouled spark plugs
Low severityConfirmed as a first-line suspect by an owner who replaced plugs/wires/coil for a diagnosed P0303 cylinder-3 misfire (though it did not fully resolve the issue here).
Reported in 1 NHTSA complaintParts:spark plugs - 3
Failing ignition coil
Medium severitySame owner also replaced the ignition coil while chasing a confirmed cylinder misfire; still a standard early check even when AFM lifters turn out to be the root cause.
Reported in 1 NHTSA complaintParts:ignition coil - 4
Clogged or leaking fuel injector
Medium severityUneven fuel delivery to one cylinder produces a repeating misfire on that cylinder.
Common causeParts:fuel injector - 5
Vacuum or intake leak
Medium severityUnmetered air leans out one or more cylinders enough to misfire, especially at idle.
Common cause - 6
Carbon-fouled intake valves
Low severityBuildup on valve backs can disrupt airflow/sealing into individual cylinders over time.
Common cause - 7
EGR valve fault
Low severityA stuck-open EGR valve dilutes the intake charge and disrupts combustion at idle/low load.
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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