P0300 on a 2015 GMC Sierra 1500
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 Sierra 1500
Ranked from 810 NHTSA owner complaints filed for this vehicle.
- 1
Active Fuel Management (AFM) / Dynamic Fuel Management (DFM) Lifter Failure
High severityThe AFM/DFM lifters collapse, leading to bent pushrods, camshaft damage, and severe engine misfires. This is confirmed by a ticking/knocking noise, loss of power, and physical inspection of the valvetrain.
Reported in 14 NHTSA complaints - 2
Worn or Damaged Spark Plugs
Low severitySpark plugs wear out over time, causing weak or inconsistent spark which leads to random misfires. Confirmed by inspecting the spark plug electrodes for wear, fouling, or incorrect gap.
Common cause - 3
Faulty Ignition Coils
Medium severityIgnition coils can fail or degrade, failing to provide enough voltage to the spark plugs. Confirmed by testing coil resistance, swapping coils to see if the misfire moves, or checking for spark.
Common cause - 4
Vacuum Leaks
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires across multiple cylinders. Confirmed by a smoke test or listening for hissing noises around the intake.
Common cause - 5
Clogged or Faulty Fuel Injectors
Medium severityFuel injectors may become clogged or fail electrically, preventing the correct amount of fuel from entering the cylinder. Confirmed by fuel pressure tests, injector balance tests, or inspecting spray patterns.
Common cause - 6
Low Fuel Pressure
Medium severityA failing fuel pump or clogged fuel filter can cause low fuel pressure, starving the engine of fuel and causing random misfires. Confirmed by attaching a mechanical fuel pressure gauge to the fuel rail.
Common cause
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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