P0300 on a 2020 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 2020 GMC Sierra 1500

Ranked from 438 NHTSA owner complaints filed for this vehicle.

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1
AFM/DFM lifter or camshaft collapse (GM 5.3L/6.2L L87 known defect) causing cylinder misfire
High severity
This is by far the dominant pattern in this batch: repeated documented lifter, camshaft, and rod-bearing failures on this exact engine/model (subject of NHTSA EA25007/PE25001), and one owner directly ties an active-fuel-management cylinder deactivation event to misfire codes appearing (odiNumber 11728804: 'dod/afm turned on and disabled a cylinder and now I'm just having codes pop up for cylinder misfires'). Rough idle, knocking, and shaking precede many of these failures.
Reported in 22 NHTSA complaints
Parts:lifters camshaft
2
Failing or clogged fuel injector
Medium severity
This batch documents an explicit fuel injector failure alongside an engine knock and O2 sensor control module failure on this exact vehicle (odiNumber 11729751).
Reported in 1 NHTSA complaint
Parts:fuel injector
3
Worn spark plugs
Low severity
A basic, common wear item that produces random misfire as gap widens or the electrode fouls.
Common cause
Parts:spark plugs
4
Failing ignition coil(s)
Medium severity
Intermittent random misfire, often worse under load or in damp conditions.
Common cause
Parts:ignition coil
5
Vacuum or intake leak
Medium severity
Unmetered air leans the mixture and causes rough idle/misfire.
Common cause
6
Carbon-fouled intake valves (direct-injection buildup)
Medium severity
Direct-injection engines don't self-clean intake valves, and buildup over time can disrupt airflow and combustion.
Common cause
7
EGR valve stuck open
Low severity
Excess exhaust gas recirculated at idle can cause rough running and misfire.
Common cause
Parts: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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Matching symptoms

Car Runs Rough Flashing Check Engine Light