Skip to content
WrenchConnect
Stop driving if it runs poorly
P0300

P0300 on a 2021 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 2021 GMC Sierra 1500

Ranked from 521 NHTSA owner complaints filed for this vehicle.

Sort by:
  1. 1

    Failed/collapsed valve lifters (AFM lifter failure - documented GM 5.3L/6.2L defect)

    High severity

    Multiple owners had dealers confirm rough running and misfire codes (including explicit P0300/P0302) traced to collapsed lifters and camshaft damage tied to the engine's Active Fuel Management system, a widely reported defect on these engines.

    Reported in 6 NHTSA complaints
    Parts:valve lifterscamshaft
  2. 2

    Rod bearing/crankshaft failure with metal debris in oil (L87/L84 recall-related)

    High severity

    Several owners describe engine shaking/knocking preceding a dealer-confirmed finding of metal debris and bearing damage requiring full engine replacement, consistent with the ongoing GM L87/L84 bearing recall on this engine family.

    Reported in 3 NHTSA complaints
    Parts:connecting rod bearings
  3. 3

    Worn or fouled spark plugs

    Low severity

    One owner's cylinder-1 misfire was first addressed by replacing all spark plugs before the underlying lifter issue was found; plug wear/fouling remains a common contributor to random misfire.

    Reported in 1 NHTSA complaint
    Parts:spark plugs
  4. 4

    Failing ignition coil

    Medium severity

    The same owner had the cylinder-1 ignition coil replaced for a persistent misfire before the lifter failure was diagnosed.

    Reported in 1 NHTSA complaint
    Parts:ignition coil
  5. 5

    Low compression from worn valve or piston components

    High severity

    One owner's mechanic found a misfire with zero compression on a single cylinder, pointing to valve or ring damage rather than an electrical fault.

    Reported in 1 NHTSA complaint
  6. 6

    Vacuum leaks

    Low severity

    A cracked intake hose or gasket lets in unmetered air, leaning out and misfiring one or more cylinders, especially at idle.

    Common cause
  7. 7

    Clogged or leaking fuel injectors

    Medium severity

    A partially blocked or dribbling injector under- or over-fuels a cylinder, producing an isolated or random misfire.

    Common cause
    Parts:fuel injector
  8. 8

    EGR valve fault

    Low severity

    A stuck-open EGR valve over-dilutes the intake charge at idle or light load, producing a lean-related misfire.

    Common cause
    Parts:EGR valve

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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

1

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.

2

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.

3

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.

4

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.

5

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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Car Runs RoughFlashing Check Engine Light