GMC-specific code — factory diagnostic data| DTC Data Sheet | |
| Code | P0300 |
| Vehicle | GMC Sierra 1500 (2007-2019) |
| Engine | 4.8L / 5.3L / 6.2L V8 (LS-family, Gen-IV, AFM) |
| System | IGNITION SYSTEM |
| Fault type | Performance |
| Official meaning | Random / Multiple Cylinder Misfire Detected |
Definition source: GMC factory description. Diagnostic guidance is based on factory-defined fault logic for this code.
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Looking for the cross-vehicle definition? Read the generic P0300 article for the SAE-defined fault logic that applies to all manufacturers.
P0300 Quick Answer
The Sierra 1500 shares the AFM-equipped 5.3L (LC9, L83) and 6.2L (L86) V8 engines with the Silverado, so P0300 has the same dominant causes: collapsed AFM lifters on cylinders 1/4/6/7, oil-fouled spark plugs from AFM oil consumption, and individual coil failures. Diagnostic order is the same; the fix list is the same; the cost band is the same.
What Does P0300 Mean on a GMC Sierra 1500?
The GMC Sierra 1500 (2007-2019) stores P0300 when the PCM detects the condition described above. This guide focuses on the 4.8L / 5.3L / 6.2L V8 (LS-family, Gen-IV, AFM) — the most common configuration on this platform. Diagnostic priorities and likely root causes differ from the generic SAE definition because of platform-specific failure patterns documented below.
Symptoms
- Rough idle (especially cold start)
- Top-end tick on AFM trucks
- Hesitation under acceleration
- Flashing CEL under load = stop driving
- Possible P030x specific-cylinder codes
- Fuel economy drop
Common Causes (Most Likely on This Model First)
The Sierra 5.3L V8 misfire failure pattern matches its Silverado twin. Order of likelihood:
- Collapsed AFM lifter on cylinder 1, 4, 6, or 7. These are the AFM-deactivation cylinders. The lifter loses oil pressure and stops following the cam profile. Tell: ticking on cold start plus P0300 plus a specific P030x for the affected cylinder.
- Stuck VLOM solenoid. Controls oil flow to AFM lifters; a stuck solenoid leaves a lifter in the wrong state.
- Oil-fouled spark plugs. AFM oil consumption contaminates the plugs on deactivated cylinders. Replace plugs at 60k-80k on AFM Sierras (not the OE 100k).
- Failing ignition coil. COP coils fail one at a time; usually a specific-cylinder P030x appears first.
- Vacuum leak at intake gasket. Gen-IV LS intake gaskets shrink at the front and rear corners.
- Failing fuel pump (older trucks). Check fuel pressure under load.
Diagnostic Approach
- Read freeze frame — note RPM and load at fault. Idle-only = AFM / ignition; load-only = fuel pressure / injector.
- Pull misfire counters per cylinder. Pattern on 1/4/6/7 = AFM. Single-cylinder dominance = coil/plug/injector for that cylinder.
- LTFT both banks > +10% = vacuum leak.
- Listen for top-end tick on AFM-equipped engines. Collapsed lifter is unmistakable.
- Pull plugs on misfiring cylinders — oily wet residue confirms AFM oil consumption.
- Swap a suspected coil to a known-good cylinder. If misfire follows, replace it.
- Smoke-test intake if both banks lean.
Possible Fixes
| Fix | When |
|---|---|
| AFM lifter replacement / AFM-delete | Tick on cold start + misfire on 1/4/6/7 |
| Spark plugs (set of 8) | Plugs > 60k miles on AFM Sierra |
| Single ignition coil | Misfire follows when swapped |
| Intake manifold gasket | Both-bank LTFT > +10% |
| VLOM solenoid | AFM-related misfire with no lifter tick |
Can I Still Drive With P0300?
Steady CEL: drive to a shop within a week. Flashing CEL: don’t drive — severe misfire damages the catalyst quickly. Extended driving with a collapsed AFM lifter can score the camshaft and turn a $1,400 job into a $4,000+ job.
How Serious Is This Code?
Moderate to high. Always investigate whether AFM lifter failure is the cause — if it is, the longer it runs, the more expensive the fix becomes.
Repair Costs
| Repair | Estimated cost (parts + labor) |
|---|---|
| Spark plug replacement (8 OE) | $180 – $340 |
| Single ignition coil | $95 – $180 |
| All 8 coils + plugs | $520 – $880 |
| AFM lifter service (one bank) | $1,400 – $2,400 |
| AFM-delete kit | $1,800 – $3,500 |
| Intake manifold gaskets | $420 – $780 |
FAQ
Why does my GMC Sierra have P0300?
On the AFM-equipped 5.3L (and 6.2L) Sierra, the leading cause is a collapsed AFM lifter on cylinders 1, 4, 6, or 7. Oil-fouled spark plugs from the same AFM oil-consumption pattern are second. Individual ignition coil failures and vacuum leaks are third and fourth.
Should I delete AFM on my Sierra 1500?
AFM-delete is the standard fix for the recurring lifter problem on 2007-2019 Sierras with AFM-equipped engines. It costs $1,800-$3,500 done correctly (cam swap, non-AFM lifters, PCM tune) and removes the failure mode permanently. Worth doing on out-of-warranty trucks past 80k miles.
Can a Sierra run with a flashing CEL?
It can, but it shouldn’t. A flashing Check Engine Light during a misfire means raw fuel is reaching the catalytic converter and overheating the substrate. The cat can fail in 15-30 minutes of continued driving. Pull over and arrange a tow.
How often should I change spark plugs on a 5.3L Sierra with AFM?
GM specs 100,000 miles, but on AFM-equipped engines the plugs foul faster from oil consumption. Replace at 60,000-80,000 miles to avoid misfire codes. All 8 at once; never half.