P0308 on GMC: How to Diagnose & Fix
Cylinder 8 Misfire Detected
1 GMC model · 1 documented issue · 1 critical · $1,500-$4,500 repair
P0308 on GMC by model: symptoms, diagnosis and fix
The Yukon XL with the 5.3L or 6.2L V8 and Active Fuel Management (AFM/DOD) system develops hydraulic valve lifter failures. The AFM lifters, which collapse to deactivate cylinders for fuel economy, fail and cause persistent ticking noise, misfires, and potential camshaft damage. The problem is most common on 2007-2019 models. GM service bulletin recommends replacing all 8 AFM lifters if ticking persists after oil change.
Common Symptoms
- Ticking noise on cold start lasting 2 seconds to 10 minutes
- Persistent engine tick at idle
- Misfires on AFM-deactivated cylinders
- Rough idle when AFM engages
- Check engine light with misfire codes
- Reduced fuel economy
How to Fix
Sources for this issue (1)
P0308 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0308 mean on a GMC?
P0308 stands for "Cylinder 8 Misfire Detected." P0308 means the engine computer detected a misfire on cylinder number 8 — that cylinder is failing to combust properly or at the correct time. The PCM identifies it by monitoring crankshaft speed variations as cylinder 8 fires. Misfires waste fuel, increase emissions, and can overheat and damage the catalytic converter, so a hard misfire may cause a flashing check engine light. Typical symptoms include rough idle, vibration, hesitation, and reduced power. On GMC it is documented across 1 model: Yukon XL.
What are the most common causes of P0308?
The most common causes of P0308 are: Worn, fouled, or incorrectly gapped spark plug on cylinder 8, Failed ignition coil or coil-on-plug for cylinder 8, Faulty, clogged, or leaking fuel injector, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged spark plug wire or boot (on applicable engines), Carbon buildup or sticking valve. The specific cause varies by vehicle.
How much does it cost to fix P0308 on a GMC?
Published repair estimates on GMC range from $1,500 to $4,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.