P0308 on GMC: How to Diagnose & Fix
Cylinder 8 Misfire Detected
2 GMC models · 2 documented issues · 1 critical · $1,500-$4,500 repair
P0308 on GMC by model: symptoms, diagnosis and fix
GM says major carbon build-up on the intake and/or exhaust valves, from fuel contamination or incomplete burning of the fuel, can cause rough idle, extended crank, crank-no-start or misfire with codes P0300-P0308, and that valves may stick. 2014-2018 GMC Sierra 1500 gasoline engines (4.3L V6, 5.3L V8, 6.2L V8); GM bulletin 16-NA-383 (June 2024) covers all 2000-2025 GM gasoline passenger cars and light/medium-duty trucks.
How to Fix
Have a technician follow the service-manual misfire diagnosis. If carbon is the cause and there is no valve damage, bulletin 16-NA-383 calls for a Top Engine Cleaner procedure through the throttle body to free sticking valves and remove carbon. GM also advises Top Tier gasoline and regular oil changes to reduce build-up. Normal warranty rules apply; this is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
On the Yukon XL with the 5.3L or 6.2L V8 and Active Fuel Management (AFM, also called Displacement on Demand or DOD), the AFM valve lifters can fail. These lifters collapse on purpose so the engine can shut off cylinders to save fuel. When one sticks collapsed, or its internal locking pin is damaged, the valve above it stops moving. The result is a ticking noise, a misfire, and a check engine light, and the camshaft lobe under the lifter can be damaged. GM service bulletin 15-06-01-002E (October 2016) covers 2015-2017 Yukon and Yukon XL with the 5.3L (L83) or 6.2L (L86). It lists three causes: an AFM lifter that is mechanically collapsed or stuck all the time, damage to the lifter's internal locking pin from oil aeration, and a lifter that has collapsed and is stuck in its bore. Owners of earlier trucks report the same kind of lifter tick. One 2007 Yukon 5.3L owner reports a tick that gets louder when the engine is warm, and in the same thread another owner says the tick came back years after a dealer replaced the lifters.
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
What to check first: if the check engine light is on, read the codes. The GM bulletin is built around DTC P0300 (random or multiple misfire) together with a tick noise. Also watch the oil pressure gauge. A 2007 Yukon owner with a warm-engine tick saw low oil pressure at idle. Another owner in that thread says the tick stopped after they replaced the oil pump and the O-ring on the oil pickup tube. That fix comes from an owner, not from GM. The dealer fix (GM bulletin 15-06-01-002E, which covers 2015-2017 Yukon and Yukon XL with the 5.3L L83 or 6.2L L86): if normal diagnosis doesn't find the cause, the technician checks whether each valve is actually moving. If one isn't, they replace the valve lifter oil manifold (VLOM) and the AFM lifters on that bank. The parts are lifter 12645725 (4 per bank) and VLOM service kit 12660737 for the L83/L86. The bulletin's parts list also includes new cylinder head gaskets and head bolts. While the lifters are out, the technician inspects the camshaft and replaces it if it is damaged. If a lifter has spun in its bore, the lifter guides are replaced too. GM lists 9.2 labor hours for the left bank and 9.3 hours for the right bank on a Yukon. Coverage: this is a technical service bulletin, not a recall. The bulletin gives labor operations for repairs done under GM Powertrain coverage, so ask a dealer to check your truck's warranty status. The bulletin doesn't cover 2007-2014 trucks. One 2007 owner bought an AFM disabler device, but the thread doesn't say whether it helped.
What you need to diagnose it
The repair instructions explicitly call for reading stored codes. Use a scanner that can read the named code families before choosing a repair.The recommended scanner supports these code families; confirm your exact year, model and module coverage before buying.
ANCEL AD310 Classic OBD-II Scanner
Read & clear engine (P) codes
$20–$40View
As an Amazon Associate, we earn from qualifying purchases. Prices are approximate and may vary.
Reading and clearing the code is DIY work. Module replacement or reprogramming is not — most modules are VIN-programmed, so a new one does nothing until a shop codes it to your car. Check for an open recall or service bulletin before buying anything.
Sources for this issue (2)
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 2 models: Sierra 1500, 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.
Looking for P0308 on a different make?
View P0308 across all makes →Join 700+ subscribers following Au7o for vehicle insights and updates.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.