P0306 on GMC: How to Diagnose & Fix
Cylinder 6 Misfire Detected
4 GMC models · 4 documented issues · 4 critical · $400-$8,000 repair
P0306 on GMC by model: symptoms, diagnosis and fix
The GMC Yukon 5.3L (L83, LC9, LMG) and 6.2L (L94, L86) V8 engines use Active Fuel Management (AFM) cylinder deactivation. The special collapsible AFM lifters on cylinders 1, 4, 6, and 7 fail prematurely, causing misfires, engine ticking, rough idle, and potential catastrophic damage. This is the single most reported mechanical failure on GMT900 (2007-2014) and K2XX (2015-2020) Yukon models. GM issued TSB 18-NA-077 acknowledging the issue. Yukon models used as daily drivers with lots of highway cruising (where AFM activates most) are most susceptible. The VLOM (Valve Lifter Oil Manifold) must also be replaced as it controls oil flow to AFM lifters.
Common Symptoms
- Misfire codes P0300, P0301, P0304, P0306, P0307
- Engine tick or tap at idle that worsens with load
- Rough idle or engine vibration
- Check engine light
- Reduced engine power / limp mode
- Metal debris in oil (confirmed on drain plug inspection)
- Vibration between 30-55 mph during V4 deactivation mode
How to Fix
Replace all 16 lifters, VLOM (GM 12639516), and inspect camshaft lobes. AFM delete kit (Texas Speed or BTR) permanently eliminates the system. Range Technology AFM Disabler RA003B prevents AFM activation as a preventive measure. Change oil every 5,000 miles with Dexos1 Gen 2 full synthetic to protect lifters.
Sources for this issue (1)
The 6.0L L96 V8 gasoline engine in Sierra 2500HD (non-diesel) suffers from Active Fuel Management (AFM) lifter failures similar to the 5.3L in Sierra 1500 — though less frequently due to the higher displacement. More commonly, 6.0L engines exhibit excessive oil consumption (1 qt per 1,000-3,000 miles) from worn valve stem seals and piston rings, particularly between 100,000-150,000 miles. The L96 is a high-output truck engine subjected to heavy towing and payload, which accelerates wear on AFM components.
Common Symptoms
- Oil level dropping between changes (check every 1,000 miles)
- Blue smoke on cold startup or under heavy acceleration
- Spark plug fouling on AFM cylinders
- Engine misfire codes P0300, P0301, P0304, P0306, P0307
- Ticking from valvetrain (AFM lifter collapse)
- Oil on spark plug threads
How to Fix
AFM disabler device (Range Technology RA003B) to prevent lifter wear. Oil consumption: GM oil consumption test procedure — if over 1 qt/2,000 miles, piston ring replacement authorized. Change oil every 5,000 miles with full synthetic Dexos2 5W-30. Valve stem seal replacement for blue smoke on startup.
Sources for this issue (1)
GMC Savana vans equipped with 4.8L (L20), 5.3L (LM7), and 6.0L (L96) V8 engines with Active Fuel Management suffer the same AFM lifter collapse problem documented across the entire GM full-size truck/van lineup. The collapsible AFM lifters on deactivation cylinders fail between 60,000-120,000 miles, causing misfires, engine ticking, rough idle, and potential engine damage. Commercial Savana vans used for fleet operations are especially vulnerable due to high mileage accumulation and variable maintenance practices. The problem affects all GMT600 platform Savana/Express vans with AFM-equipped engines.
Common Symptoms
- Misfire codes P0300, P0301, P0304, P0306, P0307
- Ticking or tapping from engine
- Rough idle especially when cold
- Check engine light
- Reduced power under load
- Metal shavings on oil drain plug magnet
How to Fix
Replace all 16 lifters and VLOM (GM 12639516). AFM delete kit for permanent fix. Range Technology RA003B disabler for prevention. Increase oil change frequency to 5,000 miles with Dexos2 full synthetic for commercial use vans.
Sources for this issue (1)
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)
P0306 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 P0306 mean on a GMC?
P0306 stands for "Cylinder 6 Misfire Detected." This code means the engine computer detected a misfire on cylinder 6 — that cylinder isn't burning its fuel-air mixture properly. The computer senses misfires by watching for tiny variations in crankshaft speed each time a cylinder should fire, and when cylinder 6 repeatedly fails to contribute, this code sets. You may feel a shake or stumble, especially at idle or under load, along with reduced power and possibly a flashing check-engine light, which signals a misfire severe enough to damage the catalytic converter. A misfire comes down to a problem with spark, fuel, or compression in that cylinder. On GMC it is documented across 4 models: Yukon, Sierra 2500HD, Savana, Yukon XL.
What are the most common causes of P0306?
The most common causes of P0306 are: Worn or fouled spark plug on cylinder 6, Failed ignition coil for cylinder 6, Clogged, leaking, or failed fuel injector, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt valve, head gasket), Damaged plug wire or coil connector, Carbon buildup or worn valvetrain components. The specific cause varies by vehicle.
How much does it cost to fix P0306 on a GMC?
Published repair estimates on GMC range from $400 to $8,000 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 P0306 on a different make?
View P0306 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.