P0306 on GMC: How to Diagnose & Fix
Cylinder 6 Misfire Detected
3 GMC models · 3 documented issues · 2 critical · $1,500-$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 diagnostic bulletins PIP4568 (2007-2014 AFM V8s) and PIP5259 (2014-2015 Yukon) covering collapsed or stuck AFM lifters. Yukon models used as daily drivers with lots of highway cruising (where AFM activates most) are most susceptible. On 2007-2014 engines the VLOM (Valve Lifter Oil Manifold), which controls oil flow to the AFM lifters, is replaced along with the lifters per PIP4568.
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 the AFM lifters and plastic lifter guides and inspect camshaft lobes; PIP4568 (2007-2014) also calls for the VLOM (GM 12639516), while PIP5259 (2014-2015) calls for the AFM lifters and guide on the affected bank plus the oil pump and oil pan. 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 (2)
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)
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 3 models: Yukon, Sierra 1500, 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 $1,500 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 →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.