P0304 on GMC: How to Diagnose & Fix
Cylinder 4 Misfire Detected
4 GMC models · 4 documented issues · 3 critical · $800-$8,000 repair
P0304 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)
The LML Duramax EGR (exhaust gas recirculation) cooler is prone to coolant leaks internally, allowing coolant to enter the intake manifold and combustion chambers. Coolant in the intake causes white smoke, rough running, and potential hydrolocking of cylinders. The EGR valve also sticks open or closed from carbon buildup, causing rough idle, black smoke, and check engine lights. The EGR cooler failure can occur as early as 60,000 miles on trucks used in stop-and-go driving. GM bulletin 08-06-04-003D provides the EN-48974 adapter set for pressure-testing the EGR cooler for coolant leaks into the combustion chambers.
Common Symptoms
- White smoke from exhaust especially on startup
- Coolant loss with no visible external leak
- Rough idle or engine misfire
- Check engine light with EGR codes (P0401, P0404)
- Coolant smell from exhaust
- Overheating or coolant in oil (severe cases)
- Black smoke and poor fuel economy (EGR valve stuck open)
How to Fix
Replace EGR cooler with updated GM design or aftermarket unit (e.g. BulletProof Diesel LML EGR cooler). Clean or replace EGR valve. Flush cooling system and inspect for coolant contamination in intake. Consider EGR delete kit for off-road/race use only (illegal for street use). Prevent premature failure by maintaining proper coolant pH and avoiding extended idle periods.
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)
The 3.6L V6 with Active Fuel Management experiences failure of the switching roller finger followers (SRFF) on the deactivating cylinders 2 and 5, where a locking pin sticks or binds or the pin shelf breaks or wears, setting the check-engine light with codes P219A/P219B and, in some cases, causing a ticking noise. A damaged follower or lash adjuster can become dislodged if not addressed.
Common Symptoms
- Ticking or tapping from engine
- Misfire on specific cylinders
- Check engine light
- Rough idle
How to Fix
Inspect and replace the damaged switching roller finger followers and stationary hydraulic lash adjusters on cylinder 2 or 5 per GM bulletin 22-NA-193 (if one SRFF on a cylinder is damaged, all four on that cylinder are replaced); the condition is often misdiagnosed as faulty injectors. AFM can be disabled with an ECM tune, but V8-style AFM lifter delete kits do not apply to this overhead-cam engine.
Sources for this issue (2)
P0304 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 P0304 mean on a GMC?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On GMC it is documented across 4 models: Yukon, Sierra 2500HD, Sierra 1500, Canyon.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a GMC?
Published repair estimates on GMC range from $800 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.
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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.