P0304 on GMC: How to Diagnose & Fix
Cylinder 4 Misfire Detected
5 GMC models · 6 documented issues · 5 critical · $400-$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 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 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 issued multiple TSBs addressing EGR cooler design improvements.
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 stainless steel unit (Mishimoto MMEGR-DMAX-LML). 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 (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)
Direct injection engines do not spray fuel over the intake valves like port injection systems, allowing carbon deposits from oil and blow-by gases via the PCV system to accumulate on intake valve backs and ports. Over time, this buildup restricts airflow causing performance and efficiency loss. The 5.3L and 6.2L engines with direct injection are susceptible to this issue typically after 60,000-100,000 miles.
Common Symptoms
- Reduced fuel economy
- Rough idle
- Hesitation during acceleration
- Engine misfires
- Reduced power output
- In severe cases, engine knock or pinging
How to Fix
Walnut shell blasting of intake valves (professional service). Chemical intake cleaning treatment. Install oil catch can to reduce PCV contamination. Manual intake manifold removal and valve cleaning.
Sources for this issue (1)
The 3.6L V6 with Active Fuel Management experiences lifter failure where the AFM lifters collapse, causing misfires and a ticking noise. The collapsing lifter can damage the camshaft lobe if not addressed.
Common Symptoms
- Ticking or tapping from engine
- Misfire on specific cylinders
- Check engine light
- Rough idle
How to Fix
Replace failed lifters and inspect camshaft for damage. AFM delete kits are available that disable cylinder deactivation and replace all AFM lifters with standard lifters. Requires ECM tune to disable AFM.
Sources for this issue (1)
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 5 models: Yukon, Sierra 2500HD, Savana, 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 $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 P0304 on a different make?
View P0304 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.