P0308 on 2015-2019 GMC Yukon Denali: Cylinder 8 Misfire Causes & Fixes

What's Unique About the 2015-2019 Gmc YUKON DENALI

The 2015-2019 GMC Yukon Denali with the 6.2L V8 (L86) engine uses an Active Fuel Management (AFM) system to improve fuel economy by deactivating cylinders. This system is a known weak point, with the specialized AFM lifters being prone to mechanical collapse, sticking, or causing internal damage due to issues like oil aeration. This can lead to persistent misfire codes, including P0308, and often requires a complex and expensive repair that goes beyond typical ignition or fuel system faults. This issue is extensively documented in GM Technical Service Bulletins (like 15-06-01-002F) and numerous owner forums.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough or shaking idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Engine ticking, chirping, or knocking noise, especially if related to a lifter issue.
• Smell of raw fuel from the exhaust
• "Stabilitrak off / Service Traction Control" message appearing at idle.

Most Likely Causes

1. Faulty Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a standard wear item. Cylinder 8 is located at the rear of the passenger side, where it can be exposed to high heat, potentially shortening its life.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, carbon fouling, oil, or damage. A fuel-soaked plug may indicate an ignition or injector issue, while a damaged electrode points to a plug failure. Compare it to a plug from another cylinder.
   Typical fix: Replace the spark plug. It's best practice to replace all 8 spark plugs at the same time with high-quality Iridium plugs like the OEM ACDelco part.
   Est. part cost: $8-$20 per plug
2. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil Heat soak from the engine can cause ignition coils to fail over time. Cylinder 8's rear position makes its coil susceptible.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6, which is easier to access on the same bank). Clear the codes and drive the vehicle. If the code changes to P0306, the coil is faulty.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $40-$80
3. Loose or Damaged Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug A specific TSB (#PIP5159C) notes that spark plug wires can become loose, sometimes due to a missing 'C' clip on the wire end where it connects to the coil, causing a misfire.
   How to confirm: Visually inspect the spark plug wire for cylinder 8. Ensure it is securely connected at both the ignition coil and the spark plug. Check for the presence of the retaining clip on the coil side. A resistance test can also be performed on the wire.
   Typical fix: Secure the spark plug wire or replace the set if damaged or if the retaining clip is missing.
   Est. part cost: $50-$100 for a set
4. Failed Active Fuel Management (AFM) Lifter 🟡 Medium Probability → Shop Engine Valve Lifter The 6.2L L86 engine has a well-documented, high rate of AFM lifter failure. The lifters can collapse, get stuck in their bores, or damage the camshaft, as detailed in multiple GM TSBs (e.g., 15-06-01-002 series). This is a very common major repair on this platform, often discussed in owner forums.
   How to confirm: This requires professional diagnosis. A technician will likely perform a physical inspection of the valvetrain by removing the valve cover to check for lifters that are not moving correctly, have play, or for bent pushrods. A consistent ticking noise from the engine is a strong indicator.
   Typical fix: Replace the failed AFM lifters on the affected bank, the valve lifter oil manifold (VLOM), and the lifter guides. The camshaft must also be inspected for damage and is often replaced. Many owners and independent shops opt for an 'AFM delete' which involves replacing the AFM lifters with standard LS7-style lifters and disabling the AFM system in the ECM to prevent future failures. 🎬 Watch: How to perform a complete AFM delete on this engine.
   Est. part cost: $400-$1500 for parts, including gaskets, fluids, and potentially a new camshaft.
5. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector While less common than ignition or lifter issues, direct injectors can become clogged or fail electronically.
   How to confirm: A technician can perform a fuel injector balance test. A DIY method is to listen to the injector with a mechanic's stethoscope to ensure it's clicking. Swapping injectors is labor-intensive. A real-world diagnosis on a forum showed a failed injector causing a P0308 with raw fuel in the exhaust.
   Typical fix: Replace the faulty fuel injector. Using a quality fuel system cleaner like Techron has been reported by some owners to resolve intermittent issues.
   Est. part cost: $70-$150

Rare But Worth Checking

• Worn Camshaft Lobe: → Shop Engine Camshaft This is often a direct result of a failed lifter. TSB #19-NA-219 (which supersedes PIP4138R) specifically calls out checking for a worn camshaft lobe if other diagnostics don't find the cause. If a lifter has failed, the cam should always be inspected for damage, as metal shavings can circulate and cause further engine damage.
• Bent Pushrod or Broken Valve Spring: A collapsed lifter can lead to a bent pushrod. TSB 15-06-01-002O mentions bent pushrods as a possible finding. TSB 19-NA-219 also lists a broken valve spring as a potential, though less common, cause of misfires that can occur intermittently, especially at high RPM.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0308 and check for 🎬 See this video for a simple explanation of P0308. any other codes like P0300.
2. Cylinder 8 Location: Identify cylinder 8. On the L86 V8, cylinders are numbered 1-3-5-7 on the driver's side (front to back) and 2-4-6-8 on the passenger's side (front to back). Cylinder 8 is the rearmost cylinder on the passenger side.
3. Ignition System Check: Start with the easiest checks. Inspect the spark plug wire for cylinder 8. Ensure it is fully seated on the coil and plug. Per TSB #PIP5159C, check for a missing retaining 'C' clip on the coil end of the wire.
4. Swap Components: Swap the ignition coil from cylinder 8 with cylinder 6 (passenger side, one cylinder forward). Clear the codes and run the engine. If the misfire code moves to P0306, the coil is bad. If not, swap the spark plug. If the code still remains on P0308, the issue is not the coil or plug.
5. Fuel System Check: Listen for a consistent clicking sound from the cylinder 8 fuel injector with a mechanic's stethoscope. No sound or an irregular sound indicates a potential injector problem. A professional can perform a balance test to confirm flow.
6. Mechanical Engine Health: If ignition and fuel seem okay, the next likely cause is a mechanical engine problem. Perform a compression test on cylinder 8 and compare it to a healthy cylinder. A significant loss of compression points to internal issues.
7. Valvetrain Inspection: If compression is good but the misfire persists (especially with a ticking noise), the valvetrain must be inspected. This involves removing the passenger-side valve cover to check for a collapsed/stuck AFM lifter, a bent pushrod, or a broken valve spring. This step is critical and directly addresses the most common major failure on this engine, as detailed in TSBs #15-06-01-002F and #19-NA-219.

Parts You'll Likely Need

• Spark Plug (OEM #ACDelco 41-168) — Spark plugs are a routine maintenance item and a primary cause of single-cylinder misfires.
  Trusted brands: ACDelco (OEM), NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12
• Ignition Coil (OEM #ACDelco 12699383) — Ignition coils fail over time and are a common cause of misfires. Swapping them is an easy diagnostic step.
  Trusted brands: ACDelco (OEM), Delphi, Standard Motor Products
  OEM price range: $50-$70
  Aftermarket price range: $40-$60
• AFM/DOD Lifter Kit — The most common major mechanical failure on this engine. Often replaced as a set for one bank or the entire engine. An 'AFM Delete Kit' is a popular alternative to prevent future failures.
  Trusted brands: ACDelco (OEM), Michigan Motorsports, Elgin
  OEM price range: $300-$500 for one bank
  Aftermarket price range: $250-$450 for one bank

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) may be stored alongside P0308. This is very common with AFM lifter issues that can disrupt the entire engine bank.
• P050D — This code for 'Cold Start Rough Idle' can sometimes accompany misfire codes on these engines, as noted in various TSBs, pointing towards a deeper mechanical or fuel system issue.

Technical Service Bulletins (TSBs) & Recalls

• PIP5159C: Addresses potential for a loose spark plug wire causing a misfire due to a missing 'C' clip.
• 15-06-01-002 (series F, M, O, etc.): The primary TSB covering diagnosis and repair of failed AFM lifters causing misfires and engine noise. It instructs technicians to replace the VLOM and the affected bank of lifters.
• 19-NA-219 (supersedes PIP4138R): Provides advanced diagnostics for engine noises and misfires when initial checks fail, pointing to worn cam lobes, sticking valves, or broken valve springs as potential causes.

Platform-Specific Known Issues

• Active Fuel Management (AFM) Lifter Failure: This is the most significant known issue for the 6.2L L86 engine. The specialized lifters designed to deactivate cylinders can mechanically collapse, get stuck, or have their internal locking pins damaged. This results in a misfire (often with code P0300) and a characteristic ticking or chirping noise. GM has issued a long series of service bulletins (e.g., 15-06-01-002F/M/O) detailing the diagnosis and repair, which involves replacing the lifters, lifter guides, and the Valve Lifter Oil Manifold (VLOM). The camshaft is often damaged by the failed lifter roller and requires inspection and frequent replacement.
• Loose Spark Plug Wires: TSB #PIP5159C specifically mentions that a loose spark plug wire, potentially from a missing retaining clip on the coil side, can be a simple cause for misfire codes P0300-P0308 across many GM V8 models, including the Yukon.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading of 0 ohms indicates a short, while a significantly higher reading indicates an open circuit.
• Ignition Coil Secondary Resistance — expected: 6,000 to 15,000 ohms (6kΩ - 15kΩ). Failure: A reading outside this range suggests a faulty coil.
• Spark Plug Wire Resistance — expected: 10,000 to 12,000 ohms per foot of wire for OEM-style wires.. Failure: Significantly higher resistance or an open circuit (infinite resistance) indicates a bad wire. Total resistance should not exceed 50,000 ohms for the longest wire.
• High-Pressure Fuel Rail Pressure (Cranking) — expected: 300-400 psi. Failure: Pressure that is significantly lower or drops off quickly could indicate a failing high-pressure fuel pump.

Hidden / Shadow Codes Worth Checking

• Mode 6 Data (Test ID $53, Component ID $01-$08): Mode 6 data can show misfire counts for each cylinder before they reach the threshold to set a P030x code. A technician can view the 'Exponential Weighted Moving Average' for misfires over the last 10 drive cycles to identify a cylinder that is beginning to fail. This is invaluable for diagnosing intermittent misfires that don't set a hard code. (see via A professional scan tool with Mode 6 capabilities (e.g., GDS2, Autel, Launch) is required.)

Scan Tool Commands That Help

• GDS2 (GM Dealer Tool): Cylinder Deactivation / AFM Control — A technician can command the AFM system on and off to see if the misfire on cylinder 8 is related to the AFM lifter's operation. If the misfire only occurs when AFM is active or inactive, it strongly points to a lifter or VLOM (Valve Lifter Oil Manifold) issue.
• GDS2 (GM Dealer Tool): Injector Balance Test — This function allows the technician to fire each injector individually and measure the corresponding drop in fuel pressure. This can confirm if the cylinder 8 injector is clogged or failing to deliver the correct amount of fuel compared to the other cylinders.

Wiring & Ground Locations

• Engine Harness Chafe Points — Inspect the engine wiring harness for chafing at the ECM/TCM bracket, the driver's side upper control arm, and the shock tower bolt. These are known problem areas where engine vibration can cause wires to rub through, potentially shorting circuits for the ignition coil or fuel injector for cylinder 8.. A shorted ignition coil or injector wire can directly cause a misfire on a single cylinder.
• Main Engine Block Ground — A primary ground cable runs from the negative battery terminal to the engine block on the passenger side. Another critical ground strap is located on the rear of the driver's side of the engine block, connecting to the firewall.. Poor ground connections can cause weak spark and erratic behavior from sensors and modules, leading to misfires. While a bad ground often causes multiple issues, it can sometimes manifest as a single-cylinder misfire initially.
• G114 - Engine Harness Ground — On some 2019+ models, a specific ground point, G114, for the engine harness has been identified as a potential source of loose connections.. A loose engine harness ground can cause a variety of issues, including misfires. GM has issued bulletins for engineers to gather data on these loose connections.

Real Owner Repair Stories

• YouTube video from 'CrazedPerformanceRepair' (2016 GMC Yukon XL 6.2L with 59,000 miles) — P0300 and P0303 codes (misfire on cylinder 3, but diagnosis is identical for cylinder 8)
  ❌ Tried (didn't work) Initial diagnosis pointed towards a standard ignition or fuel issue.
  ✅ What actually fixed it The technician used a scan tool to watch misfire counts while manually adjusting the rocker arm nut on the affected cylinder. Loosening the nut slightly caused the misfire count to drop to zero, confirming a collapsed/stuck AFM lifter. The recommended fix was a new lifter and lifter guide for that cylinder. The vehicle was still under powertrain warranty and was sent to a GM dealer for the repair.
• Tahoe Yukon Forum user (2016 Yukon with 40,000 miles) — P0308 code appeared while trailering.
  ❌ Tried (didn't work) The owner was considering swapping plugs, wires, and coils.
  ✅ What actually fixed it Another user suggested trying a fuel system cleaner first. The owner ran two tanks of gasoline, each treated with Techron fuel system cleaner. The P0308 code went away and did not return, suggesting the issue was a mildly clogged or dirty fuel injector.

OEM Part Supersession History

• HL-7011 (Elgin Aftermarket) → HL-7025 (Elgin Aftermarket) — The newer HL-7025 lifter is designed for the newer Dynamic Fuel Management (DFM) engines but is backward compatible with older Active Fuel Management (AFM) engines. The original AFM lifter (HL-7011) will ONLY fit AFM applications.
  Heads up: When performing an AFM repair or delete on a 2015-2019 L86, you can use the newer DFM-style lifters (like HL-7025), but you cannot use original AFM lifters on a newer DFM engine.

Model Year Variations Within This Range

• 2019: In 2019, GM began phasing in the L87 engine to replace the L86. The L87 features Dynamic Fuel Management (DFM), a more advanced version of AFM that can deactivate any number of cylinders, not just a fixed bank of four. While the P0308 diagnosis is similar, be aware that a 2019 model could have either an L86 with an 8-speed transmission or an L87 with a 10-speed transmission and DFM. DFM lifters are different but have also been subject to failures.