P0306 on 2010-2013 GMC Sierra Denali: Cylinder 6 Misfire Causes and Fixes

What's Unique About the 2010-2013 Gmc SIERRA DENALI

The 2010-2013 GMC Sierra Denali is equipped with the 6.2L V8 (RPO L94) engine, which uses Active Fuel Management (AFM). This system deactivates cylinders—including cylinder 6—to save fuel under light loads. Unfortunately, the hydraulic lifters associated with the AFM system are a well-documented weak point and are prone to failure. A collapsed or stuck lifter prevents the valve from operating, directly causing a P0306 misfire code, often with a distinct ticking sound.

Diagnostic Flowchart

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

Symptoms You May Notice

• Rough or vibrating idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Flashing or solid Check Engine Light.
• Engine ticking or tapping noise that increases with RPM, especially if the cause is a failed lifter.
• Service StabiliTrak and Traction Control messages may appear on the dash.
• Decreased fuel economy.

Most Likely Causes

1. Failed Active Fuel Management (AFM) Lifter 🔴 High Probability The AFM lifters for cylinders 1, 4, 6, and 7 are a known failure point on this engine. They can become stuck or collapse, preventing the valve from opening and causing a persistent misfire. This can be caused by issues with the Valve Lifter Oil Manifold (VLOM) which controls oil pressure to the lifters, or internal failure of the lifter itself. 🎬 Watch: How AFM lifters operate and why they fail.
   How to confirm: A distinct ticking or tapping noise from the top of the engine is a strong indicator. A compression test on cylinder 6 may show low or zero compression. The definitive diagnosis requires removing the passenger-side valve cover to observe if the rocker arms for cylinder 6 are moving 🎬 See how to diagnose and replace a collapsed lifter. while the engine is cranked. A bent pushrod is often found along with a collapsed lifter.
   Typical fix: This is a labor-intensive repair that involves removing the cylinder head to replace 🎬 Watch this walkthrough on removing the head and lifters. the failed lifter(s), lifter guides, and often the camshaft if it has been damaged by the failed lifter. It is highly recommended to replace the Valve Lifter Oil Manifold (VLOM) at the same time. Many owners opt to replace all AFM components or install an AFM-delete kit, which requires an ECM tune.
   Est. part cost: $200-$1200
2. Worn or Fouled Spark Plug 🟡 Medium Probability
   How to confirm: Remove the spark plug from cylinder 6 and inspect it for wear, carbon buildup, or oil/fuel fouling. Compare it to a plug from a different cylinder. A wet plug can indicate a fuel or spark issue.
   Typical fix: Replace the spark plug. It is recommended to replace all 8 spark plugs at the same time for even performance, especially if they are near their 100,000-mile service interval.
   Est. part cost: $8-$20 per plug
3. Faulty Ignition Coil 🟡 Medium Probability
   How to confirm: Swap the ignition coil from cylinder 6 with another cylinder (e.g., cylinder 2 or 4 on the same passenger side bank). Clear the codes and drive the vehicle. If the code changes to P0302 or P0304, the coil is bad.
   Typical fix: Replace the faulty ignition coil.
   Est. part cost: $40-$90
4. Damaged or Loose Spark Plug Wire ⚪ Low Probability A GM Technical Service Bulletin (TSB #PIP4819A) specifically calls out inspecting for and replacing loose spark plug wires as a fix for misfire codes (P0300-P0308) on these trucks.
   How to confirm: Visually inspect the spark plug wire for cylinder 6 for any signs of damage, arcing (white marks), or a loose connection. Ensure it 'clicks' securely onto the coil and spark plug.
   Typical fix: Replace the spark plug wire. It's best practice to replace the full set.
   Est. part cost: $50-$100 for a set
5. Clogged or Failing Fuel Injector ⚪ Low Probability
   How to confirm: Swap the fuel injector from cylinder 6 with another cylinder. If the misfire code follows the injector, it is the cause. This is more labor-intensive than swapping a coil. A mechanic's stethoscope can be used to listen for a steady clicking sound from the injector to confirm operation.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120

Rare But Worth Checking

• Intake Manifold Gasket Leak: The intake manifold gasket can fail, causing a vacuum leak near a specific cylinder. A smoke test is the most effective way to confirm this.
• Low Engine Compression (Mechanical Issue): Beyond a failed lifter, issues like a bent valve, worn piston rings, a broken valve spring, or a damaged cylinder head can cause a P0306. A compression test and cylinder leak-down test are required for diagnosis.

Diagnosis Steps

1. Check for other diagnostic trouble codes in the ECM, paying close attention to P0300 or any P34xx codes.
2. If a ticking or chirping noise is present that increases with engine speed, suspect a mechanical valvetrain issue like an AFM lifter.
3. Locate Cylinder 6 (third cylinder from the front on the passenger side).
4. Swap the ignition coil from cylinder 6 to a different cylinder on the same bank (e.g., cylinder 4). Clear the codes and see if the misfire moves to P0304. If it does, the coil is faulty.
5. If the misfire remains on cylinder 6, inspect the spark plug for wear, fouling, or damage. Replace if necessary.
6. Inspect the spark plug wire for cylinder 6 for looseness or damage, ensuring it is fully seated at both ends, as noted in TSB #PIP4819A.
7. If ignition components are good, perform an engine compression test on cylinder 6. A significant loss of compression points towards a mechanical problem, most likely a failed AFM lifter.
8. For definitive lifter diagnosis, remove the passenger side valve cover and observe the rocker arms for cylinder 6 while cranking the engine. Lack of movement indicates a collapsed lifter.
9. If compression is good and valvetrain movement is normal, the issue could be fuel-related. Consider swapping the fuel injector from cylinder 6 with another cylinder to see if the code follows.

Parts You'll Likely Need

• AFM Valve Lifter (OEM #12645725) — This is the most common major failure causing a P0306 on this engine. A collapsed lifter prevents valve operation. It is recommended to replace all 8 AFM lifters during the repair.
  Trusted brands: ACDelco, General Motors, Delphi
  OEM price range: $30-$50 per lifter
  Aftermarket price range: $20-$40 per lifter
• Valve Lifter Oil Manifold (VLOM) (OEM #12571609) — The VLOM contains the solenoids that control oil flow to the AFM lifters. A failure here can starve the lifters of oil, causing them to fail. It is highly recommended to replace this part during a lifter job.
  Trusted brands: ACDelco, General Motors
  OEM price range: $150-$250
  Aftermarket price range: $100-$180
• Ignition Coil (OEM #12699382) — A failed ignition coil provides no spark, causing a misfire. It's a common failure item and easy to diagnose by swapping cylinders.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $60-$90
  Aftermarket price range: $30-$60
• Iridium Spark Plug (OEM #12683541) — Spark plugs are a routine maintenance item and can become worn or fouled, leading to a weak or nonexistent spark.
  Trusted brands: ACDelco, NGK, Denso
  OEM price range: $10-$15
  Aftermarket price range: $8-$12

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random Misfire) code may be stored alongside P0306. This is common with developing AFM lifter issues.
• P0301, P0304, P0307 — These are the other cylinders controlled by the Active Fuel Management (AFM) system. It's common for multiple AFM lifters to fail, causing misfires on these cylinders as well.
• P3441 — This code for 'Cylinder 6 Deactivation/Intake Valve Control Circuit/Open' directly points to a fault in the AFM system for cylinder 6. It strongly suggests the P0306 is caused by a faulty AFM solenoid, wiring, or the lifter itself.

Technical Service Bulletins (TSBs) & Recalls

• PIP4819A: Mentions that engine misfire codes (P0300-P0308) can be caused by loose spark plug wires, and instructs technicians to inspect and replace them as needed.
• 19-NA-219: A diagnostic tip bulletin for various engine noises and misfire codes, which details procedures for diagnosing worn camshafts, sticking valves, broken springs, and collapsed AFM lifters as potential causes for misfires.
• 10-06-01-008H: Addresses issues of excessive oil consumption which can lead to fouled spark plugs and carbon buildup. It provides procedures to install an AFM oil deflector and clean carbon from cylinders, which can be a contributing factor to lifter failure.

Platform-Specific Known Issues

• Active Fuel Management (AFM) system lifter failure is the most prominent issue for the 6.2L V8 engine in this truck. The lifters for cylinders 1, 4, 6, and 7 are prone to collapsing or getting stuck, leading to misfires and potential camshaft damage.
• Oil consumption issues, addressed in TSB 10-06-01-008H, can contribute to lifter failure by causing carbon buildup and affecting oil pressure, which is critical for AFM system operation.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance (Flex Fuel L94) — expected: 11 to 14 ohms. Failure: A reading outside this range indicates a faulty internal coil in the injector.
• Valve Lifter Oil Manifold (VLOM) Solenoid Resistance — expected: 11 to 18 ohms. Failure: Readings outside this range suggest a faulty solenoid within the VLOM, which controls the AFM lifters.
• Engine Oil Pressure (Hot Idle) — expected: Approximately 25-30 PSI. Failure: Pressure consistently below 20 PSI at hot idle can indicate oil pump issues or a bad pickup tube O-ring, which can starve AFM lifters.
• Ignition Coil Primary Resistance — expected: 0.500 ohms. Failure: Significant deviation points to a failing coil.

Scan Tool Commands That Help

• GDS2/Tech2: Injector Balance Test — To confirm if a fuel injector is delivering the correct amount of fuel. This test pressurizes the fuel rail and measures the pressure drop when each injector is commanded to open, identifying a weak or clogged injector without having to physically swap it.
• GDS2/Tech2: Cylinder Deactivation (AFM) System Test — This bidirectional command allows a technician to activate and deactivate the AFM solenoids for each cylinder individually. If commanding cylinder 6 to deactivate/reactivate does not produce a change in engine operation or sound, it points to a problem in the VLOM solenoid, wiring, or the lifter itself.

Wiring & Ground Locations

• G103 — On the front of the passenger side (right side) cylinder head.. This is a primary engine ground. A loose or corroded connection at G103 can cause erratic behavior from sensors and actuators controlled by the ECM, including the ignition coil and fuel injector for cylinder 6, potentially leading to a misfire code.
• G104 — On the rear of the driver side (left side) cylinder head.. Another critical engine-to-chassis ground. Poor connection here can affect the overall electrical stability of the engine management system, contributing to intermittent misfires or other electrical issues that may be hard to diagnose.
• Engine Block to Chassis Strap — Typically runs from the rear of the engine block (often near the transmission bellhousing) to the firewall/chassis.. This ground strap is essential for providing a solid ground path for the entire engine. If it becomes corroded, frayed, or broken, the electrical system may try to find ground through other paths (like throttle cables or shifter linkages), causing a host of electrical gremlins, including misfires.

Real Owner Repair Stories

• Reddit user in r/AskMechanics (2011 GMC Sierra, 195,000 miles) — P0306 misfire on cylinder 6.
  ❌ Tried (didn't work) Replacing spark plug, Replacing spark plug wires, Replacing ignition coil
  ✅ What actually fixed it The vehicle was taken to a shop which diagnosed failed lifters. The user reported the shop quoted $3000-$4000 for the repair, confirming it was a major mechanical issue beyond simple ignition parts.
• YouTube video from 'Peter L' (2015 Tahoe (similar 5.3L engine with AFM, same diagnostic principle)) — Flashing check engine light, P0306 misfire code.
  ❌ Tried (didn't work) Swapping spark plug between cylinders 4 and 6, Swapping spark plug wire between cylinders 4 and 6, Replacing the ignition coil on cylinder 6
  ✅ What actually fixed it After removing the valve cover, it was discovered that the intake valve lifter for cylinder 6 had completely collapsed and was not moving. The problem was confirmed to be a failed AFM lifter.

OEM Part Supersession History

• 12645725 → 12740071 — Updated design to improve reliability and address common failure modes of the original AFM lifter.

Model Year Variations Within This Range

• 2010-2013: The L94 engine was introduced for the 2010 model year, adding Active Fuel Management (AFM) and Flex Fuel capability to the 6.2L V8 used in these trucks. Earlier 6.2L truck engines (L92) did not have AFM, making the lifter failure issue specific to the L94 and later engines in this generation.