P0308 on 2009-2010 GMC Sierra 3500: Cylinder 8 Misfire Causes & Fixes

What's Unique About the 2009-2010 Gmc SIERRA 3500

For the 2009-2010 Sierra 3500, the cause of P0308 is highly dependent on which V8 engine you have. The standard 6.0L Vortec gas engine (RPO code LY6) is prone to common ignition failures, but can also suffer from valvetrain issues like a failed lifter, sometimes related to its Active Fuel Management (AFM) system. The optional 6.6L Duramax LMM diesel engine, on the other hand, does not have spark plugs; for it, a P0308 code frequently points to a more expensive and complex problem like a failing fuel injector, a known issue on these diesel platforms.

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 shaky idle
• Engine stumbling or hesitating during acceleration
• Noticeable loss of power
• Check Engine Light is on and steady
• Check Engine Light is flashing (severe misfire)
• Engine may be harder to start
• Smell of unburned fuel from the exhaust
• Persistent ticking or chirping noise from the engine (if related to lifter failure)

Most Likely Causes

1. Worn/Fouled Spark Plug (6.0L Gas Engine) 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a service life of around 100,000 miles. Cylinder 8 is at the back of the engine bay, where heat can concentrate and access is more difficult, sometimes leading to it being overlooked during service. Oil fouling is also a possibility noted in GM TSBs like #PIP4525D.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, carbon fouling, oil deposits, or damage to the electrode. Compare it to a new plug. The correct gap should be 0.040 inches for modern replacement plugs.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 plugs at once. The updated AC Delco Iridium part is 41-162, which supersedes 41-110.
   Est. part cost: $8-$20 per plug
2. Failed Ignition Coil or Spark Plug Wire (6.0L Gas Engine) 🔴 High Probability → Shop Ignition Coil Ignition coils and wires are exposed to high heat, leading to eventual failure. A known issue is coolant leaking from heater hose quick-connects on the firewall, dripping directly onto the cylinder 8 coil and wire, causing a misfire. TSB #PIP5159C also notes that loose spark plug wires can cause misfires.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and run the engine. If the code changes to P0306, the coil is faulty. Inspect the wire for cracks, burn marks, looseness, or signs of coolant contamination.
   Typical fix: Replace the faulty ignition coil and/or spark plug wire. It's often recommended to replace wires as a set.
   Est. part cost: $40-$80 for a coil, $50-$100 for a set of wires
3. Failed Fuel Injector (6.6L Diesel Engine) 🔴 High Probability → Shop Fuel Injector Duramax LMM diesel injectors are complex components operating under extremely high pressure and are a well-documented failure point that can cause misfires, rough idle, or smoke.
   How to confirm: This requires advanced diagnostics. A professional technician can perform a fuel injector balance test with a scan tool to measure flow rates. Listening for a consistent 'click' from the injector with a mechanic's stethoscope can also be a clue, but a balance test is definitive.
   Typical fix: Replace the faulty fuel injector. This is often a job for a professional due to the high-pressure fuel system and the need for specialized tools. Bosch is the OEM supplier.
   Est. part cost: $300-$600 per injector, plus a refundable core charge.
4. Clogged or Faulty Fuel Injector (6.0L Gas Engine) 🟡 Medium Probability → Shop Fuel Injector Fuel injectors can become clogged with deposits over time or fail electrically, preventing the correct amount of fuel from entering the cylinder.
   How to confirm: After ruling out ignition issues, you can test the injector. Listen for a clicking sound with a stethoscope. A more definitive test involves using a 'noid light' to check for an electrical pulse at the connector or measuring its resistance with a multimeter.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120
5. Failed AFM/DOD Lifter (6.0L Gas Engine) ⚪ Low Probability → Shop Engine Valve Lifter The Active Fuel Management (AFM) system, also known as Displacement on Demand (DOD), deactivates cylinders to save fuel. The specialized lifters used for this system are a documented weak point and can collapse or get stuck, causing a persistent misfire. This is a widely known issue on many 2007+ GM V8s.
   How to confirm: This is a difficult diagnosis, often accompanied by a persistent ticking or chirping noise from the engine. It typically requires removing the valve cover to inspect the valvetrain movement. A compression test may also show a loss of compression on the affected cylinder. TSBs like #19-NA-219 and #PIP4138R directly address this failure.
   Typical fix: Replace the failed lifter and often the camshaft, which may have been damaged by the failed lifter. This is a major and expensive engine repair. Many owners opt to install an 'AFM delete kit' to prevent future failures.
   Est. part cost: $500-$1500 for parts kit (lifters, gaskets, camshaft)

Rare But Worth Checking

• Low Compression: If ignition and fuel systems are confirmed to be working, a mechanical issue like a burnt valve, bad piston rings, or a head gasket leak could be the cause. A compression test is the primary tool to verify this.
• Vacuum Leak: A leak in the intake manifold gasket near the runner for cylinder 8 can lean out the air/fuel mixture enough to cause a misfire. On these engines, the original orange intake gaskets were prone to leaking and were superseded by an improved teal green design.

Diagnosis Steps

1. Confirm the code P0308 is present using an OBD-II scanner. Note if the check engine light is flashing.
2. Determine if you have the 6.0L gas or 6.6L diesel engine, as the steps diverge here.
3. For 6.0L Gas Engine:
4. a. Locate cylinder 8 (rear-most on the passenger side).
5. b. Swap the ignition coil from cylinder 8 with cylinder 6 (passenger side, second from front). Clear codes and run the engine. If P0306 appears, the coil is bad.
6. c. If the misfire remains on cylinder 8, inspect the spark plug for wear, fouling, or coolant contamination from a potential heater hose leak above it. Check that the spark plug gap is 0.040".
7. d. If the spark plug looks good, swap it with cylinder 6. If the misfire moves, the spark plug was the issue.
8. e. Inspect the spark plug wire for damage, a loose connection, or corrosion.
9. f. If ignition components are good, investigate the fuel injector for cylinder 8. Listen for a steady clicking sound or use a noid light to confirm it's receiving a signal.
10. g. If you hear a persistent ticking noise from the engine, suspect a mechanical issue like a failed AFM lifter. Perform a compression test on cylinder 8 to check for mechanical engine problems.
11. For 6.6L Diesel Engine:
12. a. Locate cylinder 8 (rear-most on the driver's side).
13. b. Diagnosis is more complex. Start by listening to the fuel injector on cylinder 8 with a mechanic's stethoscope to ensure it is clicking.
14. c. Check the electrical connector and wiring to the injector for damage or corrosion.
15. d. Professional diagnosis with a scan tool capable of performing an injector balance test is the most reliable method to confirm a bad injector.
16. e. If the injector is confirmed good, a compression test may be necessary to rule out internal engine issues.

Parts You'll Likely Need

• Spark Plug (6.0L Gas) (OEM #41-162) — This is the most common and basic maintenance item to fail and cause a single-cylinder misfire. The original 41-110 has been superseded by 41-162.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $5-$1

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affecting multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) code may appear alongside P0308.
• P050D — As noted in TSB #PIP5628F, a cold start rough idle (P050D) can be related to misfire codes like P0308, indicating a potential diagnostic link that may require advanced diagnostics with a scan tool.
• P0171 / P0174 — These 'System Too Lean' codes can appear if the misfire is caused by a vacuum leak, such as a failed intake manifold gasket, which affects the air-fuel ratio on one or both engine banks.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628F: A diagnostic aid for technicians when encountering misfire codes P0300-P0308 and/or P050D. It provides a template of information to collect before contacting GM's Technical Assistance Center (TAC), including relative compression test results.
• 19-NA-219 (supersedes PIP4138R): Addresses engine noises (tick, chirp) and misfires (P0300-P0308) by pointing to potential causes like worn camshaft lobes or collapsed AFM lifters.
• PIP5159C: Specifically calls out checking for loose spark plug wires as a potential cause for misfire codes, sometimes due to a missing clip on the wire terminal.
• PIP3232B: Discusses cold engine idle misfires and lean codes (P0171/P0174) caused by leaking intake manifold gaskets on Gen III/IV truck engines. Notes the original orange gaskets were prone to failure and were replaced with an improved teal green design.

Platform-Specific Known Issues

• Heater Hose Coolant Leak: The plastic quick-connect fittings for the heater core, located on the firewall above the passenger side of the engine, are known to become brittle and leak. Coolant can drip directly onto the cylinder 8 ignition coil and spark plug wire, causing corrosion and a persistent P0308 misfire.
• AFM Lifter Failure (6.0L Gas): → Shop Engine Valve Lifter While the 6.0L in the HD trucks is robust, models equipped with Active Fuel Management (AFM) are highly susceptible to lifter failure. A collapsed or stuck lifter is a common cause of a persistent single-cylinder misfire that is not resolved by ignition or fuel system repairs. This is often preceded by a distinct ticking noise from the upper engine.

Mechanic-Grade Diagnostic Values

• 6.0L Ignition Coil Resistance (Primary) — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range indicates a faulty primary winding.
• 6.0L Ignition Coil Resistance (Secondary) — expected: 6,000 to 10,000 ohms. Failure: A reading outside this range suggests a failure in the secondary winding.
• 6.6L LMM Duramax Injector Balance Rates (at idle, in Park) — expected: +/- 4.0 mm³/sec. Failure: A reading consistently exceeding +/- 4.0 mm³/sec on a cylinder points to a faulty injector. A negative value suggests leaking/over-fueling, while a positive value suggests low compression or a clogged injector.
• 6.6L LMM Duramax Calculated Fuel Rate (CFR) at idle — expected: 7-9 mm³. Failure: A CFR significantly lower than this range (e.g., 2-3 mm³) can indicate that the ECM is compensating for one or more leaking injectors, even if the balance rates appear to be within spec.

Scan Tool Commands That Help

• Tech2 / GDS2: Injector Balance Test — For the 6.6L Duramax diesel, this is the primary diagnostic function to identify a failing injector causing P0308. It measures how much the ECM is adjusting fuel to each cylinder to maintain a smooth idle.
• Tech2 / GDS2: Misfire Data / Misfire Graphic — For both gas and diesel engines, this live data display shows current and historical misfire counts for each cylinder, helping to confirm that cylinder 8 is the primary and consistent offender.

Wiring & Ground Locations

• G104 — Front of the left (driver's side) cylinder head on the 6.0L V8.. This is a primary engine ground. A loose or corroded connection here can cause erratic behavior in the ignition system and other engine sensors, potentially leading to misfire codes.
• G102 — On the lower left front of the engine block (for V8s).. This ground point is specifically listed as the ground for all 8 ignition coils on the 6.0L V8. A fault at this single point can cause issues that may manifest as a single-cylinder misfire before affecting other cylinders.
• Cylinder 8 Injector Connector (6.6L LMM) — Rear-most injector on the driver's side. The signal wire is Yellow/Black (YEL/BLK).. Corrosion or damage to this specific connector or its wiring can cause a P0308 code. Checking for a solid connection and clean terminals is a key step before replacing the injector.
• J108 — In the ignition coil jumper harness, located between ignition coils 5 and 7 on the 6.0L V8.. This is a harness splice point. While not a ground, a failure at this junction could interrupt signals to the cylinder 7 and by extension, potentially affect the nearby cylinder 8 wiring, making it a place to check for harness integrity issues.

Real Owner Repair Stories

• Go-Parts.com repair documentation (GMC Sierra (similar GMT900 platform)) — Persistent P0308 misfire code.
  ❌ Tried (didn't work) Swapping the ignition coil from cylinder 8 to other cylinders (2 and 6) did not move the misfire code.
  ✅ What actually fixed it The wiring harness connector for the cylinder 8 ignition coil was found to have corrosion from a past coolant leak. Repairing the connector resolved the P0308 code.
• DuramaxDiesels.com forum user (GMC Sierra with 6.6L LMM Duramax) — P0308 code and a ticking noise at lower RPM.
  ✅ What actually fixed it The user was advised to first check the harness connections to the injector before replacing it. The combination of the code and the ticking noise strongly suggested a failing injector, which is the most common resolution for this scenario on an LMM.

OEM Part Supersession History

• 12570616 → 12611424 (also known as D510C) — Standard part evolution for improved durability and performance. These are the square-shaped coils used on the 6.0L LY6 engine.
  Heads up: The original part 12570616 was replaced by 12611424, which was later superseded by 12739916. All are generally interchangeable for this application.

Model Year Variations Within This Range

• 2010: For the 2010 model year, the standard 6.0L V8 engine received a slight power increase to 360 horsepower, up from 353 in 2009. Additionally, a USB port was added to uplevel sound systems. These changes do not directly affect the diagnosis of P0308 but distinguish the model years.