P0308 on 2002-2006 GMC Sierra Denali: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2002-2006 Gmc SIERRA DENALI

The 2002-2006 Sierra Denali is part of the GMT800 generation and exclusively used a 6.0L V8 engine (LQ4 or LQ9). While standard ignition component failure is common, these Gen III V8 engines have a documented history of potential valvetrain issues. Manufacturer TSBs specifically mention that a P030x code can be caused by a worn camshaft lobe or a faulty lifter, making it crucial to not automatically assume it's just a simple spark plug issue if basic fixes don't work. This issue is often preceded by a distinct 'chirp, squeak, squeal or tick' noise from the engine.

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
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing Check Engine Light (indicates a severe misfire)
• Solid Check Engine Light
• Strong smell of fuel from the exhaust
• A distinct, consistent 'chirp, squeak, squeal or tick' noise from the engine, which points towards the more serious camshaft/lifter issue.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil The coil-on-plug ignition system is reliable, but individual coils can fail with age and heat cycles. Cylinder 8 is at the rear of the passenger side engine bay, a location subject to significant heat and tight access, which can contribute to failure.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and drive. If the code changes to P0306, the coil is bad.
   Typical fix: Replace the faulty ignition coil. Many owners replace the corresponding spark plug at the same time.
   Est. part cost: $30-$70
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a routine maintenance item. While GM specifies a 100,000-mile interval, they can fail sooner. The original spec was a 0.060" gap, but GM later revised this to 0.040" for better idle and performance on iridium plugs like the ACDelco 41-162. Using the wrong gap or plug type can cause issues.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, carbon buildup, oil/coolant fouling, or damage. Compare it to a plug from a working cylinder.
   Typical fix: Replace the spark plug. It is often recommended to replace all 8 plugs at the same time 🎬 See this DIY walkthrough for replacing your spark plugs. for consistent performance. Use pre-gapped iridium plugs and do not attempt to gap them, as the tip is fragile.
   Est. part cost: $8-$15 per plug
3. Bad Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug The spark plug wires on these engines can degrade over time due to high engine temperatures, leading to cracks, increased resistance, or a poor connection at the coil or plug.
   How to confirm: Visually inspect the wire for cracks, burns, or corrosion. Test its resistance with a multimeter; it should be within the manufacturer's specification (typically a few thousand ohms per foot).
   Typical fix: Replace the faulty spark plug wire. It's good practice to replace the full set.
   Est. part cost: $40-$80 for a set
4. Clogged or Failing Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Use a 'noid light' to confirm the injector is receiving an electrical pulse. You can also swap the injector with another cylinder to see if the misfire follows, though this is more labor-intensive. Sometimes, running a high-quality fuel system cleaner like Techron for a couple of tanks can resolve a minor clog.
   Typical fix: Clean or replace the fuel injector for cylinder 8.
   Est. part cost: $50-$120

Rare But Worth Checking

• Worn Camshaft Lobe and/or Lifter: → Shop Engine Camshaft This is a known issue documented in GM Technical Service Bulletin #19-NA-219 (which supersedes PIP4138L). It often presents with a distinct ticking or squeaking noise 🎬 Watch: How to diagnose a bad hydraulic lifter. that doesn't go away when disabling injectors. If basic ignition and fuel system checks do not resolve the P0308 code, this becomes a strong possibility. The repair is labor-intensive and expensive, often costing thousands of dollars.
• Low Engine Compression: Caused by issues like a bad head gasket, worn piston rings, or a problem with the cylinder head valves (e.g., a sticking valve or broken valve spring). A compression test and/or a cylinder leak-down test is needed to diagnose this.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the intake runner for cylinder 8 can lean out the air/fuel mixture and cause a misfire. On these LS-based engines, the intake is a 'dry' design, so a leak will cause driveability issues and lean codes (P0171/P0174) rather than coolant loss. A hissing sound may be audible.
• Corroded Engine Ground: A GM bulletin notes that a corroded main engine wiring harness ground terminal (G103), which attaches to the right-side cylinder head, can cause unexplained misfires on multiple cylinders, including P0308.

Diagnosis Steps

1. Read the codes with an OBD-II scanner to confirm P0308 and check for any other codes 🎬 Watch: Step-by-step guide to diagnosing a 6.0L LS misfire. like P0300, P0171, or P0174.
2. Listen to the engine while running. Is there a consistent ticking, squeaking, or chirping noise? This could point directly to the TSB-related valvetrain issue.
3. Visually inspect the wiring and connector for the cylinder 8 ignition coil and fuel injector. Check the main engine ground strap (G103) on the passenger-side cylinder head for corrosion.
4. To quickly test the ignition coil, swap the coil from cylinder 8 with one from another cylinder (like cylinder 6). Clear the codes, run the engine, and see if the misfire code moves to P0306. If it does, the coil is the problem.
5. If the misfire remains on cylinder 8, remove and inspect the spark plug. Look for excessive wear, fouling (oil, fuel, or coolant), or damage. Ensure the replacement is the correct part (ACDelco 41-162) and is pre-gapped to 0.040".
6. Inspect the spark plug wire for any visible damage or test its resistance with a multimeter.
7. If ignition components are good, test the fuel injector. Use a noid light to check for an electrical signal. Consider running a fuel injector cleaner through the system.
8. If the cause is still not found, perform a compression test on cylinder 8 to check for mechanical engine problems like bad rings or valve issues. A leak-down test can further pinpoint the source of compression loss.
9. If compression is good but a persistent ticking noise is present, refer to TSB #19-NA-219 and inspect the valvetrain for a worn camshaft lobe or lifter. This step is invasive and may require a professional mechanic.

Parts You'll Likely Need

• Ignition Coil (OEM #12570616) — Ignition coils are a very common failure point for misfire codes. Heat and vibration cause them to fail over time. The OEM part is often cross-referenced with D510C.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Iridium Spark Plug (OEM #41-162) — Spark plugs are a maintenance item and the direct source of ignition in the cylinder. A worn or fouled plug is a primary cause of misfires. This part number supersedes the older 41-110 and 41-985 plugs. The recommended gap is 0.040".
  Trusted brands: ACDelco, NGK
  OEM price range: $8-$15
  Aftermarket price range: $6-$12
• Spark Plug Wire Set (OEM #9748HH) — Wires carry the high voltage from the coil to the plug. Degradation can cause a weak spark, especially on the hot, rear cylinders.
  Trusted brands: ACDelco, NGK, MSD
  OEM price range: $60-$100
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire Detected) may be stored alongside P0308.
• P0171 / P0174 — If the misfire is caused by a vacuum leak, such as a bad intake manifold gasket, the engine will run lean and may trigger these codes for 'System Too Lean' on Bank 1 and/or Bank 2.

Technical Service Bulletins (TSBs) & Recalls

• 19-NA-219: Supersedes older bulletins. Details diagnosis for engine misfire (P0300-P0308) accompanied by a chirp, squeak, or tick noise. Points to worn camshaft lobes, lifters, sticking valves, or broken valve springs as potential causes.
• PIP4138L: The older, commonly referenced TSB that first documented the link between P030x codes and worn camshafts/lifters. Now replaced by 19-NA-219.
• PIP5628E: Mentions P0300-P0308 codes in the context of diagnosing engine misfires and providing necessary information to GM's Technical Assistance Center (TAC).

Platform-Specific Known Issues

• A known issue, documented in GM TSB #19-NA-219 (superseding PIP4138L), points to a worn camshaft lobe and/or lifter as a potential cause for P03

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 55-62 PSI. Failure: Pressure below 55 PSI or pressure that bleeds off rapidly can indicate a weak fuel pump or leaking injector.
• Ignition Coil Primary Resistance — expected: ~0.4 - 2.0 Ohms. Failure: A reading of 0 or infinite ohms indicates a failed coil, though resistance tests are not always conclusive for these coils.
• Ignition Coil Secondary Resistance — expected: 6,000 - 10,000 Ohms (6k-10k Ω). Failure: Readings significantly outside this range suggest an internal coil winding issue.
• Engine Compression (at sea level) — expected: 120-150 PSI, with less than 15% variation between cylinders.. Failure: A reading significantly below 120 PSI on cylinder 8 points to a mechanical issue like worn rings, valves, or a damaged piston.

Hidden / Shadow Codes Worth Checking

• Misfire Current/History Counters: While not a separate DTC, a GM Tech2 or similar advanced scan tool can display 'Misfire Current Counters' and 'Misfire History' for each cylinder. This data is more sensitive than the trigger for a P0308 and can show low-level misfires on other cylinders that haven't yet set a code, which can help diagnose a systemic issue like low fuel pressure. (see via GM Tech2, GDS2, or other professional-grade scan tool with GM-specific software.)

Scan Tool Commands That Help

• GM Tech2 / GDS2: Crankshaft Position (CKP) Variation Relearn — This procedure is required after replacing the crankshaft position sensor, ECM, or making major engine repairs. It synchronizes the ECM with the minor variations of the crankshaft reluctor wheel. Failure to perform this can result in inaccurate misfire detection and may cause false P0300-series codes.
• GM Tech2 / GDS2: Fuel Injector Balance Test — This test helps diagnose a faulty or clogged fuel injector when swapping the injector is difficult. The tool individually fires each injector and measures the drop in fuel rail pressure. If the pressure drop for cylinder 8 is significantly different from the others, it indicates an injector problem.
• GM Tech2 / GDS2: Cylinder Power Balance — This function allows the user to disable one cylinder at a time and observe the drop in engine RPM. A cylinder that causes little or no RPM drop when disabled is not contributing power, confirming the misfire location and severity in real-time.

Wiring & Ground Locations

• G103 — On the front of the passenger-side (right side) cylinder head. It is a ground stud with one or more ring terminals attached.. This is a primary ground point for the ignition coil harness on Bank 2 (which includes cylinder 8). A loose or corroded connection at G103 can cause a weak or intermittent spark for all cylinders on that bank, often leading to misfire codes like P0308.
• G104 — On the rear of the driver's-side (left side) cylinder head.. While not directly related to cylinder 8, a poor connection here can cause erratic behavior in the ECM and other sensors, indirectly contributing to misfire detection issues. It's a key engine ground worth checking.
• Cylinder 8 Ignition Coil Connector — The electrical connector on the ignition coil for the rearmost cylinder on the passenger side of the engine.. The plastic locking tab can become brittle from heat and break, leading to a poor connection. The wiring itself can also become chafed or damaged in this tight, hot location.

Real Owner Repair Stories

• Forum User Reports (Various 6.0L GM Trucks (Silverado, Sierra)) — Persistent P030x misfire code, often accompanied by a 'ticking' or 'sewing machine' sound from the engine.
  ❌ Tried (didn't work) Replacing spark plugs, Replacing spark plug wires, Swapping and replacing ignition coils, Replacing fuel injectors
  ✅ What actually fixed it Replacement of a collapsed or stuck lifter and the corresponding camshaft, which had a worn-down lobe. This matches the issue described in GM TSB #PIP4138L.
• Forum User Reports (High-mileage 6.0L GM Truck) — Persistent P0308 misfire that was not resolved by ignition or fuel system parts.
  ❌ Tried (didn't work) Spark plugs, wires, ignition coil, Fuel injector swap
  ✅ What actually fixed it A compression and leak-down test revealed a major internal problem. Upon engine disassembly, a cracked piston skirt was found on cylinder 8. The fix required replacing the piston and honing the cylinder.
• Reddit user r/ChevyTrucks (2011 Suburban with 220k miles (similar engine architecture)) — P0308 misfire code.
  ❌ Tried (didn't work) Initial thought was to replace ignition components.
  ✅ What actually fixed it The user discovered that the heater core quick-connect fittings were leaking coolant directly onto the #8 spark plug wire. Fixing the coolant leak and cleaning the wire resolved the misfire without replacing any ignition parts.

OEM Part Supersession History

• 12558693 (ACDelco D581/D585) → 12570616 (ACDelco D510C) — Design change from the early 'heat sink' or 'round' style coil to the more common 'square' style coil.
  Heads up: While electrically compatible, the mounting brackets and physical shape are different. Some enthusiasts consider the older D585 'heat sink' coils to be more durable and seek them out, but the D510C is the standard modern replacement.

Model Year Variations Within This Range

• 2002-2006: The Sierra Denali could be equipped with either the LQ4 (9.4:1 compression) or the higher-output LQ9 (10.0:1 compression) 6.0L V8. The LQ9 used flat-top pistons to achieve its higher compression. While diagnosis for P0308 is identical, it's a key difference in engine hardware. The VIN's 8th digit can identify the engine ('U' for LQ4, 'N' for LQ9).
• 2005-2006: Starting in 2005, the LQ4 and LQ9 engines were equipped with stronger, full-floating connecting rods compared to the pressed-pin design of earlier years. This does not affect P0308 diagnosis but is a notable internal engine change.