P0308 on 2000-2007 GMC Sierra 2500 (6.0L V8): Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2007 Gmc SIERRA 2500

On the V8 engines in these GMC Sierra 2500 trucks (primarily the 6.0L LQ4), a P0308 code is most often a straightforward ignition system problem. However, this platform has a known, more serious issue documented in Technical Service Bulletin PIP4138R (later superseded by 19-NA-219). If basic ignition and fuel system checks 🎬 Watch: A professional walkthrough for diagnosing misfires on the 6.0L engine. don't solve the misfire, the cause could be internal engine wear, specifically a worn camshaft lobe and/or a failed lifter. This issue can present with a persistent tick, chirp, or squeak from the engine. Another platform-specific issue is the location of cylinder 8 at the rear passenger side, where leaking heater core coolant lines at the firewall can drip onto the coil pack and wire, causing corrosion and misfires.

Diagnostic Flowchart

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

Generation note: This range covers two generations: the first generation GMT800 (2000-2006 and 2007 "Classic") and the second generation GMT900 (2007). The primary V8 engine, the 6.0L LQ4, was used throughout this period. While engine designs are similar, there were different ignition coil designs (square vs. round style), so it's important to verify the correct part for your specific year.

Symptoms You May Notice

• Rough or shaking idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Check Engine Light is on solid or flashing.
• Smell of raw fuel from the exhaust.
• A persistent ticking, chirping, or squeaking noise from the engine, which could indicate a camshaft/lifter issue.

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are normal wear items. GM recommends replacement at 100,000 miles, but they can fail sooner, causing a single-cylinder misfire. Cylinder 8 is a common failure point.
   How to confirm: Remove the spark plug from cylinder 8 (rear passenger side) and inspect it. Look for a worn electrode, heavy carbon buildup, oil/fuel fouling, or a cracked insulator. Compare it to a plug from a working cylinder.
   Typical fix: Replace the spark plug for cylinder 8. It is best practice to replace all 8 spark plugs at the same time with quality plugs like ACDelco Iridium or Platinum.
   Est. part cost: $8-$20 per plug for OEM-grade Iridium plugs.
2. Faulty Spark Plug Wire 🔴 High Probability
   How to confirm: Inspect the wire for cylinder 8 for any visible damage, cracks, or burn marks, especially where it routes near hot exhaust manifolds. With the engine running in the dark, you can sometimes see arcing from a bad wire to the engine block. A coolant leak from the firewall heater connections is known to drip onto the #8 plug wire, causing failure.
   Typical fix: Replace the spark plug wire. It's highly recommended to replace the entire set of 8 wires at the same time.
   Est. part cost: $40-$80 for a quality aftermarket set (e.g., NGK, Delphi).
3. Defective Ignition Coil 🟡 Medium Probability The individual coil-on-plug design is robust but susceptible to failure from age, heat, and vibration. There were different coil styles used (square vs. round), so visual confirmation is needed before ordering a replacement.
   How to confirm: Swap the ignition coil from cylinder 8 with a coil from another cylinder (e.g., cylinder 6, which is easier to access). Clear the codes and drive the truck. If the code changes to P0306, the coil is bad.
   Typical fix: Replace the faulty ignition coil. Many owners choose to replace just the single failed coil.
   Est. part cost: $30-$70 for a single OEM-quality coil (e.g., ACDelco, Delphi).
4. Clogged or Failing Fuel Injector 🟡 Medium Probability
   How to confirm: Listen to the injector with a mechanic's stethoscope or a long screwdriver; you should hear a consistent clicking sound. You can also test the injector's resistance with a multimeter. A user on a Chevrolet forum noted that the #8 injector, being last on the fuel rail, can be prone to clogging from sediment in the fuel system.
   Typical fix: First, try a high-quality fuel system cleaner (like Seafoam or Techron). If that fails, replace the fuel injector for cylinder 8.
   Est. part cost: $50-$150 for a single injector.

Rare But Worth Checking

• Worn Camshaft Lobe and/or Lifter: This is a well-documented issue for this engine family, detailed in GM TSB #PIP4138R and its successor, 19-NA-219. It often presents as a persistent misfire on a single cylinder that isn't resolved by ignition or fuel system parts. A key symptom is a rhythmic chirp, squeak, or tick noise that follows cam speed (half of engine RPM). Diagnosis requires removing the valve cover to inspect valve train movement. Repair is labor-intensive and expensive, often costing $2,500-$6,000+ at a shop.
• Low Engine Compression: Caused by issues like a leaking head gasket, worn piston rings, or a problem with the cylinder head valves (bent valve, broken spring). A compression test is the definitive way to diagnose this mechanical issue.
• Intake Manifold Gasket Leak: A vacuum leak near the intake runner for cylinder 8 can lean out the air/fuel mixture enough to cause a misfire. This is a common issue on Vortec engines. You can test for this by spraying brake cleaner or starter fluid around the intake manifold gasket for cylinder 8 with the engine running and listening for a change in idle speed.
• Corroded Ignition Coil Wiring Harness: In some cases, particularly after a coolant leak from the heater hoses near the firewall, the electrical connector for the cylinder 8 ignition coil can become corroded, causing an intermittent connection and misfire. A visual inspection of the connector for green or white corrosion is necessary.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0308 is present and note any other codes.
2. Visually inspect the area around cylinder 8 (rear passenger side). Look for obvious issues like a disconnected plug wire, a cracked coil, or signs of coolant leakage from the nearby heater hoses.
3. Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive. If the misfire code moves to the new cylinder (P0306), the coil is faulty.
4. If the coil is good, inspect the spark plug from cylinder 8. If it's old or fouled, replace it. It's best practice to replace all 8.
5. Inspect the spark plug wire for cylinder 8. Check for damage or corrosion. Consider swapping it with another cylinder's wire if you suspect it's faulty.
6. Listen to the fuel injector for cylinder 8 with a stethoscope or long screwdriver. It should have a regular, audible clicking sound. If silent or irregular, it may be faulty.
7. If ignition and fuel components are confirmed to be good, perform a compression test on cylinder 8 to check for mechanical engine problems like bad rings, valves, or a head gasket leak.
8. If all other tests pass and you hear a persistent ticking or chirping noise, refer to GM TSB #PIP4138R/19-NA-219 and investigate the possibility of a worn camshaft lobe or lifter. This may require removing the valve cover for inspection.

Parts You'll Likely Need

• Spark Plug (OEM #ACDelco 41-962 (Professional, Platinum) or 41-110 (Iridium)) — This is the most common failure point for a single-cylinder misfire and is a routine maintenance item.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$25
  Aftermarket price range: $5-$15
• Ignition Coil (OEM #ACDelco D585 (Round 'LS Truck' style, common) or D581/12558693 (Square 'Meltric' style). Verify visually.) — The individual coils on these engines are a frequent cause of misfires due to heat and age.
  Trusted brands: ACDelco, Delphi, Bosch
  OEM price range: $50-$90
  Aftermarket price range: $30-$70
• Spark Plug Wire Set — Wires degrade over time, causing weak spark. It's best to replace them as a set. Cylinder 8's wire is exposed to heat and potential coolant leaks.
  Trusted brands: ACDelco, NGK
  OEM price range: $60-$100
  Aftermarket price range: $40-$80
• Fuel Injector — Can become clogged or fail electronically, preventing the correct amount of fuel from reaching cylinder 8.
  Trusted brands: ACDelco, Bosch, Standard Motor Products
  OEM price range: $90-$180
  Aftermarket price range: $30-$70

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) code may appear alongside P0308.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138R: Addresses engine misfire codes P0300-P0308 possibly caused by a worn camshaft lobe and/or lifter. Notes symptoms can include a chirp, squeak, or tick noise. This TSB was later superseded by 19-NA-219.
• 19-NA-219: Replaces PIP4138R and contains updated model years but the same diagnostic advice regarding misfires caused by internal engine issues like worn cams/lifters.
• PIP5628G: Mentions codes P0300-P0308 in the context of diagnosing engine misfires, indicating it's a common starting point for technician diagnosis.

Platform-Specific Known Issues

• A known potential cause for P0300-P0308 codes on this platform is a worn camshaft lobe and/or lifter, as detailed in GM TSB #PIP4138R. This should be investigated after more common causes are ruled out, especially if a persistent ticking or squeaking noise is present.
• The location of cylinder 8 makes its ignition components (wire, coil) vulnerable to damage from coolant leaking from the heater core inlet/outlet hoses at the firewall.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 55-62 PSI. Failure: Pressure below 55 PSI can cause a lean misfire.
• Ignition Coil Primary Resistance (ACDelco D585 Round Style) — expected: ~0.820 ohms. Failure: A reading significantly higher or lower (open or shorted) indicates a faulty coil.
• Ignition Coil Secondary Resistance (ACDelco D585 Round Style) — expected: ~7.10k ohms. Failure: A reading significantly outside this range suggests an internal coil failure.
• Fuel Injector Resistance (High Impedance Type) — expected: 11-14 Ohms (typically ~12.5 Ohms). Failure: A reading outside of this range indicates a failed injector coil.

Hidden / Shadow Codes Worth Checking

• Misfire Counters (per cylinder): A professional scan tool can display live and historical misfire counts for each individual cylinder, even if the count is not high enough to set a specific DTC. This helps identify intermittent issues or misfires on other cylinders that accompany P0308. (see via GM Tech2, GDS2, or equivalent professional-grade scan tool. A user reported using a Tech2 to see misfire counters on cylinder 1 and 8 when only a P0300 was present.)

Scan Tool Commands That Help

• Tech2 / GDS2 / Professional Scan Tool: Cylinder Power Balance / Fuel Injector Cut-off — This is used to confirm a misfire at idle. The command disables one cylinder at a time (usually by cutting fuel). A good cylinder will cause a noticeable drop in engine RPM; the misfiring cylinder (Cylinder 8) will cause little or no drop in RPM.
• Tech2 / GDS2 / Professional Scan Tool: Crankshaft Position System Variation Learn (CASE Relearn) — This procedure should be performed after replacing a crankshaft position sensor, PCM, or flexplate, or if a persistent P0300/P0308 code exists after all other repairs have failed. The PCM uses this learned profile to accurately detect misfires, and if its stored data is incorrect, it can falsely report them.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine; grounds all 8 ignition coils.. A poor connection at this specific ground can cause weak or erratic spark on one or more cylinders, including cylinder 8, leading to a misfire code.
• G103 / G104 — G103 is on the cowl above the brake booster; G104 is often on the rear of the passenger-side cylinder head.. These are critical grounds for the PCM and various engine sensors. A poor connection can cause erratic PCM behavior and incorrect sensor readings, potentially leading to a false misfire code or poor engine performance.
• PCM Connector C1 (Blue), Pin 43 — On the Powertrain Control Module (PCM), which is typically located in the engine bay on the driver's side fender.. This is the specific pin for the 'Fuel Injector 7 Control' (Note: GM cylinder numbering can be confusing, some diagrams label cylinder 8 as injector 7). For a 2003-2007 truck, the wire is typically Red/Black. Testing for a pulsed ground signal at this pin can confirm if the PCM is commanding the injector to fire.

Real Owner Repair Stories

• The Hull Truth Boating and Fishing Forum (2004 Silverado 6.0L) — P0300 random misfire at speeds over 60mph, flashing check engine light, torque converter unlocking.
  ❌ Tried (didn't work) Replaced air filter, Replaced AC Delco spark plugs and wires, Used fuel injector cleaner, Cleaned MAF sensor, Checked for intake leaks with starting fluid
  ✅ What actually fixed it A dealership performed a 'Crankshaft Relearn' procedure, a firmware update, and a throttle body/injector cleaning service. The misfires stopped immediately after this service.

When the Usual Fixes Don't Work

• While the most common fixes involve replacing the spark plug, wire, or coil for cylinder 8, there are documented cases where these repairs did not solve the problem. In one instance involving a 6.0L engine with a persistent misfire code, the final solution was a 'Crankshaft Relearn' procedure performed with a dealer-level scan tool. This highlights that the PCM's software and its learned values for engine operation can be the root cause, especially if other work has been performed or the battery has been disconnected for a long period.

OEM Part Supersession History

• ACDelco D581 (Square Style) → ACDelco D585 (Round Style) — Design change by GM during the production run.
  Heads up: The coils are not interchangeable without changing the wiring harness connector and bracket. Crucially, the spark plug wires for the square coils are a different length than those for the round coils. You must visually verify which coil style is on the engine before ordering wires.

Model Year Variations Within This Range

• 2000-2007: The ignition coils changed from a square design (Delphi/Melco) to a round 'LS truck' style (ACDelco D585) around 2003-2005. There was an overlap period, so a 2004 model could have either. The mounting brackets, wiring connectors, and required spark plug wire lengths are different between the two styles. Visual inspection is mandatory.