P0308 on 2000-2007 Chevrolet Silverado 2500: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2007 Chevrolet SILVERADO 2500

On the V8 engines in this generation of Silverado 2500, cylinder #8 is tucked away at the rear of the engine bay on the passenger side. 🎬 Watch: This trick makes reaching the #8 spark plug much easier. Its location makes it difficult to access and exposes its ignition components (spark plug, wire, coil) to significant heat from the exhaust manifold, increasing the failure rate of these parts. Forum users have specifically noted coolant from failing heater core quick-connects dripping onto the #8 plug wire, causing a misfire. While ignition faults are most common, these specific GM V8s (especially the 6.0L LQ4) are also subject to internal engine issues, such as worn camshafts or lifters, which can cause this code as documented in official service bulletins. [⭐] Bulletin #PIP4138P

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 Silverado generations: the GMT800 (2000-2006 and 'Classic' 2007) and the GMT900 (new body style 2007). While the fundamental causes of P0308 are similar, the GMT900 platform introduced more complex electronics. Some later GM V8s featured Active Fuel Management (AFM), which could lead to lifter-related misfires, a possibility noted in TSBs for this code family. [⭐] Bulletin #PIP4138P, [⭐] Bulletin #PIP5628F However, AFM was less common on the heavy-duty 6.0L and 8.1L engines in the 2500 series during this specific timeframe.

Symptoms You May Notice

• Rough or vibrating idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Flashing or solid Check Engine Light.
• Decreased fuel economy.
• Smell of raw fuel from the exhaust.
• Engine jerking or bucking under heavy load, like passing.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability The coil-on-plug design is generally reliable, but individual coils can fail from age and heat cycles. Cylinder #8's rear location makes its coil susceptible to high temperatures from the exhaust manifold.
   How to confirm: Swap the ignition coil from cylinder #8 with another cylinder (like #6 on the same bank). Clear the codes and drive the truck. If the code changes to P0306, the coil is the culprit. This is the most common first diagnostic step recommended by owners. 🎬 See how to diagnose a 6.0L misfire using a scan tool.
   Typical fix: Replace the faulty ignition coil. It is held on by a single bolt. The ACDelco D585 is a common OEM replacement.
   Est. part cost: $30-$60
2. Worn or Fouled Spark Plug 🔴 High Probability
   How to confirm: Remove the spark plug from cylinder #8. Inspect it for wear on the electrode (should be sharp, not rounded), cracks in the white porcelain, or heavy deposits of carbon (black, sooty) or oil (wet, black).
   Typical fix: Replace the spark plug. It's recommended to replace all 8 spark plugs if they are near their service interval (typically 100,000 miles). The OEM-recommended ACDelco Iridium or Platinum plugs (e.g., 41-962) are pre-gapped and should not be adjusted.
   Est. part cost: $10-$20 per plug
3. Defective Spark Plug Wire 🟡 Medium Probability The wire for cylinder #8 is long and routed near the hot exhaust manifold. Over time, the wire's insulation can break down, causing the spark to arc to the engine block instead of reaching the plug. Coolant drips from heater hoses have also been reported to damage this specific wire.
   How to confirm: With the engine running in a dark, well-ventilated area, look for visible blue sparks arcing from the wire to a metal engine part. Inspect the wire for physical damage, burns, or brittleness. Test its resistance with a multimeter set to ohms (Ω); a common rule of thumb is under 12,000 ohms per foot of length.
   Typical fix: Replace the spark plug wire. It's best practice to replace the entire set of 8 wires at the same time.
   Est. part cost: $50-$100 for a set
4. Clogged or Failed Fuel Injector ⚪ Low Probability On returnless fuel systems, debris can sometimes accumulate at the end of the fuel rail, which is where the cylinder #8 injector is located.
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should make a consistent ticking sound. You can also check its resistance with a multimeter or use a 'noid light' to confirm the electrical signal is reaching it. A professional can perform a fuel balance test for a definitive diagnosis.
   Typical fix: Replace the fuel injector for cylinder #8. This requires depressurizing the fuel system and removing the fuel rail. Many mechanics recommend replacing all injectors on that bank or all 8 at once for balanced fueling.
   Est. part cost: $40-$80

Rare But Worth Checking

• Worn Camshaft Lobe or Failed Lifter: This is a significant mechanical failure but is a documented issue on these engines, particularly those with Active Fuel Management (AFM), though it can occur on non-AFM engines as well. If basic ignition and fuel system checks do not resolve the misfire, this should be investigated. A compression test is the next step. This issue is specifically mentioned in GM Technical Service Bulletin #PIP4138P as a cause for P0300-P0308 codes, often accompanied by a chirp or tick noise. [⭐] Bulletin #PIP4138P
• Intake Manifold Gasket Leak: A vacuum leak near the intake port for cylinder #8 can lean out the air/fuel mixture, causing a misfire. This was a common issue on the 1999-2006 (GMT800) engines, often presenting as a rough idle when cold that improves as the engine warms up. Diagnosis can be done by spraying brake cleaner near the gasket with the engine running and listening for an RPM change, or by using a smoke machine. The original orange gaskets were prone to failure and were replaced by an updated teal green design.
🎬 Watch: Step-by-step guide to replacing your intake manifold gaskets.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0308 and check for any other codes.
2. Note if the Check Engine Light is flashing. If so, avoid driving to prevent catalytic converter damage.
3. Perform a visual inspection of the cylinder 8 area (rear passenger side). Look for loose connectors, damaged wires, or signs of coolant or oil leaks.
4. Swap the ignition coil from cylinder #8 to cylinder #6 (one forward on the same side). Clear codes, run the engine, and see if the misfire code changes to P0306. If it does, the coil is bad.
5. If the code remains P0308, swap the spark plug from cylinder #8 to cylinder #4. If the code moves to P0304, the plug is bad. Inspect the old plug for wear, fouling, or damage.
6. If the code remains P0308, inspect the spark plug wire. Check for arcing in the dark or test resistance with a multimeter (should be <12k ohms/foot).
7. If ignition components are good, check the fuel injector. Listen for a steady clicking. Use a noid light to verify the electrical pulse from the ECM.
8. If the cause is still not found, perform a compression test on cylinder #8 to rule out mechanical engine problems like a bad valve, piston ring, or a worn camshaft/lifter. Low compression points to an internal engine issue.
9. If compression is good, check for a vacuum leak around the intake manifold gasket for cylinder #8 using a smoke machine or by carefully spraying carb cleaner and listening for an engine RPM change.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D585 (GM 10457730)) — This is one of the most common failure points for a single-cylinder misfire and is easy to diagnose by swapping cylinders. The D585 is a well-known, robust coil design used on many GM V8s.
  Trusted brands: ACDelco, Delphi
  OEM price range: $45-$70
  Aftermarket price range: $30-$50
• Spark Plug (OEM #ACDelco 41-962 (Platinum) or 41-110 (Iridium)) — Spark plugs are a routine maintenance item and a frequent cause of misfires. They are inexpensive to replace. The original plugs have a 100,000-mile service life.
  Trusted brands: ACDelco Iridium/Platinum, NGK Iridium IX
  OEM price range: $12-$20
  Aftermarket price range: $8-$15
• Spark Plug Wire Set — The wire for cylinder #8 is prone to heat damage. Wires are a common wear item and should be replaced with the spark plugs if they are old or show any signs of damage.
  Trusted brands: ACDelco, NGK
  OEM price range: $70-$120
  Aftermarket price range: $50-$100

Related Codes That Often Appear With This One

• P0300 — P0300 means 'Random/Multiple Cylinder Misfire.' It often appears alongside a specific cylinder code like P0308 if the misfire is severe or intermittent enough to affect other cylinders or be difficult for the ECM to isolate.
• P0171 or P0174 — These codes indicate a 'System Too Lean' condition on Bank 1 or Bank 2. They can appear with P0308 if the root cause is a vacuum leak, such as a bad intake manifold gasket, which allows unmetered air into the engine.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138Q: (Supersedes PIP4138P) Notes that for P0300-P0308 codes where the cause isn't found via normal diagnostics, technicians should inspect for worn camshaft lobes, lifter rollers, sticking valves, or broken valve springs.
• PIP5628F: Mentions that specific diagnostic information may be required for P0300-P0308 codes related to misfires.
• 15-06-01-002H: Discusses engine misfire (P0300) and tick noises caused by failed Active Fuel Management (AFM) lifters, a known issue on some GM V8s.

Platform-Specific Known Issues

• Per TSB #PIP4138Q, if standard diagnostics do not find the cause, a worn camshaft lobe and/or lifter should be inspected as a potential cause for a P0308 code, especially if a ticking or chirping noise is present.
• The physical location of cylinder #8 at the rear passenger side makes access for diagnosis and repair more difficult than for other cylinders. A variety of socket extensions and swivels may be required.
• The intake manifold gaskets on GMT800 models (1999-2006) were a known failure point, with the original orange gaskets being replaced by an improved teal-colored design (GM P/N 89017852). A leak here often causes a cold-start misfire that improves as the engine warms up.

Mechanic-Grade Diagnostic Values

• Ignition Coil Secondary Resistance (ACDelco D585 type) — expected: Approximately 6.1k Ohms. Failure: Significantly higher or lower resistance, or an open circuit (infinite resistance).
• Fuel Injector Coil Resistance (OEM type) — expected: 12 to 12.5 Ohms. Failure: Resistance outside of this range indicates a failing injector coil.
• Fuel Pressure (Key-On, Engine-Off) — expected: 55 to 62 PSI. Failure: Pressure below this range can indicate a weak fuel pump, which may contribute to lean misfires under load.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test ID $A9 (Cylinder 8 Misfire Data): Professional scan tools can access Mode 6 data, which stores raw, non-CEL-triggering information. Test ID $A9 tracks the misfire count for cylinder 8 for the current and last 10 driving cycles. This can reveal intermittent misfires that haven't yet met the threshold to set a P0308 code. (see via A professional-grade scan tool with Mode 6 capability.)

Scan Tool Commands That Help

• Tech2 / GDS2 (GM Dealer Tools): Crankshaft Position (CKP) Variation Learn — This procedure is required if the crankshaft position sensor, engine, or harmonic balancer is replaced. Failure to perform the relearn can result in inaccurate misfire detection and may set a P1336 code, preventing the resolution of misfire codes like P0308. The procedure involves bringing the engine to operating temperature and revving it to a specific RPM (approx. 4000 RPM) until the fuel cut-off is activated.
• Tech2 / GDS2 (GM Dealer Tools): Fuel Injector Balance Test — This bidirectional command allows the technician to individually fire each injector and measure the corresponding drop in fuel pressure. If cylinder #8 shows a significantly different pressure drop compared to others, it confirms a faulty or clogged injector.

Wiring & Ground Locations

• G102 / G103 / G104 — These are primary engine and chassis grounds. G103 and G104 are typically found on the back of the cylinder heads. Another critical ground strap runs from the firewall to the back of the engine block on the driver's side. A key PCM ground is located on a stud on the front-left of the engine block.. The ignition coils are grounded to the cylinder heads via their mounting brackets and a dedicated ground wire in the harness. A corroded or loose ground at the back of the engine (G103/G104) can cause a weak or erratic spark specifically on the rear cylinders, including #8, leading to a persistent misfire that is often misdiagnosed as a bad coil.

Real Owner Repair Stories

• Reddit user in r/ChevyTrucks (2011 Suburban (similar 5.3L engine and layout)) — P0308 code.
  ❌ Tried (didn't work) The owner was considering replacing ignition components.
  ✅ What actually fixed it The heater core quick-connect fittings were dripping coolant directly onto the #8 spark plug wire. Fixing the coolant leak and cleaning the wire resolved the P0308 code without replacing any ignition parts.

OEM Part Supersession History

• GM 89017589 (Orange/Black Gaskets) → GM 89060413 (Green/Metal Gaskets) — The original composite intake manifold gaskets were prone to deterioration from heat and fluids, causing vacuum leaks. The updated part is a more robust design.
  Heads up: The new part is a direct replacement for the old one.
• ACDelco D585 / GM 10457730 (Round Coil) → While not a direct supersession, it's critical not to use the square-style coils (like D581/UF271) from later GMT900 trucks on these GMT800 models. — The mounting bracket and heat dissipation characteristics are different.
  Heads up: The square coils may physically bolt on but can overheat and fail prematurely on the GMT800 intake manifold because they were designed for the different airflow and heat characteristics of the newer body style trucks. Always use the round D585/UF262 style coil.

Model Year Variations Within This Range

• 2007: In 2007, GM sold both the old GMT800 platform as the 'Silverado Classic' and the new GMT900 platform as the 'New Body Style'. While both could have a 6.0L V8, they are fundamentally different trucks with different frames, electronics, and some engine components. When ordering parts for a 2007, it is critical to confirm if it is a 'Classic' (looks like a 2006) or the redesigned new model.