P0308 on 2005-2009 Chevrolet Trailblazer V8: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2005-2009 Chevrolet TRAILBLAZER

This code only applies to V8-equipped Trailblazer models (5.3L and 6.0L SS). While standard ignition part failures are common, the 2005-2009 5.3L V8 (engine code LH6) uses an Active Fuel Management (AFM) system to save fuel. This system deactivates four cylinders (1, 4, 6, and 7) under light load conditions using special, two-piece hydraulic lifters. These lifters are a well-documented failure point; they can become stuck or collapse due to oil pressure problems or internal mechanical failure. When a lifter on an AFM cylinder fails, it can no longer properly open the valve, leading to a persistent misfire that a simple tune-up will not fix. The issue is often linked to the Valve Lifter Oil Manifold (VLOM), which directs pressurized oil to activate/deactivate the lifters.

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 shaking idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Decreased fuel economy
• A rhythmic ticking or chirping noise from the engine that increases with RPM, especially on 5.3L models, strongly indicating a lifter issue
• Possible fuel smell from the exhaust
• On a V8, symptoms may be less noticeable than on a 4 or 6-cylinder engine, as seven other cylinders are still functioning.

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability The coil-on-plug ignition coils on GM's V8 engines are a common failure item due to age and constant exposure to high heat cycles in the engine bay. Cylinder 8 is at the back of the engine bay, where heat can be more concentrated.
   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 run the engine. If the misfire code changes to P0306, the coil is faulty.
   Typical fix: Replace the failed ignition coil. It is recommended to use a quality OEM brand like ACDelco. The common part number is D510C or UF413.
   Est. part cost: $30-$70
2. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a routine maintenance item. If the vehicle has high mileage and they haven't been replaced according to the 100,000-mile schedule, they are a likely cause. Oil consumption issues related to the AFM system can also foul the plugs prematurely.
   How to confirm: After ruling out the coil, remove the spark plug from cylinder 8. Inspect it for wear (rounded electrode), fouling (black, oily, or white deposits), or an incorrect gap. A fouled plug on an AFM cylinder can be a clue to a larger oil consumption or mechanical issue.
   Typical fix: Replace the spark plug for cylinder 8. It is best practice to replace all 8 spark plugs at the same time. Use the recommended Iridium plugs from a brand like ACDelco or NGK.
   Est. part cost: $8-$20 per plug
3. Failed Active Fuel Management (AFM) Lifter 🟡 Medium Probability This is a significant issue specific to the 5.3L LH6 V8 engine. The complex lifters used by the AFM system can collapse or get stuck due to oil pressure issues, contamination, or internal failure, preventing the valve from opening and causing a persistent misfire. This can be caused by a faulty Valve Lifter Oil Manifold (VLOM) or its gaskets.
   How to confirm: This is a mechanical failure. A key symptom is a rhythmic ticking or chirping noise from the engine that increases with RPM. If ignition and fuel system checks do not resolve the misfire, a compression test on cylinder 8 will likely show low or zero compression. A mechanic can also remove the valve cover to observe valvetrain movement.
   Typical fix: This is a major repair that involves removing the cylinder head to replace the failed lifters. 🎬 Watch this walkthrough on diagnosing and replacing collapsed AFM lifters. Many mechanics recommend replacing all AFM lifters, the camshaft (which is often damaged by the failed lifter), and the VLOM at the same time. An alternative, more permanent fix is to perform an 'AFM delete', which involves replacing the AFM lifters, camshaft, and valley cover with non-AFM components and reprogramming the ECU.
   Est. part cost: $400-$1200+
4. Clogged or Failed Fuel Injector ⚪ Low Probability
   How to confirm: If ignition components are good, check the fuel injector. A 'noid' light can be used to verify the injector is receiving an electrical pulse. Swapping the injector with another cylinder is a definitive test, but more labor-intensive than swapping a coil.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120

Rare But Worth Checking

• Worn Camshaft Lobe: As noted in TSB #PIP4138R, a worn camshaft lobe or lifter roller should be inspected if standard diagnostics don't find the cause. This often occurs in conjunction with a failed AFM lifter on the 5.3L V8, causing a consistent chirp or tick noise and a misfire.
• Intake Manifold Gasket Leak: While more likely to cause a random misfire code (P0300), a vacuum leak from the intake manifold gasket could potentially be isolated enough to primarily affect cylinder 8, causing a lean condition.
• Broken Valve Spring: A broken valve spring can cause a tick noise and an engine misfire that may be intermittent or only occur at certain RPMs. This would also result in a loss of compression.
• Loose or Faulty VLOM (Valve Lifter Oil Manifold): On the 5.3L V8, the VLOM directs oil to the AFM lifters. If the manifold itself is faulty or its bolts have come loose, it can cause an oil pressure drop to the lifters, leading to their failure and a misfire.

Diagnosis Steps

1. Confirm the P0308 code using an OBD-II scanner and check for any other codes. Note any historical misfire data in the freeze frame data.
2. Listen for a rhythmic ticking or chirping noise from the engine bay, which strongly suggests a lifter/camshaft issue on 5.3L models.
3. Identify cylinder 8 (rearmost cylinder on the passenger side).
4. Swap the ignition coil from cylinder 8 with cylinder 6 (the next one forward on the same side). Clear the codes and run the engine. If the code changes to P0306, the ignition coil is bad.
5. If the code remains P0308, swap the spark plug from cylinder 8 with cylinder 6. If the code then moves to P0306, the spark plug was the problem. Inspect the old plug for oil or carbon fouling.
6. Inspect the wiring and connector for the cylinder 8 ignition coil and fuel injector for damage, corrosion, or looseness.
7. If ignition components are confirmed good, perform a fuel injector test using a 'noid' light or by listening for its click with a stethoscope.
8. If the misfire persists, perform an engine compression test on cylinder 8. A significantly lower reading compared to other cylinders (healthy is typically 120-150 PSI) points to a mechanical issue (lifter, valve, piston ring, head gasket).
9. If compression is low on the 5.3L V8, the most likely cause is a failed AFM lifter. Professional diagnosis is recommended to confirm before beginning this extensive repair.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D510C (alt P/N: UF413, 12570616)) — This is one of the most common failure points for a single-cylinder misfire on GM V8s due to heat and age.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Iridium Spark Plug (OEM #ACDelco 41-110 (or 41-162)) — Spark plugs are a regular wear item and are a primary cause of misfires. Iridium is the OEM material for long life.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12
• Active Fuel Management (AFM) Valve Lifter Set (OEM #12645725 (Set of 4 AFM lifters)) — On the 5.3L V8, a collapsed AFM lifter is a very common mechanical cause of a P0308 misfire that ignition parts won't fix. It is recommended to replace all AFM lifters at once.
  Trusted brands: ACDelco, General Motors

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects other cylinders, a P0300 (Random Misfire) code may appear alongside P0308. This can sometimes occur with a partially failed lifter before it fails completely.
• P0430 — This code indicates 'Catalyst System Efficiency Below Threshold (Bank 2)'. A persistent misfire on cylinder 8 (which is in Bank 2) will dump unburned fuel into the exhaust, damaging the catalytic converter and eventually triggering this code.
• P0358 — This code points to a fault in the ignition coil's primary or secondary circuit for cylinder 8. It often appears with P0308 if the coil, its wiring, or the connector is the root cause.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: Mentions diagnostic aid for DTCs P0300-P0308.
• PIP4138R: Recommends checking for a worn camshaft lobe if standard diagnostics fail to find the cause of a P0300-P0308 misfire. (Superseded by 19-NA-219)
• 19-NA-219: This bulletin replaces PIP4138R and provides diagnostic tips for misfires and engine noises, pointing towards worn cam lobes or lifter rollers as a potential cause.
• 10-06-01-008H: While this TSB on excessive oil consumption explicitly excludes the Trailblazer/Envoy/Rainier due to a different oil pan design, it details how oil consumption issues in AFM engines (caused by PCV system or oil spray from the AFM pressure relief valve) can lead to fouled spark plugs and misfires, which is relevant context for the platform.
• 23-NA-043: Discusses engine misfires (P0300-P0308) caused by an internal lifter concern, such as a collapsed lifter or one that has come apart, potentially bending a pushrod.

Platform-Specific Known Issues

• The 5.3L LH6 V8 engine is known for AFM lifter failure, which is a primary mechanical cause for a single-cylinder misfire on an AFM cylinder (1, 4, 6, or 7). While P0308 points to a non-AFM cylinder, the diagnostic process must rule out the more common ignition/fuel issues before considering rarer mechanical faults on cylinder 8.
• Correction: The initial pass 1 article incorrectly implied cylinder 8 was an AFM cylinder. Cylinders 1, 4, 6, and 7 are the AFM cylinders. P0308 on cylinder 8 is therefore most likely an ignition or fuel issue. However, the overall engine health, including oil pressure and potential for camshaft wear related to the AFM system, can still be a contributing factor to issues on other cylinders.
• Oil pressure is critical for the AFM system. A failing oil pump, clogged pickup tube, or a bad oil pressure relief valve can lead to lifter issues. Some owners report that the small filter screen under the oil pressure sensor can become clogged, restricting oil flow and contributing to lifter noise and failure.
• The Trailblazer SS with the 6.0L LS2 engine does NOT have the AFM system, so any P0308 code on an SS model will be due to standard causes like ignition, fuel, or other mechanical issues, not a failed AFM lifter.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range indicates a faulty coil winding.
• Ignition Coil Secondary Resistance — expected: 6,000 to 10,000 ohms (6k-10k Ω). Failure: A reading outside this range suggests the secondary coil winding is compromised.
• Fuel Injector Resistance (5.3L/6.0L V8) — expected: Around 12.5 Ohms. Failure: A significantly higher or lower reading, or an open circuit, indicates a failed injector coil.
• Engine Compression — expected: 120 to 150 PSI, with minimal variation between cylinders. Failure: Significantly low or zero compression in cylinder 8 points to a mechanical failure like a bad lifter, valve, or piston ring.
• Mode 6 Misfire Counts — expected: Zero or very low counts on all cylinders during a drive cycle.. Failure: A high number of misfire counts logged for cylinder 8 (Test ID $A2, Component ID $08) confirms the misfire even if a code is not yet pending.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test ID $A2-$AF, Component ID $08: This isn't a DTC, but a specific data area in the OBD-II system. It shows raw misfire counts for cylinder 8 for the current and last 10 driving cycles. A high count here confirms the misfire is happening, even before the Check Engine Light is triggered. (see via A professional scan tool or an advanced OBD-II app that can display Mode 6 data.)

Scan Tool Commands That Help

• GM Tech 2 / High-End Scan Tool: Cylinder Balance Test (or Power Balance Test) — This function allows the technician to individually disable the fuel injector for each cylinder. When cylinder 8 is disabled, if there is no change in engine RPM or smoothness, it confirms that cylinder was not contributing power in the first place. This helps isolate the issue to a specific cylinder.
• GM Tech 2 / High-End Scan Tool: Misfire Data / Misfire Graphic — This provides a live data display, often as a bar graph, showing misfire counts for each cylinder in real-time. It's used to confirm the misfire is isolated to cylinder 8 and to see if it occurs under specific conditions (idle, load, etc.).

Wiring & Ground Locations

• G103 — On the front of the passenger side (right) cylinder head.. This is a primary engine ground. A loose or corroded connection at G103 can cause erratic behavior and insufficient voltage for components on the passenger side of the engine, including the ignition coil and fuel injector for cylinder 8.
• G104 — On the left rear of the engine block/cylinder head.. This is another critical engine-to-chassis ground. While on the opposite side, it is part of the PCM's grounding scheme. A fault here can cause widespread electrical issues, including communication errors with modules that control fuel and spark, potentially leading to misfires.
• Cylinder 8 Ignition Coil Connector — Plugged into the ignition coil for the rearmost cylinder on the passenger side.. The 4-pin connector supplies power, ground, and the trigger signal from the PCM. Check for 12V+ on the pink wire (key-on), a good ground on the black wire, and test the signal wires (typically two others) for integrity back to the PCM.

Real Owner Repair Stories

• TrailVoy.com forum user (2007 Trailblazer LT, 5.3L V8, 160k miles) — P0308 code, noticeable ticking noise from the engine.
  ❌ Tried (didn't work) Replaced spark plug, Replaced ignition coil
  ✅ What actually fixed it The owner performed a compression test and found low compression on cylinder 8. The final repair was replacing a failed AFM lifter on cylinder 7 (which is an AFM cylinder) that had also caused damage to the camshaft lobe for cylinder 8. Both the lifters and the camshaft were replaced to resolve the P0308 code and the ticking noise.

OEM Part Supersession History

• 12645725 → 12740071 — This is the OEM part number for the AFM/DOD valve lifter. The number has been updated over time due to design revisions aimed at improving reliability.
  Heads up: When replacing failed AFM lifters, it is highly recommended to use the latest part number available from GM and to replace them as a set of four (or all eight hydraulic lifters, both AFM and standard).
• D510C / 12570616 → D514A / 12573190 (for some applications) — GM used several ignition coil designs during this period, including square (D510C) and round/Delphi (D514A) styles. Part numbers were updated for design improvements.
  Heads up: It is critical to visually inspect the existing coils. The square (often marked Melco/Mitsubishi) and round (Delphi) coils are not interchangeable. You must replace with the same style that is currently on the vehicle.

Model Year Variations Within This Range

• 2007-2009: The 5.3L V8 in the Trailblazer was the LH6, which did not have Variable Valve Timing (VVT). Other trucks and SUVs from GM began receiving the LMG version of the 5.3L around 2007, and VVT was added to the LMG in 2010. For the 2005-2009 Trailblazer, the diagnosis for P0308 on the 5.3L remains consistent as it always involves the non-VVT LH6 engine.