P0308 on 2008-2015 Chevrolet Impala: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2008-2015 Chevrolet IMPALA

The P0308 code is unique to the Impala SS model (2006-2009) equipped with the 5.3L LS4 V8 engine. This engine features Active Fuel Management (AFM), which deactivates certain cylinders to improve fuel economy. While AFM lifter failure is a notorious problem on these engines, cylinder 8 is not one of the AFM-controlled cylinders (AFM cylinders are 1, 4, 6, and 7). However, a known oil consumption issue related to the AFM pressure relief valve can cause oil spray within the crankcase, leading to carbon deposits on piston rings and oil fouling on spark plugs on *any* cylinder, including number 8. This makes oil-fouled plugs a more common cause on the LS4 than on other engines.

Diagnostic Flowchart

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

Generation note: The 2008-2015 year range covers the end of the 9th generation (2006-2016) and the start of the 10th generation (2014-2020) Impala. However, the P0308 code is only possible on the 9th generation Impala SS model, produced from 2006-2009, as it was the only model in this range with a V8 engine (and thus, a cylinder 8).

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough or shaking idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Smell of unburnt fuel from the exhaust
• Increased vehicle emissions

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Coil-on-plug systems are common failure points on many vehicles as they age due to heat and vibration. The rear cylinders on the transverse LS4, like cylinder 8, can experience higher temperatures, potentially accelerating wear.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty.
   Typical fix: Replace the faulty ignition coil. The OEM part is often an ACDelco D510C.
   Est. part cost: $30-$60
2. Worn or Fouled Spark Plug 🔴 High Probability The LS4 engine's AFM system can cause increased oil consumption. An AFM pressure relief valve can spray oil, leading to carbon buildup and oil fouling on spark plugs, even on non-AFM cylinders like #8. TSB #11-06-01-007 specifically addresses this issue.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for signs of wear, corrosion, or black, oily deposits. A healthy plug should be a light tan color.
   Typical fix: Replace the spark plug. It is recommended to replace all 8 plugs at the same time for even performance.
   Est. part cost: $5-$15 per plug
3. Bad Fuel Injector 🟡 Medium Probability Injectors can become clogged or fail electrically over time. On the LS4, cylinder #8's injector is located under the alternator, making it one of the most difficult to access and replace.
   How to confirm: Use a mechanic's stethoscope or a long screwdriver to listen to the injector on cylinder 8. It should have a consistent, audible clicking sound. If it's silent or irregular, it may be faulty. You can also swap the injector with another cylinder, though this is more labor-intensive for cylinder 8.
   Typical fix: Replace the faulty fuel injector. This may involve removing the fuel rail. 🎬 See this walkthrough for replacing the difficult cylinder 8 injector.
   Est. part cost: $50-$100
4. Damaged Spark Plug Wire ⚪ Low Probability Wires can degrade from heat and age. A forum user with a P0302 on an '08 Impala SS found a cut wire was the culprit, highlighting the possibility.
   How to confirm: Visually inspect the wire for cylinder 8 for any cracks, burns, or damage. Test its resistance with a multimeter; it should be within the manufacturer's specified range. Aftermarket performance wires can have a resistance as low as 40-50 ohms per foot.
   Typical fix: Replace the spark plug wire. It's often sold in a set.
   Est. part cost: $40-$80 for a set

Rare But Worth Checking

• Low Cylinder Compression: This indicates a more serious internal engine problem, such as a bad valve, bent pushrod, worn piston rings, or a head gasket leak. A compression test is needed to confirm.
• Vacuum Leak: A vacuum leak is more likely to cause a random misfire code (P0300), but if a leak is isolated near the intake runner for cylinder 8, it could cause a single-cylinder misfire.
• Wiring Harness Chafing: In some cases, the injector wiring harness can rub against engine components, causing an intermittent short. One owner of an '08 Impala SS with a P0201 (Injector Circuit Fault) found the harness had rubbed through on an intake manifold bolt.

Diagnosis Steps

1. Retrieve all stored DTCs with an OBD-II scanner to see if other codes are present.
2. Visually inspect the ignition coil, spark plug wire, and wiring harness for cylinder 8 for any obvious damage, like cracks, burns, or chafing.
3. Perform an 'ignition coil swap test'. Move the coil from cylinder 8 to a known good cylinder (like cylinder 6). Clear the codes and see if the misfire moves to P0306. If it does, the coil is bad. This is the easiest first step.
🎬 Watch: Two quick DIY methods to fix a P0308 code.
4. If the misfire remains on cylinder 8, remove and inspect the spark plug. Look for excessive wear, damage, or black, oily deposits which are common on the LS4. Replace if necessary.
5. If the plug and coil are good, check the fuel injector. Listen for a steady clicking sound with a stethoscope. A more advanced test is the 'Fuel Injector Balance Test' using a GM Tech 2 or similar high-end scan tool.
6. Check the spark plug wire for cylinder 8 for damage and proper resistance using a multimeter.
7. If all ignition and fuel components check out, perform a cylinder compression test on cylinder 8 to rule out mechanical engine problems like worn piston rings or bad valves. A healthy cylinder should typically show 120-150 PSI.
8. If the problem persists, carefully inspect the wiring harness leading to the injector and coil for any signs of rubbing or damage, as this can cause intermittent faults. A known chafe point is the injector harness rubbing on an intake manifold bolt.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D510C (GM 12611424)) — This is a very common failure point for misfires and is easy to diagnose by swapping cylinders.
  Trusted brands: ACDelco, Delphi
  OEM price range: $40-$70
  Aftermarket price range: $25-$50
• Spark Plug (OEM #ACDelco 41-962 (Iridium)) — Oil fouling from AFM-related oil consumption can cause misfires. Spark plugs are a routine maintenance item and often the culprit.
  Trusted brands: ACDelco, NGK
  OEM price range: $8-$15
  Aftermarket price range: $5-$12
• Fuel Injector — A clogged or failed injector will prevent the correct amount of fuel from entering the cylinder, causing a misfire. Cylinder 8 is particularly labor-intensive to replace.
  Trusted brands: ACDelco, Bosch
  OEM price range: $70-$120
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P0300 — If the condition causing the P0308 is intermittent or beginning to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may also be stored.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A general technical service bulletin where GM requests dealers to gather information on vehicles with misfire codes P0300-P0308, indicating manufacturer awareness of these issues.
• #11-06-01-007: Addresses engine oil consumption on LS4 engines due to the AFM pressure relief valve spraying oil, causing carbon on pistons and fouling spark plugs. The fix involves cleaning pistons and installing a new oil pan gasket. This is a key TSB for understanding a root cause of plug fouling.
• #10-06-01-008G: A similar TSB for other GM V8s with AFM, detailing oil consumption causes and fixes, including piston/ring replacement if cleaning doesn't work.

Platform-Specific Known Issues

• This code is only applicable to the 2006-2009 Impala SS with the 5.3L V8 engine.
• While direct AFM lifter failure does not cause a P0308, the system's tendency to increase oil consumption can lead to oil-fouled spark plugs on any cylinder, including #8.
• Access to components for cylinder 8 (rear bank, passenger side) can be tight. The fuel injector, in particular, is located under the alternator, making replacement more difficult than for other cylinders.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 384-425 kPa (56-62 psi). Failure: Pressure below this range can indicate a weak fuel pump; pressure that drops more than 34 kPa (5 psi) in one minute can indicate a leaking injector or check valve.
• Ignition Coil Primary/Secondary Resistance — expected: Aftermarket examples: ~0.45-0.69 ohms (Primary) and ~5.3k-8.22k ohms (Secondary).. Failure: Readings significantly outside the expected range for the specific coil being tested can indicate an internal failure. Note: OEM specs may vary.
• Cylinder Compression — expected: 120-150 PSI. Failure: A reading significantly lower than other cylinders indicates a mechanical engine problem like worn piston rings, a bad valve, or a head gasket leak.
• Injector Balance Test (Tech 2) — expected: The fuel pressure drop for each injector should be within 20 kPa (3 psi) of the average pressure drop of all cylinders.. Failure: A cylinder whose pressure drop is outside this 3 psi window indicates a fuel injector that is flowing too much or too little compared to the others.

Hidden / Shadow Codes Worth Checking

• Misfire Current Counters / Misfire Graph: This is not a traditional DTC, but rather a live data parameter available in professional scan tools like the GM Tech 2 or GDS2. It shows a real-time count of misfire events per cylinder, which is more granular than a stored P-code and can help diagnose intermittent issues. (see via Access the engine control module's live data or special functions using a GM Tech 2, GDS2, or equivalent high-end diagnostic scanner.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Fuel Injector Balance Test — Use this function when a fuel injector is suspected as the cause of the P0308 misfire. The tool individually pulses each injector and measures the resulting drop in fuel rail pressure. This can definitively identify an injector that is clogged (small pressure drop) or leaking (large pressure drop) compared to the others, without having to physically remove it for testing.

Wiring & Ground Locations

• G101 — On the lower front of the engine block, on the driver's side.. This is a primary ground for the Engine Control Module (ECM) and Transmission Control Module (TCM). A poor connection here can cause a wide range of erratic engine management issues, including misfires.
• G102 — Reported at the left rear of the engine; grounds all 8 ignition coils.. As the dedicated ground for all ignition coils, corrosion or a loose connection at this single point can cause weak spark and misfires on one or more cylinders.
• Injector Harness Chafe Point — The wiring harness for the fuel injectors can rub against the head of an intake manifold bolt.. A documented failure shows the control wire for an injector can rub through its insulation at this point, causing an intermittent short to ground. This will shut down the injector and cause a misfire code for that specific cylinder.
• ECM Location — Inside the air cleaner assembly.. Knowing the physical location is critical for inspecting the main harness connections to the ECM if a wiring issue is suspected.

Real Owner Repair Stories

• YouTube channel 'Pine Hollow Auto Diagnostics' (2008 Chevrolet Impala SS 5.3L V8) — Intermittent Check Engine Light, stored code P0201 (Injector Circuit/Open - Cylinder 1). The diagnostic principle is identical for a P0308 fault on cylinder 8.
  ❌ Tried (didn't work) A previous shop had already replaced the fuel injector pigtail, which did not solve the problem.
  ✅ What actually fixed it The technician found the fuel injector wiring harness was resting on an intake manifold bolt. Over time, engine vibration caused the harness to rub through on the bolt head, exposing the copper of the injector control wire and causing it to intermittently short to ground. The fix was to repair the damaged wire and secure the harness away from the bolt to prevent it from happening again.

When the Usual Fixes Don't Work

• While the most common causes for a P0308 are a faulty spark plug, ignition coil, or fuel injector, there are documented cases where none of these were the root cause. In one instance involving an '08 Impala SS, an intermittent misfire was ultimately traced to the fuel injector wiring harness rubbing through on an intake manifold bolt. This highlights that if standard component swaps do not resolve the issue, a thorough inspection of the wiring harnesses for physical damage and short circuits is a critical next step before suspecting a major mechanical failure.

OEM Part Supersession History

• 12570616 → 12611424 (ACDelco D510C) — Standard part evolution/update.
• 12611424 → 12739916 — Standard part evolution/update.
  Heads up: 12611424 / D510C remains widely available and is the commonly referenced part for this application.

Model Year Variations Within This Range

• 2008-2009: Compared to earlier SS models (2006-2007), the 2008 models introduced Stabilitrak stability control. The terminology for cylinder deactivation also changed from 'Displacement on Demand' (DoD) to 'Active Fuel Management' (AFM), though this was primarily a marketing name change with no functional difference. These changes do not materially affect the diagnosis of a P0308 code.