P0308 on 2016-2018 Cadillac CTS-V: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2016-2018 Cadillac CTS-V

The 2016-2018 CTS-V uses the high-output 6.2L Supercharged LT4 V8, an engine that produces immense power and heat. This environment puts significant stress on ignition components. Cylinder 8's location at the rear passenger side of the engine bay makes it particularly susceptible to heat soak from the exhaust manifold, which can accelerate the degradation of the spark plug, wire, and ignition coil. GM has issued specific Technical Service Bulletins (TSBs) for this platform related to misfires, highlighting issues with spark plug wires not seating correctly (PIP5159C) and, more seriously, potential valvetrain wear involving camshafts and lifters (PIP4138R, superseded by 19-NA-219). These platform-specific issues go beyond the typical spark plug or coil failure seen on other vehicles.

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
• Solid or flashing Check Engine Light
🎬 Watch: A quick breakdown of P0308 causes and fast fixes.
• Fuel smell from the exhaust
• Audible ticking or chirping noise from the engine (may indicate a lifter or camshaft issue)

Most Likely Causes

1. Loose or Damaged Spark Plug Wire 🔴 High Probability → Shop Spark Plug A manufacturer TSB (#PIP5159C) specifically calls out this issue across all Gen V GM V8s, including the LT4. The 'C' clip that secures the wire to the ignition coil can be missing or damaged, preventing a secure connection and causing misfires under vibration. Heat from the nearby exhaust can also degrade the wire's boot and insulation over time.
   How to confirm: Visually inspect and physically check the spark plug wire for cylinder 8 at the ignition coil. Ensure it clicks firmly into place. If it's loose, inspect the wire's connector for a missing or damaged 'C' clip as shown in the TSB diagrams.
   Typical fix: Reconnect the wire securely. If the 'C' clip is missing or the wire is damaged, replace the spark plug wire. It is often recommended to replace the full set. Upgraded heat-shielding on aftermarket wires is a popular preventative measure.
   Est. part cost: $60-$150 for a set
2. Failed Spark Plug 🔴 High Probability → Shop Spark Plug The high cylinder pressures and heat in the supercharged LT4 engine can lead to a shorter service life for spark plugs compared to naturally aspirated engines. The OEM ACDelco Iridium plugs are robust, but they are still a primary wear item.
   How to confirm: Remove the spark plug from cylinder 8. Inspect it for wear (worn electrode), carbon fouling, oil, or cracks in the porcelain insulator. To be certain, you can swap it with a plug from another cylinder (e.g., cylinder 6) and see if the misfire code follows (to P0306).
   Typical fix: Replace the spark plug. The OEM recommended plug is an ACDelco Iridium 41-128 or equivalent. It is best practice to replace all 8 plugs at the same time to ensure consistent performance. The correct gap is typically 0.040 inches.
   Est. part cost: $15-$30 per plug
3. Failed Ignition Coil 🟡 Medium Probability → Shop Ignition Coil Engine bay heat, especially concentrated at the rear cylinders like #8, can cause ignition coils to fail over time. This is a very common failure point for single-cylinder misfires on many GM V8s.
   How to confirm: Swap the ignition coil from cylinder 8 with another easily accessible cylinder (e.g., cylinder 6). Clear the codes and run the engine. If the code changes to P0306, the ignition coil is faulty. This is the definitive DIY diagnostic step.
   Typical fix: Replace the failed ignition coil. Many technicians recommend replacing the spark plug at the same time. The OEM part is ACDelco D510C (GM #12611424).
   Est. part cost: $40-$90
4. Worn Camshaft Lobe and/or Lifter ⚪ Low Probability → Shop Engine Camshaft A manufacturer TSB (#19-NA-219, which replaces #PIP4138R) indicates this as a potential cause for misfires if other diagnostics do not find a fault. The LT4 engine in the CTS-V has Active Fuel Management (AFM), which uses special lifters on cylinders 1, 4, 6, and 7 that can be a point of failure. While cylinder 8 does not have an AFM lifter, a worn camshaft lobe affecting cylinder 8 can still occur and will cause a persistent misfire and often a ticking or chirping sound. 🎬 Watch: Identifying lifter tick symptoms on a CTS-V engine.
   How to confirm: This requires advanced mechanical diagnosis. It typically involves removing the valve covers to measure valve lift and inspect the valvetrain movement. A consistent ticking noise that doesn't go away when disabling the fuel injector for that cylinder is a strong indicator. Further engine disassembly is required to visually inspect the camshaft and lifters.
   Typical fix: Replace the worn camshaft and any failed lifters. This is a significant and labor-intensive repair, often costing thousands of dollars at a shop.
   Est. part cost: $500-$1500+

Rare But Worth Checking

• Clogged or Failed Fuel Injector: → Shop Fuel Injector The LT4 uses a direct injection system. While less common than ignition issues, a fuel injector stuck open or closed will cause a persistent misfire on a single cylinder. This should be investigated if ignition components and compression are good.
• Low Cylinder Compression: Indicates a more serious mechanical engine problem like a bad valve, broken valve spring, worn piston rings, or a failed head gasket. A compression test is needed to confirm this after ruling out simpler causes.
• Water Intrusion in Coil Connector: A real-world scenario reported on CaddyInfo forums involved a P0308 misfire appearing immediately after washing the engine bay. The fix was to disconnect the cylinder 8 coil connector and thoroughly dry it with compressed air.

Diagnosis Steps

1. Read the code with an OBD-II scanner and confirm P0308 is present. 🎬 See this guide on diagnosing and fixing cylinder 8 misfires. Note if the check engine light is solid or flashing. A flashing light means stop driving.
2. Locate cylinder 8: It is the rearmost cylinder on the passenger side of the engine.
3. Visually inspect the spark plug wire for cylinder 8. Per TSB #PIP5159C, ensure it is fully seated on the ignition coil and check for a missing 'C' clip on the coil end of the wire. Check for any signs of arcing or heat damage on the boot near the exhaust manifold.
4. Swap the ignition coil from cylinder 8 with cylinder 6 (the next one forward). Clear the codes and drive the vehicle. If the code changes to P0306, the coil has failed. This is the most effective first test.
5. If the code remains P0308, swap the spark plug from cylinder 8 with another cylinder (e.g., cylinder 4). If the code moves to P0304, the spark plug is the issue. Inspect the old plug for wear, fouling, or damage.
6. If the misfire remains on cylinder 8, inspect the fuel injector. Listen for a consistent clicking sound using a mechanic's stethoscope. Advanced diagnosis may require checking injector balance rates with a high-end scan tool.
7. Perform a cylinder compression test to check for mechanical engine problems like bad rings or valves.
8. If all other tests pass and especially if a ticking noise is present, refer to TSB #19-NA-219 and investigate for potential camshaft or lifter issues, which requires professional-level disassembly and inspection.

Parts You'll Likely Need

• Spark Plug (OEM #12621258 (ACDelco 41-110) or 41-128) — Spark plugs are a common wear item, and the high-performance nature of the LT4 engine can accelerate this wear, making it a primary cause of single-cylinder misfires.
  Trusted brands: ACDelco, NGK
  OEM price range: $15-$30
  Aftermarket price range: $10-$20
• Spark Plug Wire Set (OEM #19431551 or 12716289) — A specific TSB (#PIP5159C) points to loose spark plug wires as a known cause of misfires on this engine. Heat can also degrade the wires over time.
  Trusted brands: ACDelco, MSD, Katech
  OEM price range: $60-$150
  Aftermarket price range: $80-$200

Related Codes That Often Appear With This One

• P0300 — P0300 indicates a random or multiple cylinder misfire. It can appear alongside P0308 if the underlying issue (like a failing camshaft or fuel delivery problem) is intermittent or beginning to affect other cylinders.
• P0358 — This code specifically indicates a fault in the ignition coil primary/secondary circuit for cylinder 8. If you see P0358 along with P0308, it points very strongly to a bad ignition coil, a wiring problem to the coil, or a bad connector.

Technical Service Bulletins (TSBs) & Recalls

• PIP5159C: Engine Misfire May Be Due To Loose Spark Plug Wires
• PIP4138R: SES Light With Misfire DTC And/Or Valve Train Tick Noise (Superseded by 19-NA-219)
• 19-NA-219: Diagnostic Tip for Misfire, Chirp, Squeak, Squeal or Tick Noise

Platform-Specific Known Issues

• TSB #PIP5159C: Notes that a Service Engine Soon light with DTCs P0300-P0308 may be caused by a loose spark plug wire. Technicians are advised to inspect for a missing 'C' clip on the wire end that connects to the ignition coil and replace the wire if the clip is missing or damaged.
• TSB #19-NA-219 (replaces PIP4138R): For vehicles with a persistent misfire code (P0300-P0308) and/or a tick/chirp noise where standard diagnostics do not find a cause, this bulletin directs technicians to check for a worn camshaft lobe and/or lifter.

Mechanic-Grade Diagnostic Values

• Injector Pulse Width (Direct Injection) — expected: Highly variable. At idle, can be 1.2-2.0ms. At wide-open throttle near redline, it should ideally stay below 6.0ms.. Failure: A pulse width exceeding 6.5ms under load suggests the fuel system (injectors or pumps) cannot keep up with demand, which can cause a lean misfire.
• Fuel Trims (Short Term and Long Term) — expected: Within +/- 10% at idle and steady cruise.. Failure: If a single bank's fuel trims are consistently high (e.g., +15% or more), it can indicate an unmetered air leak affecting that bank or a fuel delivery problem to that bank. A technician noted that disabling an injector can cause trims on that bank to shoot to +16%, demonstrating the ECM's attempt to compensate.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Counts: The ECU continuously monitors crankshaft speed to detect misfires. It logs a count for each cylinder in a non-continuous monitor area called Mode $06. These counts can be viewed with a capable scan tool before they are high enough to trigger a P030x code and a Check Engine Light. (see via A scan tool that can access Mode $06 diagnostic data. This is a standard OBD-II function, but not all basic code readers display it.)

Scan Tool Commands That Help

• GDS2 (GM Dealer Tool) or equivalent high-end scanner: Cylinder Power Balance — This is a definitive test to confirm a non-contributing cylinder. The tool deactivates each cylinder one by one and measures the drop in engine RPM. If deactivating cylinder 8 causes little or no RPM drop compared to the others, it confirms cylinder 8 is the source of the misfire.
• GDS2 (GM Dealer Tool) or equivalent high-end scanner: Fuel Injector Balance Test — Use this after confirming spark is good. The tool commands each injector to fire and measures the fuel pressure drop in the rail. An injector that shows a significantly different pressure drop than the others is likely clogged or failing.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This is the main grounding point for all eight ignition coils. While a failure here would likely affect multiple cylinders, corrosion or a loose connection specific to the cylinder 8 ground wire within this bundle could cause an isolated, intermittent spark issue.
• G103 — Located at the left rear of the engine compartment on the cowl, above the brake booster.. This is a primary ground for the Engine Control Module (ECM). A poor connection here can cause a wide range of unpredictable issues, including false misfire detection.

Real Owner Repair Stories

• CAMARO6 forum user 'LiqTenExp' (Chevrolet Camaro with LT4 engine (same engine as CTS-V)) — Misfire on a single cylinder (#2 in this case) at higher RPMs and under load, accompanied by some knock retard (KR).
  ❌ Tried (didn't work) Swapped spark plug, Swapped spark plug wire, Swapped ignition coil, Inspected valvetrain
  ✅ What actually fixed it The root cause was a bad fuel injector.

OEM Part Supersession History

• 12621258 (ACDelco 41-110) → Unchanged, but often cross-referenced with 41-128 and 12680072. — Standard OEM part for many Gen IV and Gen V V8s.
  Heads up: While other part numbers like 41-101 may be listed by vendors for the 'CTS', they may be for the previous generation LSA engine or non-V models. It is critical to use the spark plug specified for the LT4 engine to ensure correct heat range and performance.
• 23377989 (ACDelco M100183) → Unchanged — This is the standard in-tank low-pressure fuel pump for the LT4 engine.
  Heads up: This pump is specific to the LT4-equipped models (CTS-V, ZL1) and is different from the pump used in LT1 models. The fuel tanks are also different between the models.

Model Year Variations Within This Range

• 2017: Minor software calibrations were introduced that slightly improved performance, but these did not change the fundamental engine hardware or common causes for P0308.