P0308 on 2016-2020 Cadillac CT6: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2016-2020 Cadillac CT6

This code exclusively applies to the high-performance CT6-V and CT6 Platinum models equipped with the 4.2L LTA 'Blackwing' twin-turbo V8, a sophisticated and extremely rare engine with only about 1,200 units ever produced. Its 'hot-V' design places the turbochargers within the engine's valley, which can contribute to high temperatures at the rear of the engine bay where cylinder 8 is located. Unlike common GM V8s, widespread issues like AFM lifter failure are not the primary concern. Instead, diagnosis focuses on standard ignition and fuel system components, though some technicians have noted unique fuel injector and carbon buildup issues on this specific engine. The provided TSB #PIP5628G suggests a standardized diagnostic communication process for misfires on this platform, indicating GM's desire for specific data when troubleshooting these rare powerplants.

Diagnostic Flowchart

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

Symptoms You May Notice

• Solid or flashing Check Engine Light
• Rough or shaking engine idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Reduced fuel economy
• A smell of raw fuel from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil The 'hot-V' design of the LTA engine concentrates heat in the engine bay, which can shorten the lifespan of ignition coils. Cylinder 8 is located at the rear of the engine (passenger side), where heat can be most intense.
   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 failed ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $50-$120
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: After ruling out the ignition coil, remove the spark plug from cylinder 8. Inspect it for wear on the electrode, carbon fouling, oil deposits, or cracks in the porcelain insulator. The OEM plug is an ACDelco Iridium plug.
   Typical fix: Replace the spark plug. The OEM specified part is ACDelco 41-104IP. It is best practice to replace all 8 spark plugs as a set to ensure even performance.
   Est. part cost: $15-$30 per plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Some technicians have reported seeing injector issues on the 4.2L Blackwing engine. Direct injection systems operate under very high pressure and can be sensitive to fuel quality and deposits, potentially leading to clogs or failure.
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. A more advanced method involves a fuel injector balance test using a professional scan tool (like the GM GDS2), as recommended in TSB PIP5628G.
   Typical fix: Replace the faulty fuel injector. This is a more involved and costly job than a spark plug or coil.
   Est. part cost: $100-$250

Rare But Worth Checking

• Low Cylinder Compression: This indicates a mechanical engine problem, such as a bad valve or worn piston rings. A compression test is needed to diagnose this. Normal compression is typically between 120-150 PSI. TSB PIP5628G recommends a relative compression test using a PICO scope as a key diagnostic step.
• Wiring Harness Issue: A damaged or corroded wire or a loose connector to the ignition coil or fuel injector for cylinder 8 can cause an intermittent misfire. High heat in the engine bay can make wiring insulation brittle over time.
• Carbon Buildup on Intake Valves: As a direct-injection engine, fuel does not wash over the intake valves. Over time, oil and combustion byproducts can bake onto the valves, restricting airflow and causing misfires, especially under load. A CT6 owner on a forum noted this was suggested as a cause by their mechanic, with walnut blasting being the recommended cleaning method.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes, such as P0300 or P0174.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe misfire that can damage the catalytic converter.
3. Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Cylinder 8 is on the passenger side, at the rear of the engine. Clear the codes and run the engine to see if the misfire code follows the coil (e.g., P0306).
4. If the misfire remains on cylinder 8, inspect the spark plug (OEM Part: ACDelco 41-104IP). Check for wear, damage, or fouling. You can also swap the plug with another cylinder to see if the code follows.
5. If ignition components are good, inspect the wiring and connector for the cylinder 8 ignition coil and fuel injector for any visible damage, corrosion, or looseness.
6. Listen to the cylinder 8 fuel injector with a stethoscope. It should produce a regular, audible clicking sound.
7. Per GM guidance in TSB PIP5628G, advanced diagnosis should involve capturing freeze frame data with a GDS2 scanner and performing a relative compression test with an oscilloscope (PICO scope) to rule out mechanical issues before replacing parts.
8. Based on technician experience, consider checking for exhaust back pressure and taking a fuel sample if a simple ignition or fuel injector fault is not obvious.
9. If a mechanical issue is suspected after a failed compression test, a cylinder leak-down test can pinpoint the source of the leak (piston rings, intake/exhaust valves).

Parts You'll Likely Need

• Ignition Coil — This is one of the most common failure points for a single-cylinder misfire and is a primary wear item, especially in the high-heat environment of the LTA V8.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $80-$150
  Aftermarket price range: $50-$120
• Spark Plug (OEM #ACDelco 41-104IP) — Spark plugs are a routine maintenance item and a frequent cause of misfires as they wear out. This is the specific OEM part for the 4.2L LTA engine.
  Trusted brands: ACDelco, NGK
  OEM price range: $20-$40
  Aftermarket price range: $15-$30
• Fuel Injector — Less common than ignition parts, but a clogged or failed injector will cause a misfire. Some anecdotal evidence points to injector issues on this engine.
  Trusted brands: ACDelco, Bosch
  OEM price range: $150-$300
  Aftermarket price range: $100-$250

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0358 — This code indicates a malfunction in the ignition coil's primary or secondary circuit for cylinder 8 and strongly points to a bad coil or a wiring issue.
• P0174 — A technician on a forum noted that a P0308 on a CT6-V was accompanied by a Bank 2 (which includes cylinder 8) 'System Too Lean' code. This could point to a vacuum leak, a fuel delivery issue on that bank, or an exhaust restriction.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #PIP5628G: This is a diagnostic aid, not a repair bulletin. It instructs technicians to collect specific information before contacting the Technical Assistance Center (TAC) for misfire codes (P0300-P0308). Key steps include saving freeze frame data with a GDS2 scan tool, performing a relative compression test with an oscilloscope (PICO scope), checking fuel injector balance, and inspecting spark plugs. This indicates a structured, data-driven diagnostic approach is required by GM for this engine.

Platform-Specific Known Issues

• Real-World Diagnosis: P0308 with Lean Codes: A technician on the r/Cadillac subreddit reported a CT6-V with P0308 accompanied by bank 2 lean codes (P0174). Following GM engineer guidance, their diagnostic path included taking a fuel sample and checking exhaust back pressure, suggesting that complex fuel delivery or exhaust restrictions can be a cause, not just simple ignition parts.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel System Pressure — expected: Up to 5,075 psi (350 bar / 35 MPa). Failure: Significantly lower pressure than commanded by the ECM, especially under load, indicates a failing high-pressure fuel pump or sensor.
• Low-Pressure Fuel System Pressure (In-tank pump) — expected: 50-60 PSI (general GM spec). Failure: Pressure below 50 PSI or pressure that bleeds off quickly after priming suggests a weak in-tank pump or a leak in the low-pressure side.
• Spark Plug Internal Resistor — expected: 4,000-7,500 Ohms (4-7.5k Ohms) for the OEM ACDelco 41-104 plug. Failure: A reading significantly outside this range can indicate an internal failure of the spark plug's suppression resistor.
• Engine Compression (Relative) — expected: All cylinders should be within 15% of the highest reading.. Failure: A cylinder reading more than 15% lower than the highest recorded cylinder indicates a mechanical sealing issue (e.g., piston rings, valves, head gasket).
• Ignition Coil Secondary Resistance (General GM V8 Spec) — expected: Approximately 6,000 - 11,000 Ohms (6k - 11k Ohms). Failure: A reading significantly outside this range or an open circuit (OL) suggests the coil's secondary winding has failed. Note: This is a general specification, and LTA-specific values may vary slightly.

Hidden / Shadow Codes Worth Checking

• Misfire Counter (Live Data): This is not a stored trouble code but a live data parameter that shows a real-time count of misfire events for each individual cylinder. It is the primary tool for identifying which cylinder is misfiring when only a P0300 (Random Misfire) is present or for confirming the frequency of a specific cylinder misfire like P0308. (see via Use a scan tool with GM-specific software (like GDS2 or an advanced equivalent) and navigate to the Engine Control Module's live data list. Look for parameters like 'Cylinder X Misfire Counter' or 'Misfire Graphic'.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Cylinder Power Balance Test — This is a primary diagnostic step to confirm which cylinder is not contributing power. The tool disables fuel to each cylinder one by one and measures the drop in engine RPM. If disabling cylinder 8 results in little or no RPM drop compared to other cylinders, it confirms cylinder 8 is the source of the misfire.
• GDS2 (GM Global Diagnostic System 2): Fuel Injector Balance Test — After ruling out ignition components, this test can identify a clogged or faulty fuel injector without removing it. The tool pressurizes the fuel rail, fires a single injector for a set time, and measures the pressure drop. An inconsistent pressure drop for cylinder 8 points to an injector fault. This is a key step recommended in TSB PIP5628G.
• GDS2 (GM Global Diagnostic System 2): Crankshaft Position Variation Learn — This procedure should be performed after replacing a crankshaft position sensor or performing major engine work. It calibrates the sensor to accurately detect crankshaft speed fluctuations, which is how the ECM identifies misfires. Failure to perform this can lead to false misfire codes.

Wiring & Ground Locations

• G130 / G132 (Reference) — On similar GM V8 platforms (e.g., CTS-V), these grounds are located at the rear of the right-side cylinder head/bank.. Cylinder 8 is at the rear of the right-side bank (passenger side). Ignition coils for this bank may be grounded at this location. A loose or corroded ground here could directly cause a weak spark and misfire on cylinder 8. While not confirmed for the LTA, it is a high-probability location to inspect based on GM's design patterns.
• X50D Fuse Block - Battery — The 4.2L LTA engine has two underhood fuse blocks, one on each side of the engine compartment. The X50D is one of these primary blocks.. This fuse block is a central power distribution point for engine components. While not a specific ground, checking the main fuses and connections here is a valid step when troubleshooting electrical issues that could lead to a misfire.

OEM Part Supersession History

• ACDelco 41-104 (GM #12571165) → ACDelco 41-104IP (GM #12672175) — The 'IP' version is designated as an 'Improved Performance' Iridium Spark Plug.
  Heads up: Both parts are compatible, but the 41-104IP is the current, recommended replacement for the 4.2L LTA engine.