P0308 on 2006-2009 Cadillac XLR: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2009 Cadillac XLR

The Cadillac XLR uses the Northstar V8 engine, which features a coil-on-plug ignition system. On these engines, the ignition coils are grouped into two four-coil assemblies (often called cassettes or racks), one for each cylinder bank. While a single coil can fail, the standard repair is often to replace the entire four-coil assembly on the affected bank. Access to the rear cylinders can be tight, but cylinder 8 is generally accessible as it is the rearmost cylinder on the driver's side bank.

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 vibrating idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Check Engine Light is on and may be on and may be flashing.
• Reduced fuel economy.
• A smell of raw fuel from the exhaust.

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil The Northstar engine uses coil-on-plug assemblies which are a common failure point for single-cylinder misfires as they age. Forum users frequently report individual coil failures.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive. If the code changes to P0306, the coil is faulty. A scan tool like a Tech II can also show live misfire counts per cylinder to confirm the misfire moved.
   Typical fix: Replace the four-coil assembly on the affected bank (Bank 2, driver's side). While individual coils can be replaced, many owners and parts suppliers recommend replacing the entire cassette as an 'improved' design, and the left and right bank assemblies are often identical.
   Est. part cost: $60-$150 for a coil assembly
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item. Short-trip driving without letting the engine reach full operating temperature can cause spark plugs to become fuel-fouled, as noted in TSB #08-06-04-038 for the XLR-V.
   How to confirm: Remove and inspect the spark plug from cylinder 8. Look for excessive wear on the electrode, carbon buildup, oil/fuel fouling, or cracks in the porcelain.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 plugs at the same time to ensure even performance. The recommended OEM plugs are ACDelco Iridium or Platinum. 🎬 See this in-depth tutorial on changing Northstar spark plugs.
   Est. part cost: $10-$25 per plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Injectors can become clogged with carbon deposits or fail electronically over time, leading to an incorrect fuel mixture for a single cylinder.
   How to confirm: Listen for a consistent clicking sound from the injector with a mechanic's stethoscope. You can also swap the injector with another cylinder to see if the misfire follows, though this is more labor-intensive. An advanced scanner can perform a cylinder balance test.
   Typical fix: Replace the faulty fuel injector. It's also wise to check the fuel rail for debris and consider using a quality fuel system cleaner periodically.
   Est. part cost: $50-$120

Rare But Worth Checking

• Low Engine Compression: While less common, the Northstar engine family has a known history of head gasket issues which can cause low compression. A forum user on Cadillac XLR Forums reported a P0308 code that was ultimately diagnosed as a blown head gasket after a coolant leak was also present. A compression test and a combustion leak test (block test) are required to confirm this mechanical issue.
• Vacuum Leak: A vacuum leak from a cracked hose or a loose or faulty intake manifold gasket near cylinder 8 can lean out the air-fuel mixture and cause a misfire. One owner on a Cadillac forum noted that loose intake manifold bolts are a known issue on the 2004-2005 models and can cause various codes. This is less likely to affect only one cylinder but is still possible.
🎬 Watch: Diagnosing Northstar idle misfires without coolant loss.
• Wiring Harness Issue: A damaged or corroded wire or connector leading to the ignition coil or fuel injector for cylinder 8 can interrupt the signal from the PCM, causing a misfire. Rodent damage is a possible cause, as is oil leaking from VVT solenoids and contaminating the harness connectors, a known issue on these platforms.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 is present and check for any other codes, such as P0300 or P0358.
2. Identify Cylinder 8. On the longitudinally mounted Northstar V8 in the XLR, the cylinders are numbered 1-3-5-7 on the passenger side (Bank 1) and 2-4-6-8 on the driver's side (Bank 2), from front to back. Cylinder 8 is the rearmost cylinder on the driver's side, closest to the firewall.
3. Swap Components. The easiest first step is to swap the ignition coil from cylinder 8 with an adjacent one on the same bank, like cylinder 6. Clear the codes, run the engine, and see if the misfire code moves to P0306. If it does, the coil is bad.
4. Inspect the Spark Plug. If the coil swap does not move the misfire, remove the spark plug from cylinder 8. Inspect it for wear, damage, or fouling (black and sooty, or wet with fuel). Compare it to a plug from a working cylinder. Replace if it appears faulty.
5. Check the Fuel Injector. Listen to the injector with a mechanic's stethoscope to ensure it is clicking. Check the wiring and connector for damage, corrosion, or oil contamination from leaking VVT solenoids.
6. Check for Mechanical Issues. If ignition and fuel delivery seem okay, perform a compression test on cylinder 8 to rule out internal engine problems like bad valves or a failed head gasket. A healthy cylinder should have 120-150 PSI. Follow up with a cylinder leak-down test or a chemical block test if compression is low.
7. Inspect for Vacuum Leaks. With the engine running, carefully spray brake cleaner or starting fluid around the intake manifold gasket near cylinder 8. If the engine RPM changes, you have found a leak. Also check the torque on the intake manifold bolts, as they can loosen over time.

Parts You'll Likely Need

• Ignition Coil Assembly (Bank 2) (OEM #12573190) — This is the most common cause of a single-cylinder misfire on the Northstar engine. The coils are sold as a 4-pack assembly for one bank. This part number has been superseded by 12739916.
  Trusted brands: ACDelco (D514A), Delphi, Dragon Fire Performance
  OEM price range: $100-$180
  Aftermarket price range: $60-$120
• Iridium/Platinum Spark Plug (OEM #12592619) — A worn or fouled spark plug is another primary cause of misfires. TSB #08-06-04-038 specifically calls for replacing fouled plugs on the supercharged XLR-V. ACDelco Gold/Professional Platinum or Iridium plugs are recommended.
  Trusted brands: ACDelco (41-991), NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12

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 be stored alongside P0308.
• P0358 — This code indicates an Ignition Coil 'H' (Cylinder 8) Primary/Secondary Circuit Malfunction. It often appears with P0308 and points more directly toward a problem with the ignition coil itself or its wiring circuit.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: Communicates information that will need to be provided to TAC to aid in diagnosing if a vehicle comes in with misfire and DTC Codes P0300 - P0308 and/or P050D.
• 08-06-04-038: Addresses rough idle and misfire DTCs on the 4.4L Supercharged XLR-V due to fuel-fouled spark plugs from short-trip driving, recommending plug replacement.

Platform-Specific Known Issues

• TSB #08-06-04-038 for the XLR-V notes that short-trip driving without reaching operating temperature can lead to fuel-fouled spark plugs, causing misfire codes including P0308.
• The provided TSB #PIP5628G indicates that diagnosing misfires (P0300-P0308) on this platform can sometimes be complex, requiring specific information to be relayed to the GM Technical Assistance Center (TAC) for proper diagnosis.
• Oil from leaking Variable Valve Timing (VVT) solenoids can seep into the engine wiring harness and travel down to the coil and injector connectors, causing shorts or poor connections that lead to misfires.
• Intake manifold bolts on earlier Northstar models have been known to loosen over time, creating vacuum leaks that can cause misfires and lean conditions.

Mechanic-Grade Diagnostic Values

• Ignition Coil Secondary Resistance — expected: 5,000 to 10,000 Ohms (5k-10k Ω). Failure: A reading significantly outside this range indicates a faulty coil winding.
• Ignition Coil Primary Resistance — expected: 0.5 to 0.9 Ohms (Ω). Failure: A reading outside this range suggests a problem with the primary coil winding.
• Fuel Injector Resistance — expected: 8 to 25 Ohms (Ω) for 4.6L Northstar engines.. Failure: A reading of 0, infinity, or far outside the expected range points to a failed injector coil. A 2010 DTS with a 4.6L showed a good injector at ~14.2 Ω.
• Fuel Pressure (Engine Off) — expected: 52 PSI (359 kPa) at the fuel rail.. Failure: Pressure that is significantly lower or drops off rapidly may indicate a weak fuel pump or faulty in-tank regulator.
• Engine Compression — expected: 140 to 170 PSI per cylinder, with all cylinders reading within 4-5 PSI of each other.. Failure: Any cylinder reading less than 75% of the highest cylinder is a concern. Readings below 120 PSI suggest significant mechanical wear or damage.

Hidden / Shadow Codes Worth Checking

• Mode $06, MID $A3, TID $0B: Provides the Exponentially Weighted Moving Average (EWMA) of misfire counts over the last 10 driving cycles. This allows a technician to see historical misfire data that may not have been severe enough to set a code on a single trip. (see via An advanced scan tool capable of reading GM Mode $06 data.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Crankshaft Position Variation Learn — This procedure should be performed if misfire detection seems inaccurate, especially on high-mileage engines or after replacing the crankshaft position sensor or reprogramming the PCM. It re-calibrates the PCM's reference for crankshaft speed variations, which can greatly improve the accuracy of misfire counting and reduce false positives.
• GM Tech 2 / GDS2: Misfire Graph / Misfire Data — To view live and historical misfire counts for each individual cylinder. This is the primary method for confirming that the misfire is isolated to cylinder 8 and for verifying that a repair (like swapping a coil) has successfully moved the misfire to another cylinder.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This is the specific ground point for all eight ignition coils. Corrosion or a loose connection at this single point can cause erratic behavior, including misfires, on any or all cylinders. Since cylinder 8 is on the driver's side (left side of engine), this ground is in close proximity.
• G103 — Located on the cowl at the left rear of the engine compartment, above the brake booster.. This ground serves the Engine Control Module (ECM) and the Data Link Connector (DLC). A poor connection here can cause a wide range of engine control and communication issues, potentially including false misfire codes.

OEM Part Supersession History

• 12573190 → 12739916, which was then superseded by 12699382. — Standard part evolution for design improvements or manufacturing changes.
  Heads up: The service part is commonly sold under the ACDelco number D514A. While functionally the same, ensure the replacement coil assembly matches the vehicle's wiring harness connector style.

Model Year Variations Within This Range

• 2004-2006: Early production Northstar engines in the XLR used an ignition coil harness with a 'Vertical Connector' design. This may differ from later models, so it is important to visually inspect the existing connectors before ordering replacement harness parts.