P0308 on 2000-2005 Cadillac DeVille: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2005 Cadillac DEVILLE

The 2000-2005 Cadillac DeVille is equipped with the 4.6L Northstar V8 engine, which has a unique ignition system for these model years. Instead of individual coil-on-plugs, it uses two ignition coil "cassettes," one for each cylinder bank. 🎬 Watch: Northstar firing order and cylinder bank layout explained. Cylinder 8 is located on the front bank (closest to the radiator), making access easier than the rear cylinders. While standard ignition faults are common, the Northstar engine is also infamous for head gasket failures, which can cause misfires due to coolant entering the cylinder. This is often due to the head bolts pulling the fine threads out of the aluminum block, especially after an overheating event. This should be considered a possibility, especially on higher-mileage vehicles or those with a history of overheating.

Diagnostic Flowchart

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

Symptoms You May Notice

• Flashing or solid Check Engine Light.
• Rough or unstable idle, sometimes feeling like a vibration or shake at a stop.
• Engine hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• A smell of raw fuel from the exhaust.
• Reduced fuel economy.
• Engine may be harder to start than usual.

Most Likely Causes

1. Failed Ignition Coil Cassette 🔴 High Probability The Northstar engine from 2000-2005 uses a coil cassette system. Heat and age can cause the electronics within the cassette that fire cylinder 8 to fail, or the boot connecting to the spark plug can crack, causing the spark to arc to the cylinder head instead of the plug.
   How to confirm: Swap the front and rear coil cassettes. They are interchangeable. Clear the codes and drive the vehicle. If the misfire code changes to a cylinder on the rear bank (e.g., P0301, P0303, P0305, or P0307), the coil cassette is faulty. This is the most definitive and cost-effective diagnostic step.
   Typical fix: Replace the faulty ignition coil cassette. The front cassette (Bank 2) covers cylinders 2, 4, 6, and 8.
   Est. part cost: $80-$260
2. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a regular maintenance item with a service life of up to 100,000 miles for the OEM platinum plugs. If they are past their service life, the electrode can wear out, or they can become fouled by carbon, oil, or coolant, preventing a strong spark.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for a worn electrode, heavy carbon deposits, or wetness from oil or coolant. A cracked porcelain insulator is also a clear sign of failure. You can also swap the spark plug from cylinder 8 with another cylinder (e.g., cylinder 6) and see if the misfire code follows to P0306.
   Typical fix: Replace the spark plug for cylinder 8. It is highly recommended to replace all 8 spark plugs at the same time using the correct OEM-spec plugs. The recommended OEM plug is the ACDelco 41-987 Double Platinum.
   Est. part cost: $8-$15 per plug
3. Faulty Fuel Injector ⚪ Low Probability Fuel injectors can become clogged with deposits over time or fail electrically, preventing the correct amount of fuel from being delivered to cylinder 8.
   How to confirm: Use a mechanic's stethoscope to listen for a consistent clicking sound from the injector. You can also use a multimeter to check the injector's resistance; it should be within 0.5 ohms of the other injectors on the engine. A 'noid light' can also verify it's receiving an electrical pulse from the PCM.
   Typical fix: Replace the fuel injector for cylinder 8. Sometimes running a quality fuel injector cleaner through the system can resolve minor clogging.
   Est. part cost: $60-$150

Rare But Worth Checking

• Head Gasket Failure: This is a well-documented, serious issue with Northstar engines. A failed head gasket can allow coolant to leak into the cylinder, fouling the spark plug and causing a misfire. The root cause is often the head bolt threads failing in the aluminum block. Check for symptoms like unexplained coolant loss, white smoke from the exhaust, or milky-looking oil. A chemical block test can detect exhaust gases in the coolant, confirming the failure.
• Intake Manifold Gasket Leak: A vacuum leak near the intake runner for cylinder 8 can lean out the air/fuel mixture, causing a misfire. This can sometimes be detected by spraying brake cleaner or carburetor cleaner near the gasket with the engine running and listening for a change in idle speed. A hissing sound may also be audible.
• Low Cylinder Compression: Internal engine damage, such as a burnt valve, worn piston rings, or damaged cylinder walls can cause a loss of compression, leading to a misfire. A compression test is required to diagnose this. Healthy cylinders should typically show 150-185 PSI, with no cylinder being less than 140 PSI.

Diagnosis Steps

1. Verify the code P0308 is present using an OBD-II scanner. Check for any other codes that may provide more context, such as P0300.
2. Locate cylinder 8. On the transversely mounted Northstar, it is the cylinder on the passenger side, closest to the radiator.
3. Perform the Ignition Coil Cassette Swap. Disconnect the battery. Unplug and unbolt the front ignition coil cassette (covering cylinders 2-4-6-8) and the rear cassette (covering 1-3-5-7). Swap their positions. Reconnect everything and drive the vehicle. If the code changes to a misfire on the rear bank (e.g., P0301, P0307), the coil cassette is bad and needs replacement. This is the most crucial diagnostic step for this platform.
4. If the misfire remains on cylinder 8, inspect the spark plug. Remove the plug from cylinder 8. Check for excessive wear, fouling (black and sooty, wet with oil, or white from coolant), or a cracked insulator. The recommended spark plug gap is between 0.024 and 0.071 inches. Consider swapping the plug with cylinder 6 to see if the code moves to P0306.
5. If the misfire still remains on cylinder 8, check the fuel injector. Listen for a steady clicking with a stethoscope. Use a noid light to confirm it's getting a signal. Check its resistance with a multimeter and compare it to an adjacent injector.
6. Check for Vacuum Leaks. With the engine running, carefully spray carburetor cleaner around the intake manifold gasket near cylinder 8. If the engine idle changes, you have found a leak.
7. If ignition and fuel components check out, perform a cylinder compression test to rule out mechanical engine problems like bad rings or valves.
8. If you suspect a head gasket issue (e.g., persistent coolant loss, overheating), perform a chemical block test to check for combustion gases in the cooling system.

Parts You'll Likely Need

• Ignition Coil Cassette (Front/Bank 2) (OEM #ACDelco D590, GM 19418991) — This is the most common failure point for a single-cylinder misfire on this specific engine design.
  Trusted brands: ACDelco, Delphi
  OEM price range: $150-$260
  Aftermarket price range: $80-$150
• Spark Plug (OEM #ACDelco 41-987 (or GM 12679799)) — Spark plugs are a routine wear item and a frequent cause of misfires. The OEM part is a Double Platinum plug designed for long life.
  Trusted brands: ACDelco, NGK, Autolite
  OEM price range: $10-$18
  Aftermarket price range: $8-$12
• Fuel Injector (OEM #Bosch 0280155923) — Can become clogged or fail electrically, preventing fuel delivery to the cylinder.
  Trusted brands: ACDelco, Bosch
  OEM price range: $100-$180
  Aftermarket price range: $60-$120

Related Codes That Often Appear With This One

• P0300 — If multiple cylinders are misfiring randomly due to a systemic issue like a major vacuum leak or low fuel pressure, a P0300 (Random/Multiple Cylinder Misfire) may accompany the P0308 code.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: This bulletin notes that diagnostic information may be needed for DTCs P0300-P0308, confirming the code's relevance for GM vehicles. [⭐ MANUFACTURER TSB — highest authority]

Platform-Specific Known Issues

• Northstar Head Gasket Failure: These engines are known for head bolt threads failing in the aluminum block, leading to head gasket leaks. A misfire code can be an early symptom if coolant is seeping into a cylinder. This is a very expensive repair often requiring specialized tools (like timeserts or studs) and should be professionally diagnosed if suspected.
• Clogged Purge Line: A clogged coolant purge line (a small hose running from the water pump crossover to the coolant reservoir) can create air pockets and lead to localized overheating. This overheating can stress engine components, including the head gaskets, and potentially contribute to misfires or more severe damage. The hollow bolt connecting the line to the crossover is a common point for clogging.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 41-47 PSI. Failure: Pressure below 41 PSI or an immediate drop of more than 5 PSI after the pump primes indicates a weak pump or leak.
• Fuel Injector Coil Resistance — expected: 12.0 - 12.5 Ohms. Failure: A reading significantly higher (open circuit) or lower (short circuit) than the 12.5 Ohm specification indicates a failed injector coil. All injectors should measure within about 0.8 to 1.0 ohm of each other.
• Cylinder Compression — expected: 150-185 PSI. Failure: Any cylinder reading less than 140 PSI indicates a mechanical sealing issue (valves, rings, head gasket).

Scan Tool Commands That Help

• GM Tech2 / GDS2: Cylinder Power Balance Test — This test is used to pinpoint a weak cylinder when a misfire is felt but may not be consistently setting a code. The tool disables one injector at a time and measures the RPM drop. A cylinder with little to no RPM drop is not contributing power and is the source of the misfire.
• GM Tech2 / GDS2: Fuel Injector Balance Test — This test is used after a weak cylinder is identified to determine if the cause is a faulty injector. With a fuel pressure gauge attached, the tool pulses a single injector and measures the pressure drop. All cylinders should have a similar pressure drop (within 1.5 PSI). A cylinder with significantly less or more pressure drop indicates a clogged or leaking injector, respectively.
• GM Tech2 / GDS2: Misfire Graphic Display — To visually confirm which cylinder is misfiring in real-time. The live data graph will show misfire counts for each cylinder, making it easy to confirm the P0308 is accurate and to see if other cylinders are also misfiring intermittently.

Wiring & Ground Locations

• Main Engine to Chassis Ground — A large, heavy-gauge ground strap runs from the engine block (near the transmission) to the right frame rail, underneath the firewall cowl area.. This is the primary ground for the entire engine. Corrosion or a loose connection here can cause a host of electrical problems, including weak spark and erratic sensor readings that could lead to misfires.
• PCM Grounds — The Powertrain Control Module (PCM) has multiple ground pins. On a common GM PCM, pins 1 and 40 of the main connector (often blue or black) are dedicated PCM grounds.. The PCM controls both the ignition coil cassettes and the fuel injectors. A poor ground at the PCM itself can cause it to send faulty signals or have insufficient power to operate the coil or injector for cylinder 8 correctly.
• Cylinder Head to Firewall Grounds — There are typically ground straps running from the back of each cylinder head to the firewall.. These grounds ensure the cylinder heads, which house the spark plugs and provide the ground path for the spark, have a solid connection to the vehicle's chassis ground. A poor connection could result in a weak or inconsistent spark.

Real Owner Repair Stories

• YouTube channel 'The Flat Rate Mechanic' (Cadillac DeVille with 4.6L Northstar engine) — Engine misfire, P0308 code present, later showed P0300 after clearing.
  ❌ Tried (didn't work) Swapping the ignition coil from cylinder 8 to cylinder 7 did not move the misfire; the misfire remained on cylinder 8.
  ✅ What actually fixed it Swapping the spark plug from cylinder 8 to cylinder 6 caused the misfire to move to cylinder 6 (P0306). The final fix was replacing the bad spark plug.
• YouTube channel 'WTBM123 Car repair' (2003 Cadillac DTS (DeVille Touring Sedan) with Northstar V8) — P0300 Random Misfire, but scan tool data showed the misfires were almost exclusively on cylinder #1, and only at idle.
  ❌ Tried (didn't work) Initial diagnosis focused on common causes for a single cylinder misfire.
  ✅ What actually fixed it The cause was found to be a vacuum leak from a deteriorated PCV hose where it connected to the intake plenum. Replacing the leaking vacuum hose and the intake plenum boot resolved the idle misfire.

Model Year Variations Within This Range

• 2000-2005: Unlike earlier Northstar engines that used a separate, remotely mounted Ignition Control Module (ICM), the 2000-2005 models have the ignition control circuitry integrated directly into the two coil cassettes. This simplifies the system by removing a separate failure point (the ICM) but makes the cassettes themselves more complex and central to the ignition system's operation.