P0308 on 2000-2003 Oldsmobile Aurora 4.0L V8: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2003 Oldsmobile AURORA

This code is exclusive to the 4.0L L47 V8 engine in the second-generation Aurora, an engine derived from the Cadillac Northstar V8. While common causes like ignition components are the first place to look, this engine family has a known history of more complex issues, such as head gasket failures caused by cylinder head bolts pulling out of the aluminum block threads. The provided NHTSA technical service bulletin (PIP5628G) also suggests that diagnosing misfires on this platform can sometimes be complex enough to require manufacturer technical assistance.

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 shaking engine idle
• Engine hesitation or stumbling during acceleration
• Reduced engine power
• Poor fuel economy
• A noticeable smell of gasoline from the exhaust
• Shuddering sensation when cruising or off-throttle, as described by owners.

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil The coil-on-plug ignition system places coils directly on the spark plugs, exposing them to significant heat and vibration over time, leading to eventual failure. This is the most common failure point for a single-cylinder misfire.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder on the same bank, such as cylinder 6. Clear the codes and run the engine. If the misfire code changes to P0306, the ignition coil is faulty. This is a widely confirmed diagnostic step in owner forums.
   Typical fix: Replace the failed ignition coil. It is often recommended to replace the spark plug at the same time. Use a quality replacement like the OEM part to avoid premature failure.
   Est. part cost: $40-$90
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a normal wear item. The manufacturer recommends replacement at 100,000 miles, but issues like oil consumption or a rich running condition can cause them to foul and fail prematurely.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 8. 🎬 See this walkthrough on accessing Aurora spark plugs Inspect it for wear (worn electrode), carbon fouling, oil, or coolant contamination. A cracked insulator or heavy deposits confirm it's bad.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 plugs at the same time using high-quality Iridium or Platinum plugs for longevity.
   Est. part cost: $8-$20 per plug
3. Failed Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Fuel injectors can become clogged with deposits over time or fail electrically, preventing the correct amount of fuel from reaching cylinder 8. This is a known, though less common, cause of single-cylinder misfires on this platform.
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should make a consistent clicking sound. You can also test the injector's resistance with a multimeter (compare to a known good injector) or use a 'noid' light to confirm it's receiving a pulse from the PCM.
   Typical fix: Replace the faulty fuel injector. It's also a good idea to clean the fuel rail during replacement.
   Est. part cost: $50-$120

Rare But Worth Checking

• Head Gasket Failure (Cylinder Head Bolt Pull-out): A known Achilles' heel of the Northstar engine family is the tendency for head bolts to pull their threads from the aluminum block, causing a head gasket leak. This can allow coolant to enter the cylinder, fouling the spark plug and causing a misfire. Key symptoms include unexplained coolant loss, white smoke from the exhaust, and engine temperature spikes under load. The definitive diagnosis is a combustion leak test (block test), which checks for exhaust gases in the coolant. A test kit can be rented from auto parts stores and will show a positive result (fluid turns yellow) if the gasket is compromised.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the intake runner for cylinder 8 can lean out the air/fuel mixture and cause a misfire. This may also be accompanied by a P0171 or P0174 (System Too Lean) code.
• Low Cylinder Compression: Internal engine problems like a worn piston ring or a burnt valve can cause a loss of compression, leading to a persistent misfire that ignition or fuel parts won't fix. This is diagnosed with a compression test after ruling out more common causes.
• Clogged Catalytic Converter: While usually a result of long-term misfires, a clogged or failing catalytic converter can create excessive exhaust backpressure, leading to misfires, often on multiple cylinders. This would typically be accompanied by a significant loss of power, especially at higher RPM.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 is present.
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. Locate cylinder 8. On the Aurora 4.0L V8, the cylinders are numbered 2-4-6-8 on the front bank (radiator side) and 1-3-5-7 on the rear bank (firewall side). Cylinder 8 is the rearmost cylinder on the driver's side, closest to the radiator.
4. Perform the 'Coil Swap Test': Swap the ignition coil from cylinder 8 with cylinder 6 (the next one over on the same bank). Clear the codes, run the engine, and rescan. If the code is now P0306, the coil is bad. This is a very common fix confirmed in owner forums.
5. If the code remains P0308, inspect the spark plug for wear or fouling. You can also swap the spark plug with another cylinder to see if the misfire follows the plug.
6. Inspect the ignition coil's electrical connector and wiring for any damage or corrosion.
7. If ignition components are good, check the fuel injector for cylinder 8. Listen for a steady clicking sound with a stethoscope or test the electrical circuit with a noid light.
8. If the cause is still not found, and especially if you are experiencing coolant loss, perform a combustion leak test ('block test') to check for a head gasket failure.
9. If the block test is negative, perform a cylinder compression test to check for mechanical engine problems like a bad valve or worn rings. Low compression would require further diagnosis with a cylinder leak-down test.

Parts You'll Likely Need

• Ignition Coil (OEM #12558693) — This is the most frequent point of failure for a single-cylinder misfire on a coil-on-plug engine. The OEM ACDelco part is D581.
  Trusted brands: ACDelco (D581), NGK, Delphi
  OEM price range: $60-$100
  Aftermarket price range: $40-$80
• Spark Plug (OEM #41-987) — As a primary ignition component, spark plugs wear out and are a common cause of misfires. They should be replaced as a set at the recommended service interval.
  Trusted brands: ACDelco, NGK, Denso, Champion
  OEM price range: $10-$18
  Aftermarket price range: $8-$15
• Fuel Injector — A clogged or failed injector is the next logical cause after ruling out spark-related issues. It's a common enough failure to be considered a primary diagnostic path.
  Trusted brands: Bosch, Standard Motor Products, ACDelco
  OEM price range: $80-$150
  Aftermarket price range: $50-$100

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 🎬 Watch: Diagnosing Northstar idle misfires without coolant loss (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0301-P0307 — If other specific cylinder misfire codes are present, it could indicate a more widespread problem, such as a vacuum leak, low fuel pressure, or a failing crankshaft position sensor.
• P0171 / P0174 — These 'System Too Lean' codes for Bank 1 and Bank 2 can appear if the misfire is caused by a large vacuum leak, such as a failed intake manifold gasket.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: Communicates information that will need to be provided to the manufacturer's Technical Assistance Center (TAC) to aid in diagnosing if a vehicle comes in with misfire and DTC Codes P0300 - P0308 and/or P050D

Platform-Specific Known Issues

• The 4.0L V8 is a derivative of the Cadillac Northstar engine and is susceptible to the infamous 'Northstar head gasket' problem. This is typically not a gasket failure itself, but the coarse head bolt threads pulling out of the aluminum engine block, causing a loss of clamping force. Anyone diagnosing a persistent P0308, especially with coolant loss, should perform a combustion leak test before investing in more expensive repairs.
• Access to the rear bank of cylinders (1-3-5-7) for spark plug or coil replacement is notoriously difficult, but cylinder 8 is on the front bank and is relatively easy to access.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 41-47 PSI. Failure: Pressure below this range can indicate a weak fuel pump or clogged filter, potentially causing a lean misfire.
• Fuel Injector Resistance — expected: ~12-16 Ohms. Failure: A reading significantly outside this range, or that differs by more than 1 Ohm from the other injectors, points to a failed injector coil winding.
• Cylinder Compression — expected: 145-160 PSI, with minimal variation between cylinders.. Failure: A reading on cylinder 8 that is significantly lower than the others indicates a mechanical engine problem, such as worn piston rings or a valve sealing issue.
• Ignition Coil Connector - Pin B (Dark Green/White wire) — expected: A square wave signal when testing with an oscilloscope while the engine is running.. Failure: Absence of a signal indicates a problem with the wiring from the PCM or the PCM's internal driver for that cylinder.

Scan Tool Commands That Help

• GM Tech 2 / High-End Scan Tool: Misfire Data / Misfire Graphic — This function provides real-time and historical misfire counts for each individual cylinder. It is the most definitive way to confirm that cylinder 8 is the source of the misfire and to observe if the misfire occurs under specific conditions (e.g., idle, load, cold/hot).
• GM Tech 2 / High-End Scan Tool: Crankshaft Position (CKP) System Variation Learn (CASE Relearn) — This procedure must be performed after replacing the crankshaft position sensor, PCM, or performing major engine work. Failure to perform the relearn can cause the PCM to inaccurately detect misfires and set false codes like P0308.
• GM Tech 2 / High-End Scan Tool: Injector Balance Test — This bidirectional test allows the technician to command each fuel injector to fire individually while monitoring the corresponding drop in fuel pressure. If commanding the cylinder 8 injector results in a smaller pressure drop compared to other cylinders, it confirms a clogged or failing injector.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This is the primary ground point for all 8 ignition coils. Corrosion or a loose connection at this ground can cause weak spark and lead to random or specific cylinder misfires.
• G104 — On the lower left side of the engine, near the transmission connector.. This ground serves the Powertrain Control Module (PCM). A poor connection here can cause a wide range of erratic engine management issues, including false misfire detection.
• Cylinder 8 Ignition Coil Connector — On the ignition coil for the rearmost cylinder on the driver's side of the engine bay (radiator side).. Testing for power, ground, and signal at this connector is crucial. The Dark Green/White wire (Pin B) is the Ignition Control (IC) signal from the PCM. A break in this specific wire will cause a P0308 with no spark.

Real Owner Repair Stories

• Automotive Forums Car Chat (2001 Oldsmobile Aurora 4.0L V8, 111,000 miles) — Flashing check engine light with P0300, with a scan tool showing slight misfires on cylinders 5 and 8.
  ❌ Tried (didn't work) Replaced all 8 spark plugs with AC Delco platinums., Replaced all 8 spark plug boots with new AC Delco parts.
  ✅ What actually fixed it The owner's mechanic diagnosed two bad coil packs (one on each bank of the V8). Replacing the coil packs resolved the misfire issue.
• Cadillac Forums (Northstar expertise) (Oldsmobile Aurora 4.0L V8) — Persistent P0308 misfire.
  ❌ Tried (didn't work) Swapping ignition coil., Replacing spark plug.
  ✅ What actually fixed it The problem was ultimately a faulty fuel injector on cylinder 8. Replacing the injector resolved the code.
• General automotive repair forum anecdote (Oldsmobile Aurora 4.0L V8) — Persistent P0308 after multiple part swaps.
  ❌ Tried (didn't work) New ignition coil, New spark plug, New fuel injector
  ✅ What actually fixed it A technician traced the wiring harness from the PCM to the cylinder 8 ignition coil and found a broken Ignition Control (IC) signal wire inside the loom. Repairing the broken wire fixed the misfire.

OEM Part Supersession History

• 12558693 → ACDelco D581, GM 19355500, GM 19418993 — Standard part evolution and branding changes over time.
  Heads up: The original coil is a square-style coil. While many GM vehicles use this style, it's important to confirm the shape. Some aftermarket listings note a '1st Design' stamped with 12558693. Using cheap, unverified aftermarket coils is a common cause of premature and repeat failures.
• ACDelco 41-987 → ACDelco 41-104 — Updated Iridium plug design.
  Heads up: The Northstar engine family is known to be sensitive to spark plug heat range and type. Using the correct OEM-spec Iridium plug is highly recommended for performance and longevity.