P0304 on 2007-2009 Saturn Aura: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2007-2009 Saturn AURA

The Saturn Aura was offered with three different engines: a 2.4L four-cylinder (LE5), a 3.5L V6 (LZ4 pushrod), and a 3.6L DOHC V6 (LY7). For both V6 models, which have a front and rear bank of cylinders, cylinder 4 is located on the front bank, closest to the radiator. This is a significant advantage for DIY repair, as the front bank is easily accessible, unlike the rear bank which requires removing the intake manifold to service. This makes diagnosing and fixing a P0304 code much simpler 🎬 Watch: A simple explanation of the P0304 misfire code and cheaper than a misfire on a rear cylinder (e.g., P0301, P0303, P0305). The 3.6L LY7 V6, in particular, is known for potential timing chain stretch issues that can manifest as misfire codes.

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 shaky idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Solid Check Engine Light
• Flashing Check Engine Light (indicates a severe misfire)
• Reduced fuel economy
• Smell of unburned fuel from the exhaust

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability All Aura engines use a coil-on-plug (or coil-near-plug on the 3.5L) system where each cylinder has its own coil or coil output. Like any electronic component, these can fail with age and heat cycles. It's a very common failure point for single-cylinder misfires.
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 2). Clear the codes and drive the vehicle. If the code changes to P0302, the ignition coil is faulty. This is the most effective and common no-cost diagnostic step.
   Typical fix: Replace the faulty ignition coil. It is typically held on by a single 10mm bolt.
   Est. part cost: $40-$150
2. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a maintenance item with a finite lifespan. The original ACDelco iridium plugs are rated for up to 100,000 miles, but can fail sooner. They can wear out, foul with oil or carbon, or develop a cracked insulator, leading to a weak or non-existent spark.
   How to confirm: After ruling out the ignition coil, remove and inspect the spark plug from cylinder 4. Look for heavy carbon buildup, oil fouling, a wet appearance (fuel), or a worn-down center electrode. Compare it to a plug from a healthy cylinder. Even new plugs can be faulty out of the box.
   Typical fix: Replace the spark plug. It's highly recommended to replace all spark plugs at the same time to ensure consistent performance and avoid future misfires. Apply dielectric grease to the inside of the coil boot upon reinstallation. 🎬 Watch: How to replace spark plugs on a Saturn Aura
   Est. part cost: $10-$25 per plug
3. Faulty Fuel Injector ⚪ Low Probability Fuel injectors can become clogged with debris over time or fail electronically, preventing the correct amount of fuel from being delivered to cylinder 4. This can lead to a lean misfire.
   How to confirm: Listen for a consistent clicking sound from the injector using a mechanic's stethoscope. A more definitive test involves checking the injector's electrical signal with a 'noid' light or swapping the injector with another cylinder (a more involved job) to see if the misfire code moves.
   Typical fix: Replace the faulty fuel injector. This is a more involved job than replacing a coil or plug, especially on the rear bank of V6 engines.
   Est. part cost: $50-$150

Rare But Worth Checking

• Stretched Timing Chain (3.6L V6): The 3.6L LY7 engine is known for premature timing chain stretching. This can cause camshafts to go out of sync with the crankshaft, leading to misfires. This is a serious mechanical issue and is often accompanied by other codes like P0008, P0016, or P0017, but can sometimes present initially as a single cylinder misfire.
• Vacuum Leak: A vacuum leak from a cracked hose or a bad intake manifold gasket near cylinder 4 can lean out the air/fuel mixture and cause a misfire. This is often more noticeable at idle.
• Low Engine Compression: This is a more serious mechanical issue, such as a worn piston ring, a burnt valve, or a leaking head gasket. A compression test is needed to diagnose this. A forum user on a platform-mate Pontiac G6 with a 2.4L engine reported a persistent P0304 at idle that was ultimately due to low compression (90psi).
• Wiring Harness Issue: The wiring or connector to the ignition coil or fuel injector for cylinder 4 could be damaged, corroded, or have a poor connection, causing an intermittent signal.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0304 is the primary code. Note any other codes present.
2. Identify the engine (2.4L, 3.5L, or 3.6L) and locate cylinder 4. On the 2.4L I4, it's the cylinder on the passenger side. On the 3.5L and 3.6L V6s, it's the middle cylinder on the front bank (near the radiator).
3. Swap the Ignition Coil: This is the most crucial first step. Swap the ignition coil from cylinder 4 with the coil from cylinder 2 (the middle cylinder on the rear bank for V6s, or the second from the driver's side on the I4). Use a 10mm socket to remove the retaining bolts.
4. Clear the trouble codes with the scanner.
5. Drive the vehicle until the check engine light comes back on.
6. Re-scan for codes. If the code is now P0302, the ignition coil you moved is bad and needs to be replaced. This is the most common fix.
🎬 See this walkthrough on how to fix a P0304 misfire
7. If the code remains P0304, the coil is likely good. Proceed to the next step.
8. Inspect/Swap the Spark Plug: Remove the spark plug from cylinder 4 (requires a 5/8 inch spark plug socket) and inspect it for wear, fouling, or damage. If its condition is questionable, replace it. To be certain, you can swap the plug with cylinder 2 and repeat the drive test. If the code moves to P0302, the spark plug is the culprit.
9. If the code is still P0304, the problem is likely in the fuel system (injector), a wiring issue, or a mechanical engine problem. At this point, further diagnostics like checking the fuel injector signal with a noid light or performing an engine compression test are needed.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D515C (for 3.6L V6)) — This is the most frequent cause of a single-cylinder misfire on the Saturn Aura. They are individual components that are easy to access and replace for cylinder 4.
  Trusted brands: ACDelco, Delphi, NGK, Denso
  OEM price range: $80-$150
  Aftermarket price range: $40-$80
• Spark Plug (OEM #ACDelco 41-107 (Iridium, for 3.6L V6)) — As a primary component of the ignition system and a regular maintenance item, a worn or fouled spark plug is the second most likely cause of P0304.
  Trusted brands: ACDelco, NGK, Denso
  OEM price range: $15-$25
  Aftermarket price range: $10-$20

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 P0304. The provided TSB #PIP3333C notes that P0300 and P0304 can appear together.
• P0008, P0017 — On the 3.6L V6 engine, if P0304 is caused by a stretched timing chain, it is often accompanied by engine position performance codes like P0008 or camshaft/crankshaft position correlation codes like P0017.

Technical Service Bulletins (TSBs) & Recalls

• PIP3333C: Addresses intermittent engine misfires and mentions that DTC P0304 may be found. It advises technicians to follow standard diagnostics if the cause is not immediately obvious.
• PIP4568H: While not directly for the Aura, this TSB for similar GM engines discusses diagnosing intermittent misfires after a cold start, which could be relevant.

Platform-Specific Known Issues

• An official GM Technical Service Bulletin (TSB), #PIP3333C, was issued for intermittent engine misfires that could trigger codes P0300, P0301, and/or P0304. This indicates that the manufacturer is aware of potential hard-to-diagnose misfire conditions on this platform.
• On V6 models, while cylinder 4 is easy to access, the rear bank of cylinders (1, 3, 5) is notoriously difficult to service due to the intake manifold being in the way. This makes a P0304 code significantly less intimidating and less costly to repair than a misfire on the rear bank.
• For the 3.6L LY7 V6 engine, a P0304 code should prompt awareness of the common timing chain stretch issue. While less likely to cause a single cylinder misfire than a random misfire (P0300), it remains a possibility if other causes are ruled out.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (3.6L V6) — expected: 384-425 kPa (56-62 psi) with pump running. Failure: Pressure below this range can cause lean misfires, though it would typically affect multiple cylinders.
• Fuel Injector Resistance (3.6L V6) — expected: 11-14 ohms between 10-32°C (50-90°F). Failure: A reading outside this range, or a variance of more than 3 ohms between injectors, indicates a faulty injector.
• Injector Pressure Drop (Injector Balance Test) — expected: All cylinders should have a similar pressure drop.. Failure: If one cylinder's pressure drop varies by more than 20 kPa (3 psi) from the average, the injector may be clogged or faulty.

Hidden / Shadow Codes Worth Checking

• Mode 6 Misfire Counters: This is not a specific code, but a data function within the OBD-II system. Mode 6 can show raw misfire counts for each cylinder, often before the count is high enough to trigger a P0304 trouble code. This is extremely useful for diagnosing intermittent misfires. (see via A professional scan tool or an advanced consumer OBD-II scanner with Mode 6 capability.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2 or equivalent professional scanner: Cylinder Power Balance Test — Use this after basic checks (plugs, coils) fail. The tool disables one fuel injector at a time and measures the RPM drop. If disabling cylinder 4 causes little or no RPM drop compared to other cylinders, it confirms cylinder 4 is not contributing power, pointing to a fuel, spark, or compression issue on that specific cylinder.
• GM Tech 2 / GDS2 or equivalent professional scanner: Fuel Injector Balance Test — This is a more precise fuel system test. The tool pulses each injector for a set time and measures the corresponding drop in fuel rail pressure. It can identify a clogged or restricted injector on cylinder 4 if its pressure drop is significantly less than the others.
• GM Tech 2 / GDS2 or equivalent professional scanner: Crankshaft Position Variation Learn (CASE Relearn) — This procedure should be performed if the crankshaft position sensor, ECM, or engine block has been replaced. An incorrect learn value can cause the ECM to misinterpret normal crankshaft speed changes as misfires. If P0304 appears after major repairs and other causes are ruled out, performing this relearn procedure may resolve the issue.

Wiring & Ground Locations

• G111 (3.6L) — Top rear right side of the engine.. Provides a ground path for various engine components. A loose or corroded ground here can cause erratic behavior in the ignition or fuel systems.
• G113 (3.6L) — On the center left side cylinder head.. This ground is critical for the cylinder head and its associated sensors and actuators. Poor grounding can lead to weak spark or incorrect sensor readings, potentially causing misfires.
• Ignition Coil Connector — On top of the valve cover, connecting to the ignition coil for cylinder 4.. The connector itself or the wiring leading to it can become brittle, corroded, or damaged, causing an intermittent loss of signal to the ignition coil. This can cause a P0304 that seems to disappear and reappear.

Real Owner Repair Stories

• Reddit r/MechanicAdvice (2008 Saturn Aura XR (3.6L V6)) — Chugging, misfiring, and multiple codes including 'Service ESC/Traction'.
  ❌ Tried (didn't work) The user initially suspected a stretched timing chain, but no specific timing codes (like P0008) were present.
  ✅ What actually fixed it The user replaced a faulty ignition coil and the Mass Airflow (MAF) sensor. After clearing codes, the misfire and the electronic stability control warnings were resolved.
• 2CarPros Forum (2008 Saturn Aura 3.6L V6 with 150,000 miles) — A 'really bad mis and shake', along with hard starting when the engine is hot.
  ❌ Tried (didn't work) A fuel pressure test was performed which showed some fluctuations but did not pinpoint the cause of the misfire.
  ✅ What actually fixed it The owner found and replaced a bad coil-on-plug, which fixed the misfire and shaking problem.

OEM Part Supersession History

• 12590990, 12610626 → ACDelco D515C (GM 12632479) — Standard part evolution for improved durability and performance. The coil design for the GM 3.6L V6 has been updated several times.
  Heads up: While older part numbers may still be found, D515C is the recommended and most current OEM service replacement for the 2007-2009 Aura 3.6L V6.