P0308 on 2005-2011 Cadillac STS: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2005-2011 Cadillac STS

The Cadillac STS V8 models use a coil-on-plug ignition system, where each cylinder has its own dedicated ignition coil. While this design is efficient, individual coil failure is a common source of misfires. A unique quirk on this platform is that the ignition coils are powered in two banks, 'odd' and 'even'. A single blown fuse labeled 'EVEN COILS' in the underhood fuse box can disable all four cylinders on one bank (2, 4, 6, and 8), causing a significant misfire issue that can be mistaken for a more complex problem. For the longitudinally mounted Northstar V8 in the STS, the even-numbered cylinders are on the driver's side (Bank 2).

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
• Flashing or solid Check Engine Light
• Smell of fuel from the exhaust
• Engine knocking or pinging noise

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems have eight individual coils, and engine bay heat and age cause them to fail over time. It's a common wear item on these engines and the most frequent cause of a single-cylinder misfire.
   How to confirm: Swap the ignition coil from cylinder 8 (rearmost on driver's side) with another cylinder (e.g., cylinder 6, second from front on driver's side). Clear the codes and run the engine. If the code changes to P0306, the coil is faulty.
   Typical fix: Replace the failed ignition coil. Many owners choose to replace all eight coils if they are original and high-mileage to prevent future failures.
   Est. part cost: $35-$80 per coil
2. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a service life of around 100,000 miles. If they are past their service life, the gap can become too large or electrodes can become fouled, causing a weak or inconsistent spark.
   How to confirm: After confirming the coil is good, inspect the spark plug for cylinder 8. Look for wear, carbon fouling, or oil/coolant contamination. A heavily carboned plug was noted in one owner's P0308 diagnosis. Swap the plug with another cylinder to see if the misfire follows.
   Typical fix: Replace the spark plug for cylinder 8. Best practice is to replace all eight spark plugs at the same time with the correct OEM part, such as ACDelco 41-987.
   Est. part cost: $8-$20 per plug
3. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Fuel injectors can become clogged with deposits over time, preventing the correct amount of fuel from being delivered to cylinder 8. This can lead to a lean misfire.
   How to confirm: Listen for a clicking sound from the injector with a mechanic's stethoscope. A more advanced test involves checking the injector's resistance with a multimeter or performing a fuel injector balance test with a capable scan tool. Swapping injectors is more labor-intensive.
   Typical fix: Replace the faulty fuel injector. It's recommended to replace the O-rings on adjacent injectors during the service. GM issued a bulletin (20-NA-098) advising technicians to consult the parts catalog to ensure the correct injector is used during replacement.
   Est. part cost: $50-$150
4. Vacuum Leak ⚪ Low Probability The intake manifold gaskets and various PCV vacuum hoses on Northstar engines can become brittle and crack from age and heat, creating a vacuum leak. A significant leak can lean out the air-fuel mixture enough to cause misfires. A common failure point on related Northstar engines is the rubber coupling between the intake plenum and throttle body.
   How to confirm: With the engine running, carefully spray short bursts of brake cleaner or starting fluid around the intake manifold gasket near cylinder 8. If the engine RPM changes or stumbles, a leak is present.
   Typical fix: Replace the leaking gasket or hose. Intake manifold gasket replacement is a more involved job.
   Est. part cost: $20-$100 for gaskets/hoses

Rare But Worth Checking

• Blown Ignition Coil Fuse: → Shop Ignition Coil A video for a 2005 STS showed that a single 15-amp fuse labeled 'EVEN COILS' in the under-hood fuse box had blown, disabling cylinders 2, 4, 6, and 8. Always check this simple fix before beginning major repairs. 🎬 Watch: How to fix a rough idle by checking the coil fuse. The fuse box is located on the passenger side of the engine bay.
• Low Cylinder Compression: If ignition and fuel components are confirmed good, a mechanical issue like a worn piston ring, valve seat issue, or (less commonly on this later Northstar) a head gasket problem could be the cause. A compression test is needed to verify this.
• Wiring Harness Issue: The wiring harness or connector going to the ignition coil or fuel injector for cylinder 8 can become damaged, frayed, or corroded, causing an intermittent connection and misfire. Wiggling the harness while the engine is running may reveal the fault.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes, such as P0300 or other cylinder misfires.
2. Check the simple things first: Inspect the 'EVEN COILS' fuse (typically 15-amp) in the under-hood fuse box on the passenger side.
3. Identify Cylinder 8: On the longitudinally mounted Northstar V8, it is the rearmost cylinder on the driver's side (Bank 2).
4. Swap the ignition coil from cylinder 8 with cylinder 6 (another cylinder on the same bank). Clear codes and test drive. If the code changes to P0306, the coil is bad.
5. If the code remains P0308, swap the spark plug from cylinder 8 with cylinder 6. Clear codes and test drive. If the code changes to P0306, the spark plug is bad.
6. If the code remains P0308, inspect the wiring and connector for the ignition coil and fuel injector on cylinder 8 for any damage, corrosion, or loose connections.
7. Check for vacuum leaks near cylinder 8's intake runner using the starting fluid or brake cleaner method while the engine is idling. Listen for a change in engine RPM.
8. If all of the above check out, the issue is likely a faulty fuel injector or a mechanical problem. Consider testing the fuel injector or performing a cylinder compression test.

Parts You'll Likely Need

• Ignition Coil (OEM #12585765) — This is the most common failure part for a single-cylinder misfire on the STS V8.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $60-$90
  Aftermarket price range: $35-$65
• Spark Plug (OEM #ACDelco 41-987) — A worn or fouled spark plug is the second most likely cause. It is a standard maintenance item that wears out over time.
  Trusted brands: ACDelco (OEM), NGK, Denso
  OEM price range: $15-$25
  Aftermarket price range: $8-$15

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent, affects multiple cylinders due to a vacuum leak, or is in its early stages, a P0300 (Random Misfire) code may appear alongside P0308.
🎬 See this guide on diagnosing and fixing random misfire codes.
• P0302, P0304, P0306 — If the 'EVEN COILS' fuse is blown, you will likely see misfire codes for all even-numbered cylinders along with P0308, as they are all on the same circuit.
• P0174 — A vacuum leak on Bank 2 (the driver's side, where cylinder 8 is located) can cause a P0174 (System Too Lean Bank 2) code to be set along with the misfire code.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A bulletin sent to GM technicians indicating that diagnosing misfire codes P0300-P0308 may require collecting specific information for the Technical Assistance Center (TAC). It provides a template for technicians to follow, including checking fuel quality, performing compression tests, and inspecting spark plugs, suggesting these issues can sometimes be complex to diagnose.
• 20-NA-098: An information bulletin regarding fuel injector replacement, advising technicians to refer to the parts catalog to ensure the correct injector part number is used for the specific cylinder being serviced.

Platform-Specific Known Issues

• A known issue, documented by owners, involves a blown fuse for one of the two ignition coil banks (e.g., 'EVEN COILS'). This will cause four cylinders (2, 4, 6, 8) to misfire simultaneously and can be diagnosed by checking the under-hood fuse box before replacing any parts.
• The rubber intake plenum to throttle body coupler can crack and cause a major vacuum leak, leading to rough idle and multiple codes including P0300, P0171, and P0174.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.5 to 1.5 Ohms. Failure: A reading outside this range (especially an open circuit) indicates a faulty coil.
• Ignition Coil Secondary Resistance — expected: 5,000 to 10,000 Ohms (5k-10k Ω). Failure: A reading significantly outside this range suggests the coil's secondary winding has failed.
• Fuel Injector Resistance — expected: Around 14.1-14.3 Ohms at room temperature.. Failure: An open or shorted reading on the multimeter. Resistance should be consistent across all injectors.
• Fuel Pressure (Key on, Engine off) — expected: Approximately 50-60 PSI. Failure: Significantly lower pressure indicates a weak fuel pump, clogged filter, or faulty regulator, which could cause a lean misfire.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $0B/$0C: Provides access to current and historical misfire counts per cylinder before they are high enough to set a P030x code. For cylinder 8, the Monitor ID would be $A9. TID $0B shows EWMA (Exponentially Weighted Moving Average) misfire counts for the last 10 driving cycles, while TID $0C shows counts for the current/last driving cycle. (see via A professional scan tool (like a GM Tech2 or equivalent) or an advanced OBD-II scanner with Mode $06 capability. A high count on cylinder 8 ($A9) when no code is present can point to an emerging problem.)

Scan Tool Commands That Help

• GM Tech2 / GDS2 or equivalent: Injector Balance Test — This bidirectional command allows the technician to fire each injector individually while monitoring fuel pressure drop. A smaller pressure drop for cylinder 8 compared to others indicates a clogged or failing injector.
• GM Tech2 / GDS2 or equivalent: Misfire Graphic — Provides a real-time graph of misfire counts for each cylinder, which is useful for seeing if wiggling a harness or making other small changes has an immediate effect on the misfire count for cylinder 8.

Wiring & Ground Locations

• G107 — On the rear side of the right (passenger side) cylinder head.. This is a primary engine-to-chassis ground. A loose or corroded G107 can cause erratic behavior in sensors and actuators, including the ignition system.
• G102 — Located at the left rear of the engine.. This ground is specifically cited as the grounding point for all eight ignition coils. Corrosion or a loose connection here can directly cause weak spark and misfires on any cylinder, including cylinder 8.
• Cylinder 8 Injector Control Wire — ECM Connector Pin for 'Fuel Inj 8 Ctrl'. The specific pin number can vary by year and ECM, requiring a service manual diagram for confirmation.. To diagnose a suspected PCM driver failure or wiring break, a technician can check for the grounding pulse signal on this wire at the PCM connector while the engine is running.

Real Owner Repair Stories

• CadillacForums user (2006 Cadillac STS 4.6L Northstar V8) — Persistent P0308 misfire code, rough running.
  ❌ Tried (didn't work) Replaced spark plug on cylinder 8, Replaced ignition coil on cylinder 8, Replaced fuel injector on cylinder 8, Cleaned carbon from piston head
  ✅ What actually fixed it The final cause was determined to be a weak valve spring on cylinder 8. The issue was resolved by replacing the valve springs for that cylinder, which restored proper compression and valve seating at all RPMs.

OEM Part Supersession History

• 12585765 → This part number is listed by GM as discontinued. It has been replaced by aftermarket equivalents and potentially by other GM numbers like 89017778, though direct interchange is not always guaranteed without checking VIN. — Part lifecycle, potential minor design updates.
  Heads up: When a part is discontinued, it is critical to use a high-quality aftermarket replacement (e.g., Delphi, NGK) or verify a newer GM part number with a dealer to ensure compatibility.
• ACDelco 41-987 → While still widely available and specified, some parts catalogs show 12571535 as a related or prior number. However, 41-987 remains the correct and commonly used service part for this application. — Standard part number updates and branding changes.

Model Year Variations Within This Range

• 2005-2007: Early V6 models (not applicable to the V8 P0308 code) were known for timing chain issues. The 4.6L Northstar V8 was the primary V8 option.
• 2008-2010: The STS received a mid-cycle refresh in 2008 with updated styling and a new 6-speed automatic transmission paired with the V8.
• 2011: The 4.6L Northstar V8 engine option was discontinued for the final model year of the STS. Therefore, a P0308 code is not applicable to a 2011 model, which was V6 only.