P0308 on 2006-2011 Cadillac DTS: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2006-2011 Cadillac DTS

The 2006-2011 Cadillac DTS uses the 4.6L Northstar V8 (L37), which features a coil-on-plug ignition system. Unlike earlier Northstar engines that used coil packs or cassettes, this model has an individual ignition coil for each spark plug. This design simplifies diagnosis, as a faulty coil can be easily swapped with one from another cylinder to confirm the failure. Fortunately, cylinder 8 is on the front bank of the engine, making it significantly easier to access for diagnosis and repair compared to the rear cylinders located near the firewall. This accessibility is a major advantage for DIY repair.

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 vibrating idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Reduced fuel economy
• A smell of raw fuel from the exhaust
• Harsh or abrupt transmission shifting

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are a common failure point on many modern engines due to constant exposure to heat and vibration. The Northstar V8 is no exception, and coils are a frequently reported failure item leading to misfires.
   How to confirm: Swap the ignition coil from cylinder 8 with another easily accessible coil (e.g., cylinder 6). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty. This is the most definitive and common DIY diagnostic step.
   Typical fix: Replace the faulty ignition coil. It is held in by a single T30 Torx bolt and is a simple plug-and-play replacement. 🎬 Watch: Step-by-step guide to replacing your ignition coil. No special tools are required beyond a basic socket set.
   Est. part cost: $60-$100
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item with a finite lifespan. If they have not been replaced according to the 100,000-mile service interval, they can wear out, leading to a weak or inconsistent spark that causes a misfire.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 8. Inspect it for wear (worn electrode), carbon buildup, oil fouling, or a cracked insulator. The recommended spark plug gap is between 0.024 and 0.071 inches. Compare it to a new plug to spot wear.
   Typical fix: Replace the spark plug in cylinder 8. It is highly recommended to replace all 8 spark plugs at the same time for consistent performance and to avoid future misfires on other cylinders. 🎬 See this in-depth tutorial on changing your spark plugs.
   Est. part cost: $10-$25 per plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Fuel injectors can become clogged with carbon deposits over time or fail electrically, preventing the correct amount of fuel from being delivered to the cylinder. This can lead to a lean misfire.
   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 or perform an injector balance test with a capable scan tool. Swapping the injector with another cylinder is also possible but more involved than swapping a coil.
   Typical fix: Replace the faulty fuel injector. This is more involved than a coil or plug replacement as it requires depressurizing and removing the fuel rail.
   Est. part cost: $60-$120

Rare But Worth Checking

• Vacuum Leak: A vacuum leak from a cracked hose or a leaking intake manifold gasket near the runner for cylinder 8 can lean out the air/fuel mixture and cause a misfire. This is less common for a single-cylinder misfire but still possible. A smoke test is the most effective way to find a vacuum leak.
• Low Cylinder Compression: Internal engine issues like a worn piston ring, a bad valve, or a failing head gasket can cause a loss of compression, leading to a misfire. This is the most serious and expensive potential cause. A case on a 2007 DTS with a misfire on cylinders 6 and 8 revealed cylinder 8 had 175 PSI while cylinder 6 had zero due to a broken valve spring. This should be investigated only after ignition and fuel systems are ruled out.
• Damaged Wiring or Connector: The wiring harness or the electrical connector for the ignition coil or fuel injector can become damaged from heat, vibration, or even rodents, causing an intermittent or total loss of signal to the component. A visual inspection of the harness leading to cylinder 8 is a worthwhile check.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308 and check for any other codes.
2. Locate cylinder 8 on the front-right of the engine (passenger side).
3. Swap the ignition coil from cylinder 8 with the coil from cylinder 6 (another easily accessible front cylinder). Use a T30 Torx socket to remove the coil bolt. Clear the codes, run the engine, and re-scan. If the code is now P0306, the coil is bad.
4. If the code remains P0308, move the spark plug from cylinder 8 to cylinder 6. Clear codes and re-scan. If the code is now P0306, the spark plug is bad.
5. 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 looseness.
6. Check the fuel injector. Listen for a steady clicking with a stethoscope. If you have an advanced scan tool, perform a fuel injector balance test.
7. If all ignition and fuel components check out, perform a compression test on cylinder 8 to check for mechanical engine problems. According to a Cadillac service manual, a healthy reading should be no less than 140 PSI, and the lowest cylinder should not be less than 70% of the highest. A 2007 DTS with a healthy cylinder 8 showed 175 PSI.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D598A) — This is the most common cause of a single-cylinder misfire on the Northstar engine. Heat and vibration cause the internal windings to fail over time.
  Trusted brands: ACDelco, Delphi, NGK, Denso
  OEM price range: $90-$100
  Aftermarket price range: $60-$85
• Iridium Spark Plug (OEM #ACDelco 41-162) — As a routine maintenance item, a worn spark plug is a frequent cause of insufficient spark, leading to a misfire. The original equipment is a long-lasting iridium plug.
  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 Detected) code may appear alongside P0308.
🎬 Watch: How to diagnose and fix a P0300 random misfire.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A bulletin noting that specific diagnostic information may be needed from GM's technical assistance for misfire codes P0300-P0308. It provides a template for technicians to follow for complex cases, emphasizing checking compression, fuel delivery, and spark before escalating.

Platform-Specific Known Issues

• The provided NHTSA Technical Service Bulletin #PIP5628G indicates that diagnosing misfires (P0300-P0308) on this platform can sometimes be complex enough to require communication with GM's Technical Assistance Center (TAC). The bulletin outlines a specific 'Misfire template' of information a technician should gather before calling TAC, including fuel trim values, compression test results, and whether components were swapped.
• In one documented case, a 2008 DTS owner experienced severe transmission shifting issues, including harsh engagement from drive to reverse, which were ultimately resolved by replacing a single faulty ignition coil that was causing a misfire. This highlights how the engine and transmission control modules are interconnected.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance — expected: Approximately 12 ohms. Failure: Significantly higher or lower resistance, or an open circuit (infinite resistance), indicates a failed injector coil.
• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range suggests the primary winding is faulty.
• Ignition Coil Secondary Resistance — expected: 6,000 to 10,000 ohms (6k-10k Ω). Failure: A reading outside this range indicates a failure in the secondary winding.
• Fuel Pressure (Key on, Engine Off) — expected: 40-45 PSI. Failure: Pressure significantly below this range could indicate a weak fuel pump or a clogged filter, though this would typically cause multiple/random misfires, not just on cylinder 8.

Scan Tool Commands That Help

• Tech2/GDS2 (GM Dealer Tool): Cylinder Power Balance Test — This function allows a technician to disable individual fuel injectors one by one while monitoring the RPM drop for each cylinder. If disabling cylinder 8 causes little or no change in RPM, it confirms that cylinder is not contributing power, which is the definition of a misfire. This is a definitive way to confirm the misfire is currently active before swapping parts.
• Tech2/GDS2 (GM Dealer Tool): Misfire History / Misfire Graphic — This allows viewing of the number of misfire events per cylinder over different engine cycles (e.g., last 1000 revolutions). This is useful for identifying intermittent misfires that may not be severe enough to set a code immediately but are still affecting performance. It can help confirm if the issue is isolated to cylinder 8.

Wiring & Ground Locations

• G102 — On the left side of the engine, below the thermostat housing. A separate document describes it as being at the left rear of the engine.. This ground point is specifically listed as the ground for all 8 ignition coils. A loose or corroded G102 could cause a weak or intermittent spark on one or more cylinders, leading to misfires. While a bad ground here would likely affect multiple cylinders, it could present initially on a single cylinder.
• G100 — Located on the rear of the right cylinder bank (front of the car, passenger side).. This is a primary engine ground. A poor connection here can cause a host of issues, including problems for the Engine Control Module (ECM) and its ability to properly control spark and fuel. A bad block ground can cause strange voltage readings and lead to misdiagnosis.
• Ignition Coil Connector — Directly on top of the ignition coil for cylinder 8.. The 4-pin connector and its wiring are subject to engine heat and vibration. The terminals can become loose or corroded, and the plastic locking tab can become brittle and break. A poor connection here will cause a misfire on cylinder 8. A high resistance condition on an ignition control circuit can cause a misfire code without setting a specific circuit code like P0358.

OEM Part Supersession History

• Ignition Coil Harness for 2006 models → 12602860 — The original ignition coil harness on 2006 models may not have a locator tab present on the harness connector.
  Heads up: When replacing an ignition coil on a 2006 DTS with the newer ACDelco D598A coil, you may also need to replace the harness with part number 12602860 if the original harness lacks the necessary locator tab for the new coil design.

Model Year Variations Within This Range

• 2009-2011: Bluetooth for hands-free calling was added as a feature starting around the 2009 model year and was not available on the 2006-2008 models. This does not affect the P0308 diagnosis but is a notable difference in the vehicle's features.
• 2006-2011: The DTS was available with two versions of the 4.6L Northstar V8: a standard version with 275 hp (VIN Y) and a Performance package version with 291 hp (VIN 9). The fundamental engine architecture and ignition system related to a P0308 code are identical between them.