P0355 on 2008-2009 Cadillac STS: Ignition Coil 'E' Circuit Malfunction Causes and Fixes

What's Unique About the 2008-2009 Cadillac STS

The 2008-2009 Cadillac STS was available with either a 3.6L LLT V6 or a 4.6L Northstar V8 engine. Both use a coil-on-plug ignition system, where each cylinder has its own dedicated coil. While the P0355 code applies to both engines, the location of cylinder #5 and the specific part number for the coil will differ. For the 3.6L V6, the cylinders are numbered 1-3-5 on the passenger side (right bank, front to back) and 2-4-6 on the driver's side (left bank). For the 4.6L Northstar V8, the firing order and cylinder location can be found on service diagrams, but cylinder 5 is generally on the rear bank. The diagnostic process, however, remains the same for both powertrains.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on or flashing
• Rough or shaky engine idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Engine misfiring
• Increased fuel consumption

Most Likely Causes

1. Failed Ignition Coil on Cylinder 5 🔴 High Probability Ignition coils are wear items that operate in a high-temperature environment and can fail over time due to internal shorts or winding degradation. Heat exposure is a primary cause of failure.
   How to confirm: The easiest way to confirm a bad coil is to swap the ignition coil from cylinder 5 with one from another cylinder (e.g., cylinder 1). Clear the codes and drive the car. If the code changes to P0351, the coil is faulty. A user on Reddit confirmed this method worked for them when a P0355 code appeared after they moved a suspected bad coil to cylinder 5.
   Typical fix: Replace the ignition coil on cylinder 5. It's held in place by a single 10mm bolt. 🎬 Watch: Step-by-step ignition coil replacement for the 3.6L V6 engine. Apply dielectric grease to the inside of the new coil boot to protect against moisture.
   Est. part cost: $70-$110
2. Worn or Faulty Spark Plug 🟡 Medium Probability A spark plug with an excessive gap or a fouled electrode forces the ignition coil to work harder, which can lead to coil failure or trigger a circuit code. TSB #PIP5038B notes that worn or incorrect spark plugs can cause EMI transients that are reflected back to the ECM, potentially setting coil circuit codes.
   How to confirm: After removing the ignition coil, remove the spark plug from cylinder 5. Inspect it for wear, damage, or heavy carbon deposits. Oil fouling on the plug is a strong indicator of a leaking valve cover gasket.
   Typical fix: Replace the spark plug. It's good practice to replace all spark plugs at the same time if they are near the end of their service life. Ensure the new plug is gapped correctly.
   Est. part cost: $10-$25
3. Damaged Wiring or Loose Connector ⚪ Low Probability Engine vibration and heat can cause wiring to become brittle or connectors to loosen over time. The plastic locking tabs on connectors can become brittle and break.
   How to confirm: Visually inspect the wiring harness and connector going to the ignition coil on cylinder 5. Look for any signs of corrosion, burnt marks, frayed wires, or a loose connection. Use a multimeter to check for 12V at the power pin on the connector with the ignition on.
   Typical fix: Repair the damaged section of wiring or clean the connector terminals. If the connector itself is broken, it will need to be replaced.
   Est. part cost: $5-$50
4. Leaking Valve Cover Gasket ⚪ Low Probability The valve cover gaskets on both the 3.6L and 4.6L engines can become brittle and leak oil over time. This oil can seep into the spark plug tubes, flooding the spark plug and saturating the ignition coil boot, causing a misfire and potential coil failure.
   How to confirm: When removing the ignition coil for cylinder 5, check for liquid oil on the coil boot or down in the spark plug well. A visual inspection around the valve cover may also show signs of external oil leaks.
   Typical fix: Replace the valve cover gasket and the spark plug tube seals. While the cover is off, clean all oil out of the spark plug tube. The affected ignition coil and spark plug should also be replaced.
   Est. part cost: $40-$100 for gasket set

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is very rare. The internal driver circuit for the coil can fail. This should only be considered after all other possibilities (coil, plug, wiring) have been definitively ruled out. TSB #PIP5038B mentions ECM replacement as a possible, though last-resort, solution.

Diagnosis Steps

1. Confirm the presence of P0355 (and any other codes) with an OBD-II scanner. Note any accompanying misfire codes like P0305.
🎬 See this video for a deep dive into P0305 misfire causes.
2. Identify the engine (3.6L V6 or 4.6L V8) to correctly locate cylinder 5. For the 3.6L V6, cylinder 5 is the rear-most cylinder on the passenger side (right bank). For the 4.6L V8, consult a service manual for the specific cylinder layout.
3. Perform a visual inspection of the coil and its connector for obvious damage or corrosion.
4. Swap the ignition coil from cylinder 5 with the coil from another cylinder (e.g., cylinder 1).
5. Clear the codes with the scanner, start the engine, and let it run or take a short drive.
6. Re-scan for codes. If the code has changed to P0351 ('Ignition Coil 'A' Circuit Malfunction'), the ignition coil you moved is faulty and needs to be replaced.
7. If the P0355 code returns, the problem is not the coil. The next step is to inspect the wiring and connector for cylinder 5. Check for 12V power at the connector's power supply pin with the key on, engine off.
8. If the wiring is good, remove and inspect the spark plug for cylinder 5. Check for oil in the spark plug tube, which indicates a leaking valve cover gasket. Replace the plug if it is worn, fouled, or damaged.
9. If the issue persists after replacing the coil, plug, and verifying the wiring, the problem may be with the PCM, which requires professional diagnosis.

Parts You'll Likely Need

• Ignition Coil (OEM #ACDelco D515C (GM #12632479) - for 3.6L V6) — This is the most frequent cause of a P0355 code, as the coil itself fails internally. The ACDelco D515C is the specified OE part for the 3.6L V6 in the 2008-2009 STS.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $90-$110
  Aftermarket price range: $70-$95
• Spark Plug — Often replaced as a preventative measure along with the coil, or if it is found to be worn or fouled during diagnosis. A bad plug can cause the coil to fail.
  Trusted brands: ACDelco, NGK, Champion
  OEM price range: $15-$25
  Aftermarket price range: $10-$18

Related Codes That Often Appear With This One

• P0305 — P0305 means 'Cylinder 5 Misfire Detected.' Since P0355 indicates a fault with the ignition coil that creates the spark for cylinder 5, it's very common for both codes to appear together.
• P0300 — P0300 means 'Random/Multiple Cylinder Misfire Detected.' A failing coil on one cylinder can sometimes cause engine instability that the PCM interprets as a random misfire, as noted in TSB #PIP5038B.
• P0205 — P0205 is 'Injector Circuit/Open - Cylinder 5'. In some cases, a significant electrical issue or a failing PCM driver can affect multiple components on the same cylinder, leading to both injector and coil codes appearing together, as seen in a CaddyInfo forum thread.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #PIP5038B: Mentions that P0355 can be found with other misfire and ignition coil codes, directing technicians to follow standard diagnostics. It emphasizes that issues like worn spark plugs, corroded grounds, or loose battery connections can be the underlying cause, reflecting an EMI transient back to the ECM.

Platform-Specific Known Issues

• A Technical Service Bulletin (TSB #PIP5038B) was issued for the 2008 model year mentioning that P0355 can appear alongside a full suite of misfire (P030x) and other ignition coil codes (P035x). This indicates a known potential for ignition system faults on these engines. The TSB advises checking for worn plugs, bad grounds, and loose connections as root causes.
• On both the 3.6L V6 and 4.6L V8 engines, leaking valve cover gaskets can cause oil to pool in the spark plug wells, leading to ignition coil failure and misfire codes. This is a common issue on higher-mileage vehicles.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.5 to 0.9 Ohms. Failure: A reading outside this range indicates a faulty coil.
• Ignition Coil Secondary Resistance — expected: 5,000 to 10,000 Ohms (5k-10k Ω). Failure: A reading outside this range, particularly an open circuit (infinite resistance), points to a failed secondary winding.
• Ignition Coil Connector Power Supply Voltage — expected: ~12 Volts (Battery Voltage). Failure: No voltage or significantly low voltage indicates a problem with the ignition voltage supply circuit, fuse, or relay.
• Ignition Control (IC) Circuit Voltage (at ECM) — expected: Should vary between 1-4V when cranking (AC Volts setting). Failure: No signal or a static voltage suggests a wiring issue or a failed ECM driver.

Scan Tool Commands That Help

• GM Tech2 / GDS2: Ignition Coil Control Circuit Test Status — While the engine is running, a technician can observe the status for 'Low Voltage', 'Open Circuit', or 'High Voltage' faults for each specific coil. This helps differentiate between a wiring fault and a component failure without manually testing wires first.
• Advanced Bi-Directional Scanners (e.g., iCarsoft CR MAX): Coil Activation Test — With the engine off and key on, this command manually fires the selected ignition coil (e.g., Coil 'E'). This allows a technician to confirm if the coil itself is capable of producing a spark (using a spark tester) and verifies the integrity of the control circuit from the ECM.

Wiring & Ground Locations

• Ignition Coil Connector (3.6L V6) — Connects directly to the top of each ignition coil.. For P0355 on the V6, this is the connector for the rearmost coil on the passenger side. Pin D (Pink wire) is the ignition voltage supply, and Pin A is the ground. Pin B is the Ignition Control (IC) signal from the ECM.
• G102 (Northstar V8) — Located at the left rear of the engine.. This ground point is specifically for the 8 ignition coils on the V8 engine. A loose or corroded connection here could cause intermittent circuit faults for multiple coils, including P0355.
• G107 (Northstar V8) — Located on the rear side of the right cylinder head.. This is another critical engine ground. Wiring diagrams for the 4.6L engine show this as a key ground location that could affect ignition components.
• ECM (Engine Control Module) — On the left front frame rail, located below the left headlamp assembly.. All ignition control signals originate here. Verifying a clean, tight connection at the ECM connectors is a crucial step if wiring is suspected to be the fault after the coil and plug have been ruled out.

Real Owner Repair Stories

• Reddit user in r/AskMechanics (2001 Ford F-250 5.4L V8 (Note: Different vehicle, but identical diagnostic principle)) — Rough idle, underpowered, pending misfire code on cylinder 2.
  ❌ Tried (didn't work) Initially scanning showed a pending code that disappeared. The rough idle was intermittent.
  ✅ What actually fixed it The user swapped the ignition coil from the misfiring cylinder (#2) to cylinder #5. After driving, the truck ran rough again and this time threw a definitive P0355 code, confirming the coil that was moved to cylinder 5 was the faulty part.

OEM Part Supersession History

• 12632479 → ACDelco D515C — D515C is the service part number for the original GM part 12632479.
  Heads up: While D515C is the correct part for the 3.6L V6, it is not compatible with the 4.6L Northstar V8, which uses a different ignition coil. Always verify the part number by engine type.

Model Year Variations Within This Range

• 2008-2009: The 2008 model year introduced a new, more powerful direct-injected 3.6L V6 engine (LLT), replacing the previous port-injected V6. This engine carried over into 2009 with minimal changes. The ignition system components, including the coils, are specific to this direct-injection engine. The 4.6L Northstar V8 remained an option for both years.