P0306 on 2013 Audi S4: Cylinder 6 Misfire Causes and Fixes

What's Unique About the 2013-2013 Audi S4

The 3.0L TFSI supercharged engine in the B8.5 generation S4 is a direct-injection engine, which makes it inherently prone to carbon buildup on the intake valves over time. This buildup can restrict airflow, specifically to cylinders 5 and 6 which are at the rear of the manifold, causing misfires under load. While ignition components are the most frequent cause, this platform has widely documented issues with leaking supercharger intercoolers (charge air coolers) and PCV valve failures that can lead to misfires, particularly in cylinder 6.

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 power
• Flashing Check Engine Light
• Reduced fuel economy
• Smell of gasoline from the exhaust
• Sweet smell of coolant from the exhaust (if caused by intercooler leak)
• Blue smoke from exhaust on startup (if oil is fouling the plug)

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Ignition coil failure is a very common maintenance issue on Audi/VW engines of this era due to constant exposure to high heat. They are a known wear item.
   How to confirm: Swap the ignition coil from cylinder 6 with another cylinder (e.g., cylinder 5). Clear the codes and drive. If the code changes to P0305, the coil is bad. This is the definitive DIY diagnostic step.
   Typical fix: Replace the faulty ignition coil. 🎬 Watch: Step-by-step ignition coil and spark plug replacement guide. It is highly recommended to replace all six coils at the same time, as they have all experienced the same wear and heat cycles.
   Est. part cost: $35 - $75
2. Worn Spark Plug 🔴 High Probability
   How to confirm: After ruling out the coil, remove the spark plug from cylinder 6. Inspect it for wear, an incorrect gap, carbon fouling, oil, or a white/pink crusty residue indicating a coolant leak. Tuned engines are especially sensitive to correct spark plug gaps.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs as a set to ensure even performance. Always check the gap on new plugs before installation.
   Est. part cost: $15 - $30 per plug
3. Carbon Buildup on Intake Valves 🟡 Medium Probability As a direct-injection engine, fuel doesn't wash over the intake valves. Oil vapor from the PCV system bakes onto the hot valves, restricting airflow. Cylinders 5 and 6 are often the most affected due to manifold design.
   How to confirm: This requires a visual inspection of the intake ports with a borescope, which necessitates removing the supercharger. It is often diagnosed after ignition and fuel-related causes are ruled out.
   Typical fix: The intake manifold and supercharger must be removed and the valves must be manually cleaned, typically through walnut shell blasting. 🎬 See how to perform a professional walnut shell carbon cleaning.
   Est. part cost: $0 (if cleaning only)
4. Leaking Supercharger Intercooler 🟡 Medium Probability The two intercooler cores (charge air coolers) located within the supercharger assembly can develop internal leaks, dripping coolant directly into the intake ports. Cylinders 3 and 6 are particularly susceptible due to their location relative to the coolant passages.
   How to confirm: The primary symptom is mysterious coolant loss with no visible external leaks. A pressure test of the supercharger's dedicated cooling system is required. A borescope may reveal a clean, steam-cleaned piston top or white/pink crust on the spark plug. Definitive diagnosis requires removing the supercharger to check for coolant residue.
   Typical fix: The supercharger must be removed to replace the internal intercooler cores. 🎬 Watch this DIY walkthrough for replacing your supercharger intercooler cores. Many owners replace both cores as a preventative measure.
   Est. part cost: $300 - $600 for cores
5. Failed PCV Valve / Oil Separator ⚪ Low Probability The PCV valve is a very common failure point on the 3.0T engine, with a TSB issued for related oil consumption issues. A failed diaphragm creates a large vacuum leak or allows excess oil into the intake, which can foul the spark plug in cylinder 6 and cause a misfire.
   How to confirm: Symptoms include a loud whistling or sucking noise from the engine bay, high oil consumption, and oil in the spark plug wells. A definitive test is to try removing the oil filler cap with the engine running; if it is held down by extremely strong vacuum, the PCV diaphragm is likely torn.
   Typical fix: Replace the PCV valve assembly. Since this part is located under the supercharger, it is often replaced at the same time as a carbon cleaning service or thermostat replacement.
   Est. part cost: $150 - $250

Rare But Worth Checking

• Clogged or Faulty Fuel Injector: While less common than ignition issues on the 3.0T, a fuel injector can become clogged or fail electronically, preventing the correct amount of fuel from reaching cylinder 6. This can sometimes happen after a carbon cleaning service if debris enters the injector.
• Low Engine Compression: This is a worst-case scenario involving internal mechanical engine damage, such as to piston rings, valves, or cylinder walls. A compression test is required to diagnose this. Oil on the spark plug can be a sign of worn piston rings.
• Failed Thermostat: A failing thermostat is a known issue on the B8.5 S4, located under the supercharger. While it doesn't directly cause a single-cylinder misfire, an overheating engine caused by a stuck-closed thermostat can lead to multiple engine issues, including misfires.

Diagnosis Steps

1. Read the fault codes with an OBD-II scanner and note any other codes present, especially P130A ('Cylinder Disabling').
2. Swap the ignition coil from cylinder 6 to cylinder 5. Clear the codes and drive the vehicle to see if the misfire code moves to P0305. If it does, the coil is faulty.
3. If the misfire remains on cylinder 6, inspect the spark plug. Remove it and check for excessive wear, fouling (black soot, oil), or a white/pink crusty residue which indicates coolant from a leaking intercooler.
4. Inspect the spark plug tube for oil, which would indicate a leaking valve cover gasket or spark plug tube seal.
5. If you are also experiencing coolant loss with no visible external leaks, suspect a leaking supercharger intercooler. A cooling system pressure test can help confirm this, but removal of the supercharger is the definitive check.
6. Check for signs of a failing PCV valve, such as high oil consumption or a loud whistling noise from the engine. Perform the 'oil cap test' to check for excessive vacuum.
7. If ignition components are good, investigate fuel delivery. A professional may be needed to test the fuel injector for cylinder 6.
8. As a final step for persistent misfires, perform a compression test on cylinder 6 to rule out internal engine mechanical issues.
9. Use a borescope to visually inspect the intake valves for carbon buildup, a common issue on this engine that requires supercharger removal to view.

Parts You'll Likely Need

• Ignition Coil (OEM #06E905115G (supersedes 06E905115F, 06E905115E)) — This is the most frequent cause of a single-cylinder misfire on the 3.0T engine. Part numbers are frequently updated; 'G' is a recent revision.
  Trusted brands: Bosch, NGK, Eldor, Genuine Audi
  OEM price range: $50 - $75
  Aftermarket price range: $35 - $60
• Spark Plug (OEM #101905622) — Spark plugs are a regular maintenance item. A worn, fouled, or incorrectly gapped plug is a very common cause of misfires.

Related Codes That Often Appear With This One

• P130A — This Audi-specific code for 'Cylinder Disabling' is often stored alongside a misfire code. The ECM intentionally shuts down the fuel injector to the misfiring cylinder to protect the catalytic converter from damage. This is noted in TSB 2033768.
• P0300 — If multiple cylinders are misfiring, you may see a P0300 'Random/Multiple Cylinder Misfire' code. This points to a systemic issue like a massive vacuum leak (bad PCV), fuel pressure problem, or clogged catalytic converters.
• P0305 — Misfires on cylinders 5 and 6 often occur together, especially when caused by carbon buildup, as both are at the rear of the intake manifold. A user on the Ross-Tech forum reported P0305 and P0306 together, which ended up being a combination of fouled plugs and oil leaks.

Technical Service Bulletins (TSBs) & Recalls

• TSB 2033768/1 / 01-13-89: MIL on with cylinder misfire fault (P0301-P0306) and cylinder deactivation fault (P130A) stored.
• TSB 2030197/15: Addresses high oil consumption related to PCV system failure.

Platform-Specific Known Issues

• A Technical Service Bulletin (TSB #2033768, also listed as 01-13-89) exists for misfire faults (P0301-P0306) appearing with code P130A. The TSB is informational, instructing technicians to contact Audi's Technical Assistance Center (TAC) with diagnostic logs before proceeding.
• Forum discussions frequently point to carbon buildup on the intake valves as a cause for misfires on cylinders 5 and 6, particularly under heavy load.
• Leaking intercoolers inside the supercharger are a well-documented failure mode for the 3.0T engine, often causing persistent misfires in cylinders 3 and 6 that can be misdiagnosed as ignition or fuel issues.

Mechanic-Grade Diagnostic Values

• VCDS Misfire Counter (Live Data) — expected: Using VCDS, Measuring Block Group 016 should show a count of 0 for cylinder 6 at idle and under load.. Failure: A rapidly increasing number in the counter for cylinder 6 confirms an active misfire.
• Low-Pressure Fuel Pump (LPFP) Feed Pressure — expected: Approximately 4 to 6 bar (58 to 87 PSI).. Failure: Pressure dropping below this range will starve the High-Pressure Fuel Pump (HPFP), potentially causing misfires under load.
• High-Pressure Fuel Pump (HPFP) Rail Pressure — expected: Up to 1500+ PSI (over 100 bar) under load.. Failure: Significantly lower pressure than requested by the ECM, especially under acceleration, points to a failing HPFP or a leak in the high-pressure system (e.g., an injector).

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $53 (example ID): Provides raw misfire count data for each cylinder from the last 10 driving cycles, even if the misfire rate is not high enough to set a P0306 code and turn on the Check Engine Light. (see via Requires a professional-grade scan tool with Mode $06 capability. Basic OBD-II readers typically cannot display this data.)

Scan Tool Commands That Help

• VCDS (VAG-COM): Advanced Measuring Values / Measuring Blocks (Groups 015, 016) — This is the primary method for confirming a live misfire. It allows real-time monitoring of the misfire counter for each individual cylinder to see when and how often cylinder 6 is misfiring.
• VCDS (VAG-COM): Output Tests (Function 03) — After ruling out spark, this can be used to test the electrical circuit for the cylinder 6 fuel injector. The tool commands the ECU to activate the injector, and a clicking sound should be audible. This helps differentiate between a failed injector and a wiring problem. Note: The engine must be off, and you may need to press the accelerator pedal to activate the test.

Wiring & Ground Locations

• Engine Ground Strap — On the 3.0T engine, a primary ground strap is located on the right (passenger) side of the engine compartment, connecting the engine block to the vehicle's frame, often near the alternator.. A corroded, loose, or broken main engine ground can cause insufficient voltage to reach the ignition coils, leading to a weak spark and intermittent misfires that can be difficult to diagnose and may not be isolated to a single cylinder.
• Cylinder 6 Fuel Injector Connector — Connects to the fuel injector for cylinder 6, which is on the driver's side, closest to the firewall. The wiring is part of the driver's side fuel injector loom (harness part # 06E971627S as an example).. If an injector is suspected, checking for voltage at this connector's pins during an output test can confirm if the injector is receiving a signal from the ECM. A wiring diagram is needed to identify the specific power and signal pins.

Real Owner Repair Stories

• MyS4.org blog post (Audi S4 2.7T (B5 generation, but diagnostic process is identical for P0306)) — Persistent P0306 misfire on cylinder 6.
  ❌ Tried (didn't work) Replacing ignition coils, Swapping ignition coils between cylinders, Replacing the fuel injector for cylinder 6, Swapping the spark plug from cylinder 6 to cylinder 5
  ✅ What actually fixed it The owner discovered the aftermarket Denso Iridium spark plugs had a gap far too wide (0.035 - 0.045 inches). After replacing them with OEM NGK plugs gapped to a much tighter 0.022 inches (for his tuned application), the misfire was resolved. The key takeaway was to always check the gap on new spark plugs before installation.
• AudiWorld Forums user (2012 Audi S4 3.0T) — EPC light, rough running, poor acceleration, P0305 (Cylinder 5 Misfire, but same root cause applies to P0306).
  ❌ Tried (didn't work) Replacing spark plugs, Replacing ignition coils
  ✅ What actually fixed it After ignition components didn't fix the issue, the supercharger was removed. A pressure test of the intercooler system revealed a fine mist of coolant leaking from one of the internal cores directly into the cylinder. Replacing the two intercooler cores inside the supercharger assembly resolved the misfire.

"I Checked Everything" — The Actual Cause

• A failed check valve in the PCV breather hose on the 3.0T engine may not be found with a traditional intake smoke test. This specific failure allows boost pressure to enter the crankcase, causing a rough idle and misfires. The diagnosis requires removing the specific hose and testing the internal check valve's function directly, rather than relying on a smoke test which checks for unmetered air entering the intake.

OEM Part Supersession History

• 06E103547 (revisions F, Q, C) → 06E103547AC (or later, verify by VIN) — The PCV valve (oil separator) for the 3.0T has undergone multiple revisions to improve durability and address issues with the internal diaphragm tearing, which causes a major vacuum leak and can lead to misfires.
  Heads up: Some revisions changed the internal crankcase pressure regulation. Using the wrong version for your specific engine code and software can lead to oil consumption or seal issues. It is critical to match the part to your vehicle's VIN.