P0304 on 2012-2018 Audi A6 3.0T: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2012-2018 Audi A6

The 3.0L TFSI Supercharged V6 is a direct-injection engine, which makes it inherently prone to carbon buildup on the intake valves over time. This buildup can disrupt airflow and is a well-documented cause of misfires on this platform, sometimes requiring cleaning as early as 60,000 miles. While ignition components are a primary suspect, this underlying issue of carbon buildup should always be considered. Additionally, the PCV valve (oil separator) and the water pump/thermostat assembly, both located under the supercharger, are known failure points that can lead to misfires and other engine issues.

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough engine idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Increased fuel consumption
• Unpleasant odor from the exhaust (like gasoline)
• Vibration felt throughout the cabin

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems on modern high-output engines are subjected to high thermal and electrical stress, making them a common failure item. The red-topped 'R8' style coils are often used as an upgrade for a stronger spark.
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 5). Clear the codes and drive the vehicle. If the code changes to P0305, the ignition coil is faulty. This is the most definitive and cost-effective first diagnostic step. 🎬 Watch: How to diagnose misfire codes P0300, P0304, and P0305.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all six coils at the same time, as they tend to fail around the same mileage. Use genuine OEM coils for best results.
   Est. part cost: $35-$85 per coil
2. Worn Spark Plug 🔴 High Probability → Shop Spark Plug Audi's supercharged engines have specific service intervals for spark plugs, typically every 35,000-40,000 miles. Exceeding this interval or oil fouling from other issues can lead to a weak spark and misfires.
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 4. Inspect it for wear (worn, rounded electrode), fouling (black, sooty, or oily deposits), or damage to the electrode or ceramic insulator.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs at once with OEM-spec plugs, such as the NGK PFR8S8EG, to ensure consistent performance. Ensure they are torqued 🎬 See this DIY walkthrough for replacing your Audi A6 spark plugs. to the correct specification (typically 22 ft-lbs or 25-30 Nm).
   Est. part cost: $15-$25 per plug
3. Intake Valve Carbon Buildup 🟡 Medium Probability As a direct-injection (TFSI) engine, fuel is sprayed directly into the combustion chamber, not over the intake valves. This means there's no fuel to wash away crankcase oil vapors, which bake onto the hot valves, forming hard carbon deposits that restrict airflow and cause misfires, particularly on cold starts. This issue is well-documented across the 3.0T platform.
   How to confirm: This requires a visual inspection using a borescope inserted into the intake runners, or by removing the supercharger and intake manifolds. This is typically done when other causes are ruled out, often at mileages over 60,000.
   Typical fix: The intake valves must be cleaned. This is typically done via walnut shell blasting, a process where crushed walnut shells are used as a media to blast the carbon off the valves without damaging the metal surfaces. This is a labor-intensive job that requires removal of the supercharger.
   Est. part cost: $100-$200 for cleaning materials, but labor can be $800-$1500 due to the need to remove the supercharger.
4. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Direct injection systems operate at very high pressures, and injectors can become clogged or fail electronically, disrupting the fuel spray pattern. While less common than ignition issues, it is a possible cause.
   How to confirm: This is more difficult for a DIYer. A professional can perform a fuel injector balance test. Swapping injectors between cylinders is possible but is much more labor-intensive than swapping coils and requires special tools and replacement of single-use Teflon seals.
   Typical fix: Replace the faulty fuel injector. It's mandatory to replace the Teflon seals and O-rings on any injector that is removed to prevent dangerous fuel leaks.
   Est. part cost: $100-$250 per injector

Rare But Worth Checking

• Leaking Supercharger Intercooler: → Shop Turbocharger The 3.0T engine has two air-to-water intercoolers (charge air coolers) located within the 'V' of the engine, beneath the supercharger. These units can develop internal coolant leaks. The coolant can then drip into the intake ports, fouling the spark plug and causing a misfire. This is often accompanied by unexplained coolant loss. While known to commonly affect cylinders 3 and 6, it can happen to any cylinder.
• Failed PCV Valve / Oil Separator: → Shop PCV Valve The Positive Crankcase Ventilation (PCV) valve, also called an oil separator, is a known failure point located under the supercharger. A failed PCV can cause excessive oil consumption, vacuum leaks, rough idle, and misfires that may start on one cylinder and spread to others. There have been several revisions of this part.
🎬 Watch: How to replace the PCV, water pump, and thermostat.
• Low Cylinder Compression: In rare cases, a mechanical issue like a damaged valve (sometimes due to extreme carbon buildup), worn piston rings, or a compromised head gasket can cause a loss of compression in cylinder 4, leading to a persistent misfire. This is confirmed with a compression test and is the most serious potential cause.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0304 and check for any other codes.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe, catalyst-damaging misfire. Do not drive if flashing.
3. Perform an ignition coil swap. Move the coil from cylinder 4 (front-most, driver's side) to cylinder 5 (middle, driver's side). Clear the codes and drive the car. Re-scan. If the code is now P0305, the ignition coil is bad.
4. If the code remains P0304, inspect the spark plug. Remove the coil from cylinder 4 again and remove the spark plug using a 16mm spark plug socket. Check for heavy wear, damage, or fouling from oil or carbon. Replace if necessary, preferably as a set of six.
5. If the coil and plug are good, inspect the spark plug tube for oil. A leaking valve cover gasket can allow oil to pool in the tube, shorting out the ignition coil.
6. If ignition components are ruled out, consider more platform-specific issues. Check engine coolant levels. A slow, consistent drop in coolant with no visible external leak could point to a leaking supercharger intercooler.
7. Consider carbon buildup, especially if the vehicle has over 60,000 miles. A professional diagnosis with a borescope is the next logical step.
8. Check for vacuum leaks, paying close attention to the PCV system. A rough idle and hissing noise can be clues.
9. If all else fails, a compression test should be performed on cylinder 4 to check for mechanical engine health.

Parts You'll Likely Need

• Ignition Coil (OEM #06E905115G) — This is the most common failure part for a single-cylinder misfire on this engine. It's recommended to use OEM or a quality equivalent like Bosch or NGK.
• Spark Plug (OEM #NGK PFR8S8EG) — This is a routine maintenance item and the second most likely cause. Plugs can foul or wear out, causing a weak spark. Always replace as a set of six.
• PCV Valve / Oil Separator (OEM #06E103547AC) — A very common failure on the 3.0T engine. A torn internal diaphragm causes a massive vacuum leak, leading to rough idle and misfires. The part has been revised multiple times; ensure you get the latest version.

Related Codes That Often Appear With This One

• P0300 — If the underlying issue, like a major vacuum leak from the PCV system or severe carbon buildup, starts to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear with P0304.
• P0305, P0306 — Misfires on other cylinders in the same bank (Bank 2 is cylinders 4, 5, 6) can indicate a problem common to that bank, such as a leaking intercooler on that side or a clogged catalytic converter for Bank 2.
• P130A — This Audi-specific code for 'hide cylinder' or 'Cylinder Disabling' may appear with a severe misfire. It means the ECM has intentionally cut fuel to cylinder 4 to prevent unburned fuel from damaging the catalytic converter.
• P0430 — Catalyst System Efficiency Below Threshold (Bank 2). If a misfire has been occurring for a long time, unburned fuel can damage the catalytic converter on the driver's side, triggering this code.

Technical Service Bulletins (TSBs) & Recalls

• TSB 2069038 / 01 24 21: Addresses an illuminated MIL with misfire codes (P0300-P0306) and states that an ECM software update may be required to correct overly sensitive detection.
• TSB 2058038: Notes that software calibration can cause misfire codes with certain winter-blend fuels.
• Bulletin #A0124212069038_5: Provides diagnostic information for when the MIL is illuminated specifically with cylinder misfire codes P0300 through P0306.
• Bulletin #A0123912069038_5: Notes that the MIL may be illuminated with stored DTCs P0300, P0301, P0302, P0303, P0304, P0305, and P0306.
• Bulletin #A0123802069038_4: Confirms that misfire codes P0300-P0306 are associated with MIL illumination and provides relevant technical guidance.
• Bulletin #A0123612069038_3: States that the MIL may be on and DTCs P0300, P0301, P0302, P0303, P0304, P0305, or P0306 may be stored in the vehicle's memory.
• Bulletin #A0123352069038_2: Provides instructions for cases where the MIL is illuminated and one or more DTCs are stored, including P0300, P0304, and other cylinder-specific misfire codes.

Platform-Specific Known Issues

• Carbon Buildup: Direct injection engines like the 3.0T TFSI are highly susceptible to carbon deposits on intake valves, which is a leading cause of misfires as the vehicle ages.
• PCV Valve Failure: The oil separator (PCV) located under the supercharger is a common failure item that requires significant labor to replace.
• Leaking Intercoolers: The charge air coolers located under the supercharger can leak coolant internally, dripping into the intake ports and fouling spark plugs, causing misfires.
• Software Updates: In some cases, Audi has released TSBs for overly sensitive misfire detection that can be corrected with an ECM software update at a dealership.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel Rail Pressure (at idle) — expected: Approx. 35 Bar (508 PSI). Failure: Significantly lower pressure, or pressure that doesn't rise with engine load, can indicate a failing High-Pressure Fuel Pump (HPFP).
• High-Pressure Fuel Rail Pressure (under load) — expected: Up to 125 bar (1812 PSI) or higher, depending on engine load.. Failure: If 'Actual' pressure in VCDS does not track 'Requested' pressure, it points to a fuel delivery problem.
• Low-Pressure Fuel System Pressure (pre-HPFP) — expected: 4.0 to 6.0 bar (58 to 87 PSI).. Failure: Pressure dropping below this range will starve the HPFP, causing misfires, especially under high load.
• VCDS Misfire Counter (Live Data) — expected: The counters for each cylinder should remain at or near zero at idle and under load.. Failure: A rapidly increasing count on cylinder 4 confirms the misfire in real-time, which is useful for diagnosing intermittent issues.
• Long-Term Fuel Trim (VCDS Measuring Block 032) — expected: Within +/- 10%. Values close to 0% are ideal.. Failure: High positive values indicate the engine is adding fuel to compensate for a vacuum leak; high negative values mean it's removing fuel due to an over-rich condition (e.g., leaking injector).

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $A2-$AC: This data tracks misfire counts per cylinder during the current and last 10 driving cycles. Test ID $A5 would correspond to cylinder 4. This data can show misfires that haven't yet met the threshold to trigger a Check Engine Light. (see via Most advanced OBD-II scanners, including VCDS, can access Mode $06 data.)

Scan Tool Commands That Help

• VCDS (VAG-COM): Measuring Value Blocks (MVB) - Groups 015 & 016 — This is the primary method for confirming an active misfire on a specific cylinder in real-time. Use this while the engine is running to see the misfire count for cylinder 4 (and others) increase at idle or under load, which helps isolate the conditions of the fault.
• VCDS (VAG-COM): Advanced Measuring Values - Fuel Pressure — Use this function to log both 'Fuel pressure' (high pressure) and 'Fuel pump: specified value' to see if the actual pressure is meeting the requested pressure under various load conditions.
• VCDS (VAG-COM): Measuring Block 032 (Fuel Trims) — Check long-term fuel adaptations at idle and under load. Significant deviations can point towards unmetered air (vacuum leaks) or fuel delivery issues that could cause a misfire.

Wiring & Ground Locations

• Engine Block Ground Strap — On the C7 platform, a primary engine ground point is located on the right (passenger side in US) of the engine compartment, near the alternator.. A corroded or loose engine ground can cause a variety of electrical issues, including weak spark or erratic behavior from ignition coils, leading to misfires that can be difficult to diagnose.
• Ignition Coil Connector Ground — Each ignition coil has a small ground wire integrated into its harness connector.. This wire can become brittle from heat cycles and break during spark plug or coil replacement. A broken ground at the coil connector will prevent that coil from firing, causing a persistent misfire on that cylinder.
• T4fj / T56c — T4fj is the 4-pin connector at the ignition coil for cylinder 4. It routes back to the Engine Control Unit (J623) at the 56-pin connector T56c.. When diagnosing a persistent misfire after replacing parts, testing for continuity and voltage at these pins is necessary. Pin 2 of T4fj is the signal wire, and Pin 4 is the ground connection (846) in the engine prewiring harness.
• 1J0973724 — This is the OEM part number for the 4-pin ignition coil connector pigtail.. The locking tabs on these connectors often break with age, leading to a poor connection. If the connector is damaged, replacing the pigtail is often necessary to ensure a solid connection to the ignition coil.

Real Owner Repair Stories

• Ross-Tech Forums user (Audi Q7 3.0T (same engine and principles)) — Misfires on one entire bank of cylinders (4, 5, and 6).
  ❌ Tried (didn't work) Good coils and plugs were assumed, No extreme carbon buildup was present
  ✅ What actually fixed it The catalytic converter for that bank had failed internally. The matrix had cracked and partially melted, creating a major exhaust restriction (back-pressure) that caused misfires across all cylinders on that bank. The fix was to replace the catalytic converter.
• Reddit user on r/Audi (Audi S4 with 3.0T Supercharged engine) — Misfire on a single cylinder.
  ❌ Tried (didn't work) Initial diagnosis pointed to common coil/plug issues.
  ✅ What actually fixed it The final cause was a leaking supercharger intercooler core. Coolant was dripping directly into the intake port for the affected cylinder, fouling the spark and causing the misfire. The fix required removing the supercharger and replacing the internal intercooler cores.

"I Checked Everything" — The Actual Cause

• A common scenario on the 3.0T is a failed PCV valve diaphragm. This creates a massive, unmetered vacuum leak. While a smoke test should find this, the leak is often internal to the crankcase system, and mechanics sometimes miss it if they aren't specifically testing crankcase vacuum or are unfamiliar with this engine's failure mode. The symptom is often a loud whistle from the engine bay at idle.

When the Usual Fixes Don't Work

• While ignition coils and spark plugs are the most common cause, a significant number of owners on forums like Audizine and AudiWorld have chased a P0304 code by replacing these parts, only to find the true cause was one of the platform's other weaknesses. The two most common 'final fixes' after coils/plugs fail to solve the issue are carbon cleaning for high-mileage cars, and replacement of a leaking supercharger intercooler, often identified by slow coolant loss with no external drips.

OEM Part Supersession History

• 06E103547 (and revisions F, Q, C) → 06E103547AC (or later) — The PCV valve (oil separator) has undergone multiple revisions to improve the durability of the internal diaphragm, which is prone to tearing and causing a large vacuum leak.
  Heads up: It is critical to use the correct revision for your specific engine code and software version. Some revisions have different internal crankcase pressure regulation, and using the wrong one can lead to oil consumption or other issues. Always verify the part number with your VIN.

Model Year Variations Within This Range

• 2012-2015: Pre-facelift models use the CGXB or CTUA engine codes. While mechanically very similar to the later CREC engine, some early models (especially 2012-2013) were noted to have higher rates of catalytic converter failure.
• 2016-2018: Facelift models (C7.5) use the CREC engine, which features a dual-injection system (both direct and port injection) in some markets, though the US version remained direct-injection only. The CREC also has a supercharger clutch that decouples the supercharger under light load for better efficiency. While the fundamental causes of P0304 remain the same, diagnostics related to the supercharger clutch could be a factor in rare cases if it affects boost delivery inconsistently.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Thermostat and Water Pump Failure 🔴 High — Common failure item, often between 60,000 and 100,000 miles. Early C7 models were particularly prone. It is a labor-intensive job as the components are located under the supercharger. (Ref: Customer satisfaction bulletins were issued for some early models.)
• PCV Valve / Oil Separator Failure 🔴 High → Shop PCV Valve — Very common failure. Leads to high oil consumption, rough idle, and potential vacuum leaks. Like the thermostat, it's located under the supercharger, making replacement costly. (Ref: Multiple part revisions exist (e.g., 06E103547AC, 06E103547AH), indicating ongoing attempts to improve reliability.)
• Upper Timing Chain Cover Oil Leaks 🟠 Medium — The gaskets and seals for the upper timing chain covers at the rear of the engine are known to leak oil over time. While not an immediate catastrophic failure, it can create a mess and lead to oil loss.
• Engine Mount Failure 🟠 Medium — The fluid-filled electronic engine mounts are a common wear item and can fail, leading to excessive engine vibration felt in the cabin, especially at idle.
• Timing Chain Tensioner Wear 🟠 Medium — Less common than on older Audi V8s, but the timing chain tensioners can wear on higher mileage (100k+ miles) 3.0T engines. The primary symptom is a brief chain rattle on a cold start that lasts for 1-2 seconds. While Audi may consider a short rattle 'normal', it indicates wear.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, used parts are generally not recommended for the core components. However, if a larger assembly like the supercharger needs to be removed, sourcing used but tested components like intercooler cores from a low-mileage donor vehicle can be a cost-effective alternative to new OEM parts, which are very expensive.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For supercharger intercoolers: Ask the seller if they were pressure tested after removal. Look for any signs of coolant residue or staining around the ports.
• For any electronic module: Ensure the part number is an exact match. Ask for a photo of the part from the donor vehicle showing it is clean and free of corrosion.
• Verify the donor vehicle's VIN to check for accident history; avoid parts from flood or severe front-end collision vehicles.

OEM-only on this vehicle (don't cheap out):

• PCV Valve / Oil Separator: Due to multiple revisions and the critical nature of crankcase pressure, using a genuine, latest-revision OEM part is strongly advised.
• Spark Plugs: These are inexpensive, critical wear items. Never use used spark plugs.
• Fuel Injector Seals: These are single-use Teflon seals. They must be new anytime an injector is installed to prevent catastrophic fuel leaks.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Bosch, NGK, Eldor (often the OEM supplier). 'R8' red-top coils are a popular upgrade.
• Spark Plugs: NGK (PFR8S8EG is a common OEM spec), Denso.
• PCV Valve: Vaico, Hengst (if Genuine Audi is unavailable).

Brands owners have reported issues with on this vehicle:

• Unnamed/unbranded ignition coils and sensors from online marketplaces. These frequently have high failure rates and can cause more diagnostic confusion.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0300 — Random or Multiple Cylinder Misfire Detected; often accompanies P0304 when issues like carbon buildup or PCV failure affect multiple cylinders.
• P0305 — Cylinder 5 Misfire; used as a diagnostic companion when swapping coils from cylinder 4 to cylinder 5 to confirm a faulty ignition coil.
• P0306 — Cylinder 6 Misfire; another Bank 2 cylinder code that may appear alongside P0304 during TSB-related software sensitivity issues.
• P0301 — Cylinder 1 Misfire; mentioned in TSB 2069038 as part of the P0300-P0306 range requiring potential ECM software updates.

Frequently Asked Questions