P0305 on 2019-2021 Audi Q8: Cylinder 5 Misfire Causes and Fixes

What's Unique About the 2019-2021 Audi Q8

The 2019-2021 Audi Q8 uses the 3.0L V6 turbocharged EA839 engine (engine code DLZA). While ignition coil failure is a common issue on many modern Audi engines, another significant concern for these direct-injection platforms is carbon buildup on the intake valves. Because fuel is injected directly into the cylinder, it no longer washes over the intake valves, allowing oil and fuel vapors from the PCV system to bake onto them over time, restricting airflow and causing misfires. Some early EA839 engines have also seen rare but documented issues with rocker arms, though this is less common than ignition or fuel system faults.

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 (solid or flashing)
• Rough or vibrating idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Reduced fuel economy
• Fuel smell from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coil failure is a well-documented issue on the EA839 3.0T engine family used across Audi, Porsche, and other VW Group vehicles.
   How to confirm: Swap the ignition coil from cylinder 5 with another cylinder (e.g., cylinder 4). Clear the codes and drive the vehicle. If the code changes to P0304, the ignition coil is faulty. This is the standard DIY diagnostic procedure.
   Typical fix: Replace the faulty ignition coil. It is highly recommended to replace all six coils and spark plugs at the same time for preventative maintenance and to ensure consistent performance.
   Est. part cost: $40-$80 per coil
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item. The official Audi maintenance schedule for the Q8 55 TFSI calls for replacement every 40,000 miles or 6 years, whichever comes first. Worn plugs can cause a weak spark, leading to misfires.
   How to confirm: After ruling out the ignition coil, remove the spark plug from cylinder 5. Inspect it for wear (worn electrode), carbon or oil fouling, or damage. Compare it to a new plug.
   Typical fix: Replace the spark plug in cylinder 5. It is best practice to replace all six spark plugs as a set to ensure consistent performance, especially if they are near the 40,000-mile service interval.
   Est. part cost: $20-$35 per plug
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Direct injection systems operate under very high pressure, and injectors can become clogged with deposits or fail electronically. This can lead to an improper fuel spray pattern or incorrect fuel volume, causing a misfire. Forum discussions for the Q8 and its platform-mate Q7 mention injectors as a confirmed cause for misfires.
   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 a much more labor-intensive job than swapping coils or plugs. A strong fuel smell from the oil can sometimes indicate a leaking injector.
   Typical fix: Replace the faulty fuel injector. This requires removal of the fuel rail and should be done by an experienced technician or a confident DIYer. It's often recommended to replace the seals on the other injectors on that rail at the same time.
   Est. part cost: $100-$250 per injector
4. Carbon Buildup on Intake Valves 🟡 Medium Probability This is a well-documented issue on all Audi direct injection (TFSI) engines, including the EA839. Since fuel is injected directly into the cylinder, it doesn't clean the intake valves, leading to carbon deposits that restrict airflow and cause misfires, especially at idle or under load.
   How to confirm: Requires a borescope to visually inspect the intake valves, which usually involves removing the intake manifold.
   Typical fix: The intake valves need to be physically cleaned, typically through walnut shell blasting. 🎬 Watch: How to perform a walnut blast carbon cleaning service. This is a labor-intensive service best left to a professional shop specializing in German vehicles.
   Est. part cost: $0 (service only, typically $600-$1200)

Rare But Worth Checking

• PCV System Failure: A failed PCV (Positive Crankcase Ventilation) valve can cause incorrect crankcase vacuum and allow excessive oil vapor into the intake, which can foul spark plugs and contribute to carbon buildup, indirectly causing misfires.
• Low Engine Compression: This indicates a more serious mechanical engine problem, such as worn piston rings, a damaged piston, or a valve sealing issue. A compression test followed by a leak-down test is needed to confirm and diagnose the source.
• Vacuum Leak: A leak in a vacuum hose or the intake manifold gasket can disrupt the air-fuel ratio and cause misfires. This would usually be accompanied by other codes, like a lean condition code (P0171/P0174) or affect multiple cylinders.
• Rocker Arm Failure: → Shop Engine Rocker Arm Some earlier EA839 engines experienced issues with rocker arm bearings failing, causing valve train noise and misfires. While considered rare, it is a known catastrophic failure mode for this engine family.
🎬 See how to diagnose a rocker arm failure on this engine.

Diagnosis Steps

1. Read the code using an OBD-II scanner to confirm P0305 is present. Check for any other codes and review freeze-frame data.
2. Locate cylinder 5. On the Audi 3.0L V6 (EA839), Bank 1 (passenger side, right side) contains cylinders 1, 2, and 3 (front to back). Bank 2 (driver's side, left side) contains cylinders 4, 5, and 6 (front to back). Cylinder 5 is the middle cylinder on the driver's side.
3. Perform a visual inspection of the engine bay. Look for obvious issues like disconnected hoses, damaged wiring, or leaks near cylinder 5.
4. Swap the ignition coil from cylinder 5 with the coil from cylinder 4. To access the driver's side coils, the coolant expansion tank may need to be unbolted and moved aside. Clear the codes and drive the vehicle.
5. Rescan for codes. If P0304 appears, the ignition coil is bad and needs replacement.
6. If P0305 returns, swap the spark plug from cylinder 5 with the plug from cylinder 4. Clear codes and drive.
7. Rescan for codes. If P0304 appears, the spark plug was the issue.
8. If P0305 persists, the problem is likely a fuel injector, carbon buildup, or a mechanical issue. At this point, professional diagnosis with tools like a borescope or compression tester is recommended.

Parts You'll Likely Need

• Ignition Coil (OEM #06H905110P (Latest Revision)) — This is the most frequent cause of a single-cylinder misfire on this engine. The part has been superseded multiple times; older numbers include 06J905110N and 06H905110H.
  Trusted brands: Bosch (OEM Supplier), NGK, Eldor
  OEM price range: $60-$90
  Aftermarket price range: $40-$70
• Spark Plug (OEM #06K905601B) — As a primary ignition component, spark plugs are a common wear item that directly cause misfires when they fail. The service interval is 40,000 miles.
  Trusted brands: NGK, Bosch
  OEM price range: $25-$40
  Aftermarket price range: $15-$25
• Fuel Injector (OEM #06M906036AE (Verify with VIN)) — A clogged or failed fuel injector is a common cause once spark plugs and coils have been ruled out, especially on direct-injection engines.
  Trusted brands: Bosch (OEM Supplier), OEM
  OEM price range: $150-$250
  Aftermarket price range: $100-$180

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders, a P0300 (Random Misfire) code may appear alongside P0305.
• P0301, P0302, P0303, P0304, P0306 — If other cylinder-specific misfire codes are present, it points towards a more widespread issue like a failing fuel pump, major vacuum leak, systemic carbon buildup, or a software issue referenced in a TSB.
• P0171/P0174 (System Too Lean) — If the misfire is caused by a vacuum leak or a clogged fuel injector, the engine may run lean, triggering these codes in addition to the misfire code.

Technical Service Bulletins (TSBs) & Recalls

• A0124212069038_5: Provides information for when the MIL is illuminated due to cylinder misfires (P0300-P0306).
• A0123912069038_5: Provides information for when the MIL is illuminated, DTCs P0300, P0301, P0302, P0303, P0304, P0305, P0306.
• A0123802069038_4: This TSB specifically mentions a software update for the Engine Control Module (J623) to correct the logic that triggers misfire codes. It supersedes a previous bulletin and is a key step for a dealer to check.
• A0123612069038_3: This bulletin provides information for when the MIL is on and DTCs P0300, P0301, P 0302, P0303, P0304, P0305, or P0306 may be stored.

Platform-Specific Known Issues

• Audi has released TSB #A0123802069038_4 which involves a software update for the ECM (J623) to correct misfire detection sensitivity under certain conditions for P0300-P0306 codes. This should be checked by a dealer.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel Rail Pressure at Idle — expected: 35-50 bar. Failure: Significantly lower pressure, such as 25 bar, indicates a potential supply issue, HPFP fault, or leaking injectors.
• High-Pressure Fuel Pump (HPFP) Bleed-off Test — expected: After engine shutdown, rail pressure should climb from ~50 bar to 110-130 bar.. Failure: If pressure plateaus around 80-90 bar and then begins to bleed off, it can indicate the HPFP is leaking fuel into the crankcase.
• Fuel Injector Voltage (Key On, Engine Off) — expected: 11-14 V at the injector connector power pin to body ground.. Failure: No voltage indicates a problem with the power supply relay or wiring.
• Fuel Injector Control Wire Resistance to ECM — expected: Below 1 Ω. Failure: High resistance indicates an open or corroded wire in the harness between the injector and the ECM.

Hidden / Shadow Codes Worth Checking

• Misfire Counter (per cylinder): The ECU maintains a running count of misfire events for each cylinder, which can be viewed before they are frequent enough to set a P030x trouble code. (see via Using a VAG-specific scan tool like VCDS or OBDeleven, navigate to the Engine module (01) and view Live Data. On older systems, this was in Measuring Blocks 015 and 016. On newer UDS systems, search for live data channels with names like 'Misfire counter cylinder 5'.)
• Mode $06, TID $53 (example): Mode $06 data stores the results of non-continuous monitor tests. It contains misfire data for the last 10 driving cycles. A technician can see raw misfire counts for each cylinder, even if no check engine light is on. (see via A professional scan tool with Mode $06 capability. The tool will display test IDs (TID) and component IDs (CID). The misfire data will show a value and a maximum threshold. A value approaching the threshold indicates an intermittent misfire on that cylinder.)

Scan Tool Commands That Help

• ODIS/VCDS: SVM (Software Version Management) Update — Use when a TSB, such as A0123802069038_4, indicates a software update is available to fix overly sensitive or incorrect misfire detection. This should be a primary step if the cause of the misfire is not obviously mechanical.
• ODIS: Guided Fault Finding (GFF) — This is the dealer-level diagnostic routine. When a code like P0305 is present, GFF provides a step-by-step test plan, prompting the technician to check specific values and components in the correct order, which is crucial for complex diagnoses.
• VCDS/OBDeleven: Output Tests (e.g., Fuel Injector Test) — After ruling out spark, an output test can be used to command a specific fuel injector to cycle. The technician can listen for an audible click from the injector to confirm it is being actuated by the ECM, helping to isolate a wiring or ECM driver issue from a mechanically failed injector.

Wiring & Ground Locations

• 832 / 833 — Ground Connection 4 and 5 are located on the cylinder head.. The ignition coils and other critical engine sensors ground directly to the cylinder head. A loose or corroded ground connection at these points can cause erratic behavior and misfires that are difficult to trace.
• 12 — Ground Connection in the engine compartment, on the left front longitudinal member.. This is a primary chassis ground point for the engine harness. Corrosion or looseness here can cause a variety of seemingly unrelated electrical issues, including misfires.
• J623 (ECM) — The Engine Control Module (ECM) provides the switched ground signal to each individual fuel injector to fire it.. If a test light on the injector connector stays lit while cranking, it can indicate a shorted injector driver within the ECM or a short-to-ground in the wiring harness, rather than a faulty injector itself.

Real Owner Repair Stories

• Ross-Tech Forums user 'Itsmys4' (2018 Audi S4 (EA839 engine), 48,000 miles) — Ticking noise from engine, which dealers dismissed as normal injector noise.
  ❌ Tried (didn't work) Dealer diagnosis
  ✅ What actually fixed it The owner diagnosed a rocker arm failure and successfully replaced the faulty rocker arms himself over a weekend without removing the engine from the car, which resolved the issue.
• AudiWorld Forums user (2018 Audi Q7 3.0T, 53,000 miles) — P0304 (Cylinder 4 Misfire), hide cylinder code, drive system malfunction.
  ❌ Tried (didn't work) Replacing all fuel injectors
  ✅ What actually fixed it The misfire persisted after replacing injectors. A compression test revealed cylinder 4 was at 3 BAR (spec is 10+ BAR). Despite no visible damage with a borescope, the low compression indicated a severe internal engine problem, leading to a dealer recommendation for a full engine replacement.
• Reddit user r/Audi (2020 Audi SQ5 (EA839 engine), 17,000 miles, tuned with Integrated Engineering software) — Cylinder 6 misfire.
  ❌ Tried (didn't work) Initial diagnostics
  ✅ What actually fixed it A compression and leak-down test failed, with air audibly escaping into the crankcase. This pointed to piston and/or piston ring failure. The final repair was a new cylinder head. The user noted this was a recurring issue for other tuned cars in their area.

"I Checked Everything" — The Actual Cause

• In cases of severe internal engine damage like low compression from a bad valve or piston ring failure, a smoke test for vacuum leaks will come back clean. All external components (coils, plugs, injectors) will test good, but a compression and leak-down test will reveal the true mechanical failure.

OEM Part Supersession History

• 06M-prefix rocker arms → 0P2-prefix rocker arms — The original rocker arms used smaller needle bearings that were prone to failure. The updated part uses thicker, more durable needle bearings to prevent this.
  Heads up: Early production 2019 models (built before 08/06/2018) may still contain the failure-prone 06M rocker arms.
• 06M906036K, 06M906036AA, 06M906036AC → 06M906036AE (current for 3.0T) — Standard part evolution for performance and reliability.
  Heads up: The EA839 engine family has multiple compatible injectors. RS models use different part numbers (e.g., 06M906036AQ) which flow more fuel and are often used as upgrades, but require specific software tuning.

Model Year Variations Within This Range

• 2019: Early 2019 model year Q8s (manufactured before August 6, 2018) may be equipped with the older design of rocker arms (PN prefix 06M) that are known to have a higher failure rate. Later models use an updated part (PN prefix 0P2).
• 2019-2020: A specific Technical Service Bulletin (TSB 2054182, superseded by A0123802069038_4) calls for an ECM software update to correct misfire detection sensitivity. This applies to early-production vehicles and may resolve misfire codes where no mechanical fault can be found.