P0306 on 2006-2013 BMW 3-Series (E90): Cylinder 6 Misfire Causes and Fixes

What's Unique About the 2006-2013 BMW 3-Series (E90/E91/E92/E93)

The BMW N52 engine is a naturally aspirated inline-six known for its smooth operation, but it is not immune to misfires. The most common culprits for a single-cylinder misfire like P0306 are the individual coil-on-plug ignition coils, which are a known wear item. While many engines have similar issues, the N52's original-spec Bosch ignition coils are a very frequent point of failure as these cars age. BMW even issued a service bulletin (SIB 12 18 14) recommending the replacement of all Bosch coils with Delphi units if a failure occurred on certain N52 engines.

Symptoms You May Notice

• Rough or shaky idle
• Hesitation or stumbling during acceleration
• Reduced engine power
• Solid or flashing Check Engine Light
• Increased fuel consumption
• Engine vibrations that can be felt in the cabin

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability The original equipment (OE) Bosch coils on the N52 engine are a well-documented, common failure item as they age. BMW later switched to Delphi and Eldor coils, which are considered more reliable by many owners and independent shops.
   How to confirm: Swap the ignition coil from cylinder 6 with an adjacent cylinder (e.g., cylinder 5). Clear the codes and drive the car. If the code changes to P0305 (Cylinder 5 Misfire), the ignition coil is faulty. This is the universally recommended first diagnostic step.
   Typical fix: Replace the faulty ignition coil. Many owners choose to replace all six coils at the same time as a preventative measure, since if one has failed, others may be close to failing. It is recommended to use the same brand for all six coils.
   Est. part cost: $35-$70 per coil
2. Worn Spark Plug 🔴 High Probability Spark plugs are a regular maintenance item with a typical service life of 60,000-100,000 miles. If they are past their service life, the gap can become too large or electrodes can become fouled, leading to a weak or inconsistent spark.
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 6. Inspect it for wear, carbon fouling, or oil contamination. You can also swap the plug with another cylinder to see if the misfire follows.
   Typical fix: Replace the spark plug in cylinder 6. It is highly recommended to replace all six spark plugs at the same time to ensure even performance and avoid future misfires.
   Est. part cost: $15-$25 per plug
3. Leaking Valve Cover Gasket 🟡 Medium Probability The valve cover gasket on the N52 engine is known to become brittle and leak oil over time. The spark plug tube seals are integrated into this gasket, and a failure allows oil to pool in the spark plug wells, shorting out the ignition coil and fouling the spark plug.
   How to confirm: When you remove the ignition coil for cylinder 6, look for liquid oil pooled in the spark plug well. A smell of burning oil from the engine bay is also a common symptom.
   Typical fix: Replace the valve cover and valve cover gasket set. The oil-soaked ignition coil and spark plug for cylinder 6 must also be replaced. It is often recommended to replace the entire valve cover assembly as the integrated CCV system can also fail.
   Est. part cost: $50-$150 for the gasket set, or $300-$500 for a full valve cover assembly
4. Faulty Fuel Injector ⚪ Low Probability
   How to confirm: If the ignition coil and spark plug have been ruled out, the fuel injector is the next likely component. This requires more advanced diagnosis, such as swapping the injector with another cylinder (more difficult than a coil swap) or having a professional test the injector's flow and spray pattern.
   Typical fix: Replace the faulty fuel injector. It's often recommended to replace them as a set if the vehicle has high mileage.
   Est. part cost: $50-$200 per injector

Rare But Worth Checking

• Clogged or Failed CCV System: The Crankcase Ventilation (CCV) system can fail, creating a vacuum leak that leads to a lean air-fuel mixture and potential misfires. On the N52, the CCV is integrated into the valve cover. This would typically be accompanied by lean codes (P0171, P0174).
• Low Engine Compression: Internal engine issues like a burnt valve or worn piston rings can cause a loss of compression, leading to a persistent misfire. This is a worst-case scenario and should be investigated with a compression test after all other causes are ruled out.
• VANOS Solenoid Failure: Dirty or failing VANOS (variable valve timing) solenoids can cause rough idling, hesitation, and misfire codes. They are located on the front of the engine and can often be cleaned as a first step before replacement.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0306 and any other related codes.
2. Turn off the engine and locate cylinder 6, which is the cylinder at the rear of the engine, closest to the windshield.
3. Swap the ignition coil from cylinder 6 with the coil from cylinder 5.
4. Clear the fault codes using the scanner, then start the engine and let it run for a few minutes or take a short drive.
5. Re-scan for codes. If the code has moved to P0305, the ignition coil from the original cylinder 6 is faulty and must be replaced.
6. If the code remains P0306, the coil is likely good. Proceed to inspect the spark plug. Remove the plug from cylinder 6 and check for excessive wear, fouling, or damage. If it looks worn or is soaked in oil or fuel, replace it.
7. If you find oil in the spark plug tube, the valve cover gasket has failed and needs to be replaced. This is the root cause of the misfire.
8. If the coil and plug are good and there is no oil, the issue may be the fuel injector. This is a more involved diagnosis and may require professional assistance or swapping the injector to another cylinder.
9. As a final step for persistent issues, a mechanic can perform a compression test to check for internal engine problems like worn piston rings or a burnt valve.

Parts You'll Likely Need

• Ignition Coil (OEM #12138616153 (Delphi/Eldor), 12137594937 (Bosch)) — This is the most common failure point for a P0306 code on the N52 engine. The original Bosch coils are known to fail, and many owners upgrade to Delphi or Eldor.
  Trusted brands: Bosch, Delphi, Eldor
  OEM price range: $50-$70
  Aftermarket price range: $35-$55
• Spark Plug (OEM #12122158253 (NGK PLZFR6A-11S)) — A worn or fouled spark plug is the second most common cause and is a standard maintenance item.
  Trusted brands: NGK, Bosch
  OEM price range: $20-$25
  Aftermarket price range: $15-$20
• Valve Cover Gasket Set (OEM #11127581220) — Required if oil is found in the spark plug well, which is a common cause of coil and plug failure on this engine.
  Trusted brands: Elring, Victor Reinz
  OEM price range: $80-$120
  Aftermarket price range: $40-$70

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0306.
• P0301-P0305 — If other coils or plugs are failing, you may see codes for other specific cylinders. Seeing multiple codes often points to a common cause like a vacuum leak or fuel delivery issue.
• P0171 / P0174 — These codes indicate the engine is running too lean (too much air, not enough fuel). A vacuum leak from a failed CCV system or intake boot could cause both a lean condition and a misfire code like P0306.

Technical Service Bulletins (TSBs) & Recalls

• SIB 12 18 14: A BMW service bulletin addressing ignition coil failures on N51, N52, N52K, and N52T engines. It advises replacing all original Bosch coils with Delphi coils when one fails on vehicles that have been in service for over 24 months or 10,000 miles.

Platform-Specific Known Issues

• The original Bosch ignition coils are a known weak point. BMW issued Service Bulletin SIB 12 18 14, which in later revisions recommended replacing all Bosch coils with Delphi coils upon the first failure for cars over 24 months or 10,000 miles old.
• Oil leaking from the valve cover gasket into the spark plug wells is a very common N52 issue that directly causes misfires by damaging the ignition coil and fouling the plug.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System — expected: 5.0 Bar (72.5 PSI) at idle. Some fluctuation between 60-72 PSI at idle has been observed by owners, but a stable pressure is ideal.. Failure: Pressure significantly below 5.0 Bar (e.g., 3.5 Bar) or priming to only ~40 PSI can indicate a weak fuel pump or faulty regulator, potentially causing lean conditions and misfires.
• Ignition Coil Power Supply Voltage — expected: Approximately 11.4V with key-on, engine-off at the orange power supply wire to each ignition coil.. Failure: Significantly lower voltage or no voltage on the cylinder 6 coil connector could indicate a wiring problem or an issue with the DME power supply, rather than a faulty coil itself.

Hidden / Shadow Codes Worth Checking

• 29D2: DME: Misfire, cylinder 6. This is the direct BMW-specific hexadecimal code for a P0306 misfire. (see via BMW-specific scan tools like ISTA, INPA, or high-end aftermarket scanners (e.g., Foxwell, Autel).)
• 29CC: DME: Misfire, multiple cylinders. This code often appears with 29D2 if the misfire is severe or if other cylinders are also beginning to fail. (see via BMW-specific scan tools.)
• 2A2B / 2A2C: DME: Mixture control 1 / Mixture control 2. These codes indicate a fuel mixture problem (lean or rich) on Bank 1 (cylinders 1-3) or Bank 2 (cylinders 4-6). A misfire on cylinder 6 can sometimes trigger a 2A2C code for Bank 2. (see via BMW-specific scan tools.)
• 2A31 / 2A32 / 2A47: DME: Valvetronic, eccentric shaft sensor plausibility/guide/reference. These codes point to a problem with the eccentric shaft sensor. A failing sensor can cause erratic engine behavior, including misfires, even without oil in the connector. (see via BMW-specific scan tools like ISTA or INPA.)

Scan Tool Commands That Help

• ISTA/INPA: Smooth Running Test — This function provides a live data reading of the rotational consistency of each cylinder. A cylinder with a high deviation value is misfiring. This is useful for confirming the misfire on cylinder 6 and checking if other cylinders are also struggling, even if they haven't set a code yet.
• ISTA/INPA: Valvetronic Limit Position Relearn — This should be performed after replacing a valve cover, valve cover gasket, or the Valvetronic motor itself. Failure to relearn the end stops can cause rough running and misfires. On post-2006 models, this can sometimes be done by cycling the ignition on and off twice without starting the engine.

Wiring & Ground Locations

• Main Engine Ground Strap — Connects from the driver's side engine mount area to the chassis, under the car near the driver's footwell.. This is the primary ground for the engine block. Corrosion or looseness of this strap can cause a host of difficult-to-diagnose electrical issues, including weak spark, sensor reading errors, and intermittent misfires that may not seem to have a clear cause.
• Ignition Coil Ground Strap — A smaller ground strap is located on the top-right of the engine (passenger side in LHD cars) that provides a ground path for the ignition coils.. If this specific ground is loose or corroded, it can cause fluctuating voltage and misfires across all cylinders, but may present as a single cylinder misfire initially.

Real Owner Repair Stories

• Bimmerpost user (2007 E92 335i (N54, but similar diagnostic path)) — P0306 code, service engine soon light.
  ❌ Tried (didn't work) Replacing spark plugs, Replacing ignition coils
  ✅ What actually fixed it The final cause was a leaking fuel injector on cylinder 6. The user noted that after replacing plugs and coils didn't work, the next logical step was the injector.
• Bimmerpost user (E90 328xi (N52)) — Misfire codes 29D0 (Cyl 4) and 29D2 (Cyl 6), plus 29CC (multi-cylinder misfire).
  ❌ Tried (didn't work) Replacing all spark plugs with new NGKs, Replacing all ignition coils with new Delphi units
  ✅ What actually fixed it The user discovered that swapping the ignition coil *wiring harness connectors* between cylinders 5 and 6 caused the misfire to move to cylinder 5. This indicated the problem was not the coil itself, but a break or fault in the wiring leading to the cylinder 6 coil connector.
• Reddit user r/BmwTech (2006 330i with 199k miles) — Cylinder 2 misfire (P0302), severe vibration, no power.
  ❌ Tried (didn't work) New spark plug, New ignition coil, New fuel injector, Unplugging eccentric shaft sensor (no change)
  ✅ What actually fixed it This was a terminal internal engine failure. A borescope inspection revealed "gold colored shards and... impact craters" in the cylinder, indicating a catastrophic mechanical failure (likely a dropped valve or broken valve spring) was the true cause of the misfire. This is a rare but possible worst-case scenario.

"I Checked Everything" — The Actual Cause

• In some cases, a persistent misfire on a single cylinder that isn't resolved by ignition or fuel components can be caused by a localized issue that a smoke test might miss. One such example is a failing hydraulic valve lifter (HVA). A stuck or collapsed lifter can prevent the valve from opening or closing properly, leading to a loss of compression and a misfire on that specific cylinder. This is a known issue on early N52 engines, often presenting as a 'ticking' noise.

When the Usual Fixes Don't Work

• While the vast majority of P0306 codes are fixed with a new coil or spark plug, forum records show a significant number of cases where these parts are replaced without resolving the issue. In one such case, a user with misfires on cylinders 4 and 6 (including code 29D2 for cylinder 6) replaced all coils and plugs to no avail. The problem was ultimately traced to a fault in the engine wiring harness itself, where swapping the physical connectors for the coils caused the misfire code to follow the connector, not the coil. In another instance, a P0306 that resisted new plugs and coils was finally fixed by replacing a leaking fuel injector.

OEM Part Supersession History

• 12137594937 (Bosch) → 12138616153 (Often Delphi or Eldor) — The original Bosch coils had a high failure rate. BMW issued service bulletins recommending replacement with more robust Delphi or Eldor units.
  Heads up: While technically compatible, it is strongly advised not to mix coil brands. If replacing one failed Bosch coil, it is best practice to replace all six with the newer Delphi or Eldor design to ensure consistent performance and prevent subsequent failures.

Model Year Variations Within This Range

• 2006 (Early N52) vs. 2007-2013 (N52K/N52N): The early N52 (pre-06/2006) had a magnesium valve cover with an external CCV system. The later N52K (often called N52N) switched to a plastic composite valve cover with an integrated CCV system. While the plastic cover is less prone to the complex CCV failures of the early engine, the cover itself can crack. The N52K also received updates to address the 'lifter tick' that was common on early N52s.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Electric Water Pump and Thermostat Failure 🔴 High — Very common, often fails without warning between 60,000 and 90,000 miles. Failure can cause rapid overheating and engine damage.
• Oil Filter Housing Gasket (OFHG) Leak 🔴 High — Extremely common. The gasket degrades, causing an oil leak that drips onto the serpentine belt. This can cause the belt to slip off, get ingested by the front crank seal, and lead to catastrophic engine failure.
• Valve Cover & Gasket Leak 🟠 Medium — Very common. The plastic cover can crack and the gasket becomes brittle, leading to oil leaks onto the exhaust manifold (causing a burning smell) and into the spark plug wells (causing misfires).
• Hydraulic Valve Lifter (HVA) Tick 🟡 Low — Common on earlier N52 engines (pre-2009), especially on cars used for frequent short trips. Causes an audible ticking noise from the top of the engine when warm. An updated cylinder head design was the eventual fix from BMW. (Ref: SIB 11 09 07)
• VANOS Solenoid Failure 🟡 Low — Fairly common. Solenoids get clogged with oil sludge, causing rough idle, hesitation, and fault codes. Often can be resolved by cleaning, but may require replacement.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, using used parts is generally not recommended for the most common failure items. However, if a larger component like the valve cover is needed, a used OEM valve cover from a low-mileage donor car can be a viable option, provided it is inspected thoroughly for cracks.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a used valve cover, check for any hairline cracks, especially around bolt holes and the integrated CCV port.
• Ensure all mounting surfaces are flat and not warped.
• Avoid any parts that show signs of extreme heat exposure (discoloration, brittleness).

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

• Eccentric Shaft Sensor: Aftermarket versions are known to be unreliable; use only a VDO (OEM supplier) or genuine BMW part.
• Valve Cover Assembly: While some aftermarket brands are acceptable, many cheap online options are known to crack or have poor CCV integration. Stick to OEM, or high-quality OEM suppliers like Elring if just replacing the gasket.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Delphi, Eldor, Bosch (though Delphi/Eldor are often preferred over the original-spec Bosch).
• Spark Plugs: NGK, Bosch.
• Valve Cover Gasket: Elring, Victor Reinz.

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name ignition coils and valve cover assemblies sold on marketplaces like Amazon or eBay. These frequently have high failure rates right out of the box or shortly after installation.

Real Owner Stories

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

Related OBD-II Codes

• P0305 — This code indicates a misfire on cylinder 5; it is frequently seen during the diagnostic process when swapping coils from cylinder 6 to cylinder 5 to confirm a component failure.
• P0300 — A random/multiple cylinder misfire code that may accompany P0306 if multiple ignition coils or spark plugs are reaching the end of their service life.
• P0301 — Cylinder 1 misfire; related as it shares the same common failure points (Bosch coils, spark plugs, or valve cover gasket leaks) as cylinder 6.
• P0302 — Cylinder 2 misfire; often occurs alongside P0306 on high-mileage N52 engines when spark plugs or coils fail as a set.

Frequently Asked Questions