P0306 on 2007-2015 Mazda CX-9: Cylinder 6 Misfire Causes and Fixes

What's Unique About the 2007-2015 Mazda CX-9

The 2007-2015 Mazda CX-9 uses a Ford-derived 3.7L 'MZI' V6, which is part of the Ford Cyclone engine family. For a P0306 code, the key platform detail is the cylinder layout: Cylinder 6 is located on the front bank (closest to the radiator), on the driver's side. This makes it the easiest cylinder to access for diagnosis and repair, unlike the rear cylinders which require removing the intake manifold. While the misfire is usually a simple ignition part, this engine has a critical flaw with its internal, timing-chain-driven water pump that can cause much more severe issues, though a single cylinder misfire is not a primary symptom.

Symptoms You May Notice

• Rough idle or engine vibration
• Hesitation, stumbling, or jerking during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Reduced fuel economy
• Engine may stall when stopping
• A blinking check engine light, especially under load like going up an incline.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are a very common failure item on the 3.7L V6 engine, often failing due to heat and vibration causing the internal windings to break down over time.
   How to confirm: Swap the ignition coil from cylinder 6 with the coil from cylinder 5 (middle, front bank). Clear the codes and drive. If the code changes to P0305, the ignition coil is bad. This is a 15-minute diagnostic test.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all three front coils at the same time, or all six if the rear bank is being accessed for other service. Always use a quality OEM or aftermarket brand.
   Est. part cost: $40-$80 for a single coil
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug The 3.7L engine is sensitive to spark plug condition and proper gap. Worn plugs are a frequent cause of misfires, especially on vehicles with over 60,000 miles. In many owner experiences, a simple spark plug replacement is all that is needed to fix a P0306.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 6. Inspect it for a worn electrode, wide gap, carbon fouling, oil contamination, or a cracked insulator.
   Typical fix: Replace the spark plug in cylinder 6. It is best practice to replace all six spark plugs if they are near their service interval (typically 60k-100k miles), though accessing the rear three requires removing the intake manifold.
   Est. part cost: $10-$25 per iridium spark plug
3. Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Fuel injector failure is a documented issue on the 3.7L V6, often occurring after 70,000 miles. The injector can become clogged or fail electronically.
   How to confirm: If the coil and plug are good, listen to the injector with a mechanic's stethoscope; it should have a steady clicking sound. A silent or erratically clicking injector is suspect. This may also be accompanied by a P0206 code. A definitive test is to check the injector's resistance with a multimeter; a healthy injector should be between 11.3 and 12.7 ohms.
   Typical fix: Replace the fuel injector for cylinder 6.
   Est. part cost: $50-$150

Rare But Worth Checking

• Leaking Head Gasket: In a well-documented case on the Mazdas247 forum, a 2009 CX-9 with a persistent P0306 was not fixed by a new coil, plug, injector, or even a replacement PCM. The ultimate cause was found to be leaking head gaskets allowing coolant into cylinder 6, causing the misfire. Symptoms included coolant loss and white smoke from the exhaust. This should be considered a root cause if all ignition and fuel-related fixes fail.
• Failed Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) A failing ignition coil can short out and send a voltage spike back to the PCM, damaging the driver circuit for that cylinder. This is a known issue on the Ford/Mazda 3.7L platform. If a P0306 code persists with a P0356 code immediately after replacing the coil and plug, the PCM driver is likely fried. Specialized services can repair the PCM board, which is more affordable than a full replacement and reprogramming from the dealer.
• Low Engine Compression: If all ignition and fuel system checks pass, a mechanical issue like worn piston rings, a burnt valve, or issues related to the timing chain/water pump failure could be causing low compression in cylinder 6. A compression test is the definitive way to verify this. A healthy cylinder should have around 175 psi.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0306 is present.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe, catalyst-damaging misfire; do not continue to drive.
3. Locate cylinder 6. On the 3.7L V6, the cylinders are numbered 1-2-3 on the rear bank (by the firewall) and 4-5-6 on the front bank (by the radiator), from passenger to driver side. Cylinder 6 is on the front bank, driver's side.
4. Swap the ignition coil from cylinder 6 with the coil from cylinder 5 (the middle cylinder on the front bank).
5. Clear the codes with the scanner and test drive the vehicle until the Check Engine Light returns.
6. Re-scan the codes. If the code has moved to P0305, the ignition coil is faulty and must be replaced.
7. If the code remains P0306, the coil is likely good. Move the spark plug from cylinder 6 to cylinder 5, clear the codes, and repeat the test drive. If the code changes to P0305, the spark plug is the cause.
8. If P0306 returns again, the issue is likely with the fuel injector or a mechanical problem. Listen to the cylinder 6 fuel injector with a stethoscope; it should click rhythmically. Compare its sound to cylinder 5. Check the injector's resistance; it should be 11.3-12.7 ohms.
9. If ignition and fuel components appear to be working, check for signs of a head gasket leak (milky oil, coolant loss, white smoke).
10. If no other cause is found, perform a compression test on cylinder 6 to check for mechanical engine problems. Healthy compression should be around 175 psi.

Parts You'll Likely Need

• Ignition Coil (OEM #CY01-18-100A) — This is the most common cause of a single-cylinder misfire on the Mazda CX-9 3.7L engine.
  Trusted brands: Mazda (OEM), Motorcraft, NGK, Denso
  OEM price range: $70-$110
  Aftermarket price range: $40-$80
• Iridium Spark Plug (OEM #L3Y3-18-110 (Cross-references to Motorcraft SP-520 and NGK 6509)) — Worn or fouled spark plugs are the second most likely cause of a P0306 misfire. They are a standard maintenance item that can easily be replaced on the front bank.
  Trusted brands: NGK, Motorcraft, Denso
  OEM price range: $15-$25
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or other cylinders are also misfiring (e.g., due to a vacuum leak or contaminated fuel), a P0300 (Random Misfire) may be stored as well.
• P0206 — This code for 'Injector Circuit/Open - Cylinder 6' points directly to a problem with the fuel injector or its wiring. If seen with P0306, the fuel injector is the most likely culprit.
• P0356 — This code for 'Ignition Coil 'F' Primary/Secondary Circuit' indicates an electrical fault in the cylinder 6 ignition coil circuit. It strongly suggests a bad coil, a wiring issue, or a damaged PCM driver for that cylinder.

Technical Service Bulletins (TSBs) & Recalls

• TSB 13-4-17 (Ford): While a Ford TSB, it is relevant to the shared 3.7L engine. It describes a scenario where a failed ignition coil damages the PCM, requiring replacement of the PCM and all ignition coils. This is a known, though rare, failure pathway for a persistent misfire code.
• Mazda SSP 93: This is a Special Service Program, not a traditional TSB for this code, but is a critical document for owners. It extends the warranty on the power brake booster for 2007-2013 CX-9s, a common failure point on the platform.

Platform-Specific Known Issues

• Internal Water Pump Failure: The 3.7L V6 has a well-documented design flaw where the internal, timing-chain-driven water pump can fail and leak coolant into the engine oil. While this typically causes timing codes (like P0016) or random misfires (P0300), a persistent, hard-to-diagnose single misfire could be an early symptom of developing engine problems related to oil contamination or coolant fouling the spark plug.

Mechanic-Grade Diagnostic Values

• Fuel Injector Coil Resistance — expected: 11.3 - 12.7 ohms @ 20°C (68°F). Failure: A reading of infinite ohms (Open Loop), near-zero ohms (Short), or a value outside the specified range.
• Fuel Pressure (at fuel pump outlet) — expected: 55-65 psi. Failure: Pressure significantly below 55 psi indicates a weak fuel pump or clogged filter.
• Engine Compression — expected: ~175 psi per cylinder. Failure: A cylinder reading significantly lower than others, or below 100 psi, indicates a mechanical engine problem.
• Injector Connector Power Supply Voltage — expected: ~12V (Battery Voltage) with Key On, Engine Off. Failure: 0V indicates a fault in the power supply wire or the main/injector fuse.

Scan Tool Commands That Help

• Mazda IDS (or equivalent high-end scanner): Power Balance Test — To identify which cylinder is contributing less power in real-time, confirming the misfire on cylinder 6 without having to wait for a code to set. This is useful for diagnosing intermittent misfires.
• Mazda IDS (or equivalent high-end scanner): Mode 6, Test ID $82, Component ID $06 — To view the raw misfire count for cylinder 6. This can show if misfires are occurring even if they aren't frequent enough to set a P0306 code, helping to confirm an intermittent issue.

Wiring & Ground Locations

• G11 — Behind the left side of the dash.. This is a major grounding point for various interior and dash components. While not a primary engine ground, a poor connection here can cause erratic electrical behavior that could potentially affect PCM reference voltages.
• Ignition Coil Fuse — In the Fuse & Relay Box in the center front of the engine compartment, labeled 'IG COIL FUSE 25A'.. This single fuse supplies power to all six ignition coils. While a failure would likely cause multiple misfires (P0300), a loose or corroded connection could theoretically cause intermittent power drops affecting one or more coils.
• Cylinder 6 Ignition Coil Connector — The electrical connector on the ignition coil for the driver's side cylinder closest to the radiator.. The two pins are for 12V power and the PCM ground signal. Checking for 12V power at this connector (with key on) is a critical step. The PCM provides the ground to fire the coil; a damaged wire between this connector and the PCM will cause a P0306 and/or P0356.

Real Owner Repair Stories

• Mazdas247 forum user (2009 Mazda CX-9 Sport AWD, 50k miles) — Rough running, solid then blinking check engine light, heavy white smoke from exhaust, near overheating, bucking and shaking.
  ❌ Tried (didn't work) Replaced all 6 spark plugs and PCV valve., Replaced cylinder 6 ignition coil., Replaced cylinder 6 fuel injector., Replaced the Powertrain Control Module (PCM).
  ✅ What actually fixed it The final diagnosis was leaking head gaskets on both cylinder banks, which allowed coolant to enter cylinder 6 and cause the misfire. The fix was replacing both head gaskets.
• Reddit user @Shyatic (2008 Mazda CX-9, ~50k miles) — Blinking check engine light, engine misfiring while driving.
  ❌ Tried (didn't work) Initially purchased a new ignition coil but did not need to install it.
  ✅ What actually fixed it Replacing only the spark plug in cylinder 6 resolved the P0306 code and the misfire.

OEM Part Supersession History

• CY01-18-100A → CY01-18-100B — Revision for improved reliability or manufacturing change.
  Heads up: The parts are interchangeable; CY01-18-100B is the current recommended replacement for CY01-18-100A.
• L3Y3-18-110 (Mazda) → Motorcraft SP-520 — This is a cross-reference. The 3.7L MZI engine is a Ford design, and Motorcraft is Ford's OEM parts brand. SP-520 is the direct equivalent.
  Heads up: Some sources indicate SP-520 has been superseded by SP-589, but for this application, SP-520 remains the commonly cited and correct plug.

Model Year Variations Within This Range

• 2007 vs 2008-2015: The 2007 model year used a 3.5L MZI V6. Starting in the 2008 model year, the CX-9 was equipped with the larger 3.7L MZI V6. While architecturally similar, all information in this article is specific to the 3.7L engine found in 2008-2015 models.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Internal Timing-Chain-Driven Water Pump Failure 🔴 High — Common at higher mileage (typically over 80,000 miles). Failure is a matter of 'when,' not 'if.'
• AWD Transfer Case (PTU) Failure 🔴 High — Commonly reported between 70,000 and 120,000 miles on AWD models. (Ref: Mazda issued a warranty extension to 9 years / 90,000 miles in the USA, but many failures occur outside this window.)
• Power Brake Booster Failure 🟠 Medium — Common enough that it prompted an NHTSA investigation and a warranty extension. (Ref: Mazda Special Service Program (SSP 93) extended warranty to 7 years / 90,000 miles.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, a used ignition coil from a low-mileage donor vehicle is a reasonable and cost-effective choice, especially for diagnosis. Since cylinder 6 is easily accessible, the labor to replace it again if the used part fails is minimal.

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

What to inspect on the donor part:

• Check the coil's plastic housing for cracks or signs of melting/arcing.
• Inspect the rubber boot for tears, brittleness, or swelling.
• Ensure the electrical connector pins are clean and free of corrosion.
• Ask for the mileage of the donor vehicle if possible.

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

• Powertrain Control Module (PCM) - While repair services exist, if a full replacement is needed, only an OEM unit properly programmed to the vehicle will function correctly.
• Fuel Injectors - Due to the sensitivity of fuel systems and the potential for clogs in used parts, new OEM or high-quality aftermarket injectors are strongly recommended.

Aftermarket brands forum-validated for this vehicle:

• Ignition Coils: Motorcraft, NGK, Denso
• Spark Plugs: NGK, Motorcraft, Denso

Brands owners have reported issues with on this vehicle:

• Unbranded, 'no-name' ignition coil sets from online marketplaces are frequently cited in forums as having high premature failure rates. While the initial cost is low, they often do not last.

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 — The companion code for the adjacent cylinder (middle, front bank) used during the 'coil swap' diagnostic procedure to confirm a faulty ignition coil.
• P0206 — A fuel injector circuit code for cylinder 6 that may accompany a P0306 misfire if the injector has failed electronically.
• P0300 — A random/multiple cylinder misfire code that can occur alongside P0306, especially if issues like internal water pump failure or coolant leaks are affecting multiple cylinders.
• P0016 — A timing correlation code that often appears if the internal water pump fails and contaminates the engine oil, leading to timing chain issues.

Frequently Asked Questions