P0303 on 2012-2018 Mazda 3 2.0L Skyactiv-G: Cylinder 3 Misfire Causes and Fixes

What's Unique About the 2012-2018 Mazda 3

The 2012-2018 Mazda 3 with the 2.0L Skyactiv-G engine is known for a few specific issues that can cause a P0303 code. For the early Skyactiv models (2012-2014), Mazda issued a Technical Service Bulletin (TSB 01-014/16) for premature ignition coil failure from a specific supplier named 'YUMEBISHI'. Additionally, as these are direct-injection engines, they can be susceptible to carbon buildup on the intake valves over time, which can disrupt airflow and lead to misfires, an issue also covered by a TSB (01-012/19). While less common, clogged fuel injectors have also been identified as a cause after ignition components were ruled out, as documented in owner forums.

Generation note: This range covers two generations of the Mazda 3 that used the 2.0L Skyactiv-G engine: the late second generation (BL, 2012-2013) and the third generation (BM/BN, 2014-2018). The primary causes for P0303 are consistent across these years, though a specific TSB for faulty ignition coils (01-014/16) was issued for 2012-2016 models, including the Mazda 3. Another TSB regarding misfires in cold weather due to a frozen muffler (01-006/18) applies to 2014-2018 models. A TSB for carbon buildup (01-012/19) specifically targets 2012-2013 models. Manufacturer Bulletin #01-012-19-3612 further confirms that vehicles may experience the Check Engine Light with P0303 stored in memory due to these conditions.

Symptoms You May Notice

• Rough or shaking idle
• Hesitation or stumbling during acceleration
• Flashing Check Engine Light
• Noticeable loss of engine power
• Reduced fuel economy
• Engine stalling
• Fuel smell from the exhaust
• Sputtering or popping sounds from the engine

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability Mazda issued a TSB (#01-014/16) for 2012-2016 models acknowledging that original ignition coils from a supplier named 'YUMEBISHI' were prone to premature failure. These faulty coils can be identified by their 4-digit lot numbers, whereas the improved MITSUBISHI-made coils have 5-digit lot numbers. This remains the most common cause for a single-cylinder misfire across this entire year range. Manufacturer Bulletin #01-010-18-3442b notes that an engine misfire during acceleration with a flashing check engine light and P0303 may occur, even if the DTC is not immediately stored.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 2). Cylinder 3 is the third from the right (passenger side). Clear the codes and drive. If the code changes to P0302, the ignition coil is faulty. Visually inspect the coil boot for white traces or black soot, which indicates it is cracked and arcing against the engine block.
   Typical fix: Replace the faulty ignition coil. 🎬 Watch: Step-by-step guide to replacing your ignition coils. It is often recommended to replace all four coils if they are original and high-mileage, as the others may fail soon. Using OEM or reputable aftermarket brands like NGK or Denso is advised.
   Est. part cost: $35-$100
2. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a standard maintenance item. The recommended service interval for the Skyactiv-G engine is 75,000 miles. A worn, improperly gapped, or fouled plug can't create a strong enough spark, leading to incomplete combustion. Many owners replace plugs first, as it's routine maintenance.
   How to confirm: After ruling out the ignition coil, remove the spark plug from cylinder 3. Inspect it for wear (worn electrode), carbon fouling (black, sooty), or oil fouling. Compare it to a new plug. The correct OEM plug is typically an NGK Iridium plug, part number PE5R-18-110.
   Typical fix: Replace the spark plug in cylinder 3. It is best practice to replace all four spark plugs as a set to ensure even performance and avoid future misfires on other cylinders.
   Est. part cost: $10-$25 per plug
3. Clogged or Faulty Fuel Injector ⚪ Low Probability As a direct-injection engine, the fuel injectors can become clogged or fail electronically over time. Forum discussions from owners confirm that a clogged injector can be the root cause of a persistent P0303 after ignition parts have been replaced. In one documented case, a user had the same cylinder 3 injector fail four times, suggesting a potential wiring issue to the injector. Manufacturer Bulletin #01-001-23a notes that customers may complain about poor acceleration and rough idling with DTC P0303:00, though this specifically applies to turbo engines.
   How to confirm: This is more difficult to diagnose. A professional can perform a fuel injector balance test. A DIY method is to swap the fuel injector from cylinder 3 with another cylinder, but this is a more involved repair requiring removal of the fuel rail. Before swapping, one can listen to the injector with a mechanic's stethoscope to check if it's clicking like the others.
   Typical fix: Replace the faulty fuel injector. 🎬 See this DIY walkthrough for replacing a fuel injector. Sometimes professional fuel system cleaning can resolve minor clogs, but electronic failure requires replacement. If the issue recurs, the injector wiring harness should be inspected.
   Est. part cost: $70-$200

Rare But Worth Checking

• Intake Valve Carbon Buildup: Skyactiv-G engines are direct-injection, meaning fuel is sprayed directly into the cylinder, not over the intake valves. This allows carbon deposits from the PCV system to bake onto the back of the valves, disrupting airflow and causing misfires, especially on cold starts during the accelerated warm-up system (AWS) operation. Mazda has a TSB (01-012/19, superseding 01-020/15) for this issue on 2012-2013 models, outlining a cleaning procedure. This often requires professional service like walnut blasting.
🎬 Watch: How to clean carbon buildup on Skyactiv engines.
• Low Engine Compression: Though uncommon, internal mechanical problems like worn piston rings, a leaking head gasket, or damaged valves can cause a loss of compression in cylinder 3, resulting in a persistent misfire that ignition and fuel parts won't fix. A compression test is the definitive diagnostic step for this. A mechanic in one forum thread performed this test to rule out engine damage.
• Frozen Muffler / Main Silencer: For 2014-2018 models, Mazda issued TSB 01-006/18 for misfire codes that appear after starting the vehicle in freezing temperatures (below 32°F / 0°C). Water from condensation can accumulate and freeze in the muffler, restricting exhaust flow and causing backpressure that leads to a misfire. Manufacturer Bulletin #01-007-19-3541 similarly notes that frozen water accumulated in the main silencer can restrict exhaust flow and cause P0303.

Diagnosis Steps

1. Read the code with an OBD-II scanner. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe, catalyst-damaging misfire.
2. Swap the ignition coil from cylinder 3 to an adjacent cylinder (like cylinder 2 or 4). Cylinder 3 is the third from the right (belt side). Clear the codes and run the engine to see if the misfire code follows the coil (e.g., changes to P0302 or P0304). If it does, the coil is bad.
3. If the code remains P0303, inspect the spark plug in cylinder 3. Look for signs of wear, fouling, or damage. Replace if necessary. It's best to replace all four at once for even performance.
4. If the coil and plug are good, the issue may be fuel-related. Inspect the wiring to the fuel injector for cylinder 3 for any visible damage. A more advanced step is to swap the injector with another cylinder, though this is more labor-intensive.
5. If the misfire persists, especially on cold starts, consider carbon buildup on the intake valves. This may require a borescope inspection to confirm and professional cleaning (e.g., walnut blasting) to resolve.
6. Perform a compression test on cylinder 3 to rule out mechanical engine problems like bad rings or valves.
7. If the issue occurs only in freezing temperatures on a 2014-2018 model, inspect the muffler for signs of ice blockage as per TSB 01-006/18. The official fix is a replacement muffler with drain holes.

Parts You'll Likely Need

• Ignition Coil (OEM #PE20-18-100A) — This is the most frequent cause of a P0303 code on this engine, with a known history of premature failure on earlier models as per TSB 01-014/16. This part number supersedes PE20-18-100 and PE01-18-100A.
  Trusted brands: NGK, Denso, Bosch, Mazda (OEM)
  OEM price range: $120-$180
  Aftermarket price range: $35-$100
• Iridium Spark Plug (OEM #PE5R-18-110) — Spark plugs are a common wear item and the second most likely cause after the ignition coil. A fouled or worn plug in cylinder 3 will prevent a proper spark. The OEM-spec plug is NGK Iridium PE5R-18-110.
  Trusted brands: NGK, Denso
  OEM price range: $20-$30
  Aftermarket price range: $10-$25

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders due to a shared underlying cause (like carbon buildup or a failing fuel pump), a P0300 (Random/Multiple Cylinder Misfire) may be stored alongside P0303.
• U0442 / U0401 — In some complex cases, a P0303 misfire can be accompanied by communication codes like U0442 (Invalid Data Received from PCM) or U0401, which may point to deeper electrical or module issues rather than a simple component failure.
• P0316 — As noted in Manufacturer Bulletin #01-005-20-3704, vehicles may exhibit a rough run or lack of power with DTCs P0303 and P0316 stored together.

Technical Service Bulletins (TSBs) & Recalls

• 01-014/16: Premature failure of original 'YUMEBISHI' ignition coils on 2012-2016 Skyactiv models.
• 01-012/19 (supersedes 01-020/15): Misfire codes due to carbon deposits on intake valves.
• 01-006/18: Rough idle and misfire codes in freezing temperatures due to a frozen main silencer (muffler).
• 01-005/15: Misfire codes and increased engine oil level due to fuel accumulation in the crankcase during short trips in extreme cold.
• Bulletin #01-012-19-3612: Confirms Check Engine Light with DTC P0303:00 stored in memory for certain Mazda models.
• Bulletin #01-007-19-3541: Addresses P0303 caused by frozen water accumulated in the main silencer, restricting exhaust flow.
• Bulletin #01-010-18-3442b: Notes engine misfire during acceleration with a flashing check engine light and P0303:00.

Platform-Specific Known Issues

• TSB 01-014/16 identified faulty 'YUMEBISHI' brand ignition coils on 2012-2016 models, recommending replacement with MITSUBISHI-made parts if found defective.
• TSB 01-012/19 (supersedes 01-020/15) addresses misfire codes caused by carbon deposits on intake valves for 2012-2013 models, outlining a cleaning procedure to be performed if misfires occur during cold start warm-up.
• TSB 01-006/18 notes that misfires in cold weather on 2014-2018 models can be caused by a frozen muffler blocking exhaust flow. The fix is a modified muffler with drain holes.
• TSB 01-005/15: Misfire codes and increased engine oil level due to fuel accumulation in the crankcase during short trips in extreme cold.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel System — expected: Up to 3,000 PSI. Failure: Significantly lower pressure readings under load can indicate a failing high-pressure fuel pump (HPFP), which could cause a misfire.
• Ignition Coil Primary Amperage Waveform — expected: A consistent waveform pattern on a lab scope.. Failure: An inconsistent or erratic waveform indicates an internally shorted or failing ignition coil, confirming it as the source of the misfire.

Scan Tool Commands That Help

• Mazda IDS (Integrated Diagnostic System): Power Balance Test — This function allows a technician to disable one cylinder at a time to observe the corresponding drop in engine RPM. If disabling cylinder 3 causes little or no change in RPM, it confirms that cylinder is not contributing power, pinpointing the misfire without swapping parts.

Wiring & Ground Locations

• Main Engine Ground — Typically located on the engine block near the transmission housing, connecting the main engine harness to the chassis.. A loose or corroded main engine ground can cause intermittent and hard-to-diagnose electrical issues, including weak spark or erratic injector signals that can lead to a misfire code like P0303.

Real Owner Repair Stories

• Reddit user krob58 on r/mazda (2012 Mazda 3 hatchback, 2.0L Skyactiv, 140k miles) — Check Engine Light with code P0303.
  ❌ Tried (didn't work) Replaced all spark plugs and ignition coils with aftermarket parts. The code returned the next day.
  ✅ What actually fixed it The user was advised by the dealer that the issue was carbon buildup. However, multiple forum members suggested the true cause was the use of aftermarket ignition coils, which are a common point of failure, and recommended replacing the cylinder 3 coil with an OEM part before paying for a carbon cleaning service. The strong consensus pointed to the aftermarket parts being the problem.

OEM Part Supersession History

• PE01-18-100A, PE20-18-100 → PE20-18-100A — The original 'YUMEBISHI' coils (identified by a 4-digit lot number) were prone to internal failure. The updated part is from a different supplier (MITSUBISHI, with a 5-digit lot number) and is more robust.
• PE19-20-3F0 → PE19-20-3F0A — This is the part number for the high-pressure fuel pump (HPFP). While not a primary cause for a single-cylinder misfire, a failing pump can cause fuel starvation issues. Revisions often improve reliability.

Model Year Variations Within This Range

• 2012-2013: These early Skyactiv-G engines were considered 'partial Skyactiv' and did not feature the full 4-2-1 exhaust header due to space constraints in the 2nd generation chassis. This can make them slightly more susceptible to issues like carbon buildup addressed in TSB 01-012/19.
• 2014-2018: These models feature the 'full Skyactiv' engine design, including the tuned 4-2-1 exhaust header, an improved chassis, and revised transmission tuning, which may affect long-term engine characteristics compared to the earlier models.

Diagnostic Flowchart

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/Multiple Cylinder Misfire Detected. Often appears alongside P0303 during cold weather/high load events, or due to widespread carbon buildup on intake valves as noted in TSB 01-012/19.
• P0302 — Cylinder 2 Misfire Detected. Frequently seen during diagnosis when a potentially faulty ignition coil is swapped from cylinder 3 to cylinder 2 to confirm the failure.
• P0304 — Cylinder 4 Misfire Detected. Similar to P0302, this code will appear if a defective cylinder 3 ignition coil is moved to cylinder 4 during troubleshooting.
• P0301 — Cylinder 1 Misfire Detected. Shares the same root causes as P0303, such as faulty 'YUMEBISHI' ignition coils (TSB 01-014/16) or frozen mufflers in cold weather (TSB 01-006/18).

Frequently Asked Questions