P0301 on 2019-2025 Mazda 3 2.5L Turbo: Cylinder 1 Misfire Causes and Fixes

What's Unique About the 2019-2025 Mazda 3

While ignition components are common culprits for any P0301 code, the 2.5L Skyactiv-G Turbo engine (model years 2021+) has a critical, platform-specific issue. Mazda issued Technical Service Bulletin (TSB) 01-001-23 for a problem where a valve in the exhaust manifold can seize, causing excessive heat, exhaust valve seat wear, and a loss of engine compression. This turns a simple misfire code into a potential indicator of a major mechanical failure requiring extensive and expensive repairs. The TSB specifically affects 2020-2021 Mazda 3 Turbo models with VINs lower than JM1BP**300247 (Japan built) or 3MZBP**205173 (Mexico built). Additionally, manufacturer TSB Bulletin #01-001-23a notes that customers may complain about poor acceleration and rough idling alongside the P0301:00 code specifically in turbo engine applications.

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough or shaking engine idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Reduced fuel economy
• Strong fuel smell from the exhaust
• In some cases, other warning lights like Smart Brake Malfunction may appear simultaneously.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are a common failure point on modern engines due to heat and vibration. Some owners report sequential coil failures, where one fails and is replaced, only to have another cylinder's coil fail a few months later.
   How to confirm: Swap the ignition coil from cylinder 1 with another cylinder (e.g., cylinder 2). Clear the codes and drive the vehicle. If the code changes to P0302, the ignition coil is faulty. 🎬 Watch this quick guide to diagnosing and fixing a P0301 code. A Reddit user with a CX-5 confirmed this diagnostic step after experiencing failures on both cylinder 2 and then cylinder 1 two months apart.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $50-$120
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 1. Inspect it for signs of wear (worn electrode), carbon fouling, oil, or damage to the insulator. An oil leak from a failing valve cover gasket can also foul the plug and coil boot, causing a misfire.
   Typical fix: Replace the spark plug. It is best practice to replace all spark plugs as a set to ensure even performance. Use OEM-quality Iridium plugs. If oil was present, the valve cover gasket set should also be replaced to prevent a recurrence.
   Est. part cost: $20-$50
3. Carbon Buildup on Intake Valves 🟡 Medium Probability Like all direct-injection engines, the Skyactiv-G sprays fuel directly into the cylinder, so gasoline detergents don't clean the intake valves. Oil vapor from the PCV system can bake onto the valves, restricting airflow and causing misfires, especially during cold starts.
   How to confirm: This is difficult to confirm without a borescope to visually inspect the intake valves. It is usually diagnosed after ignition and fuel injector issues have been ruled out. A misfire that only occurs during a cold start and disappears when the engine warms up can be an indicator.
   Typical fix: Intake valve cleaning (walnut blasting or chemical cleaning) performed by a professional shop. 🎬 Watch a DIY walnut blasting demonstration to clear carbon buildup.
   Est. part cost: $400-$800
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Listen to the injector with a mechanic's stethoscope for a consistent clicking sound. You can also test the injector's electrical resistance with a multimeter; it should be between 1.6 and 2.52 ohms. Swapping injectors between cylinders is another method, but is more labor-intensive. Sometimes, using a quality fuel system cleaner like Chevron Techron or Berryman B-12 can resolve a minor clog.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $150-$350

Rare But Worth Checking

• Exhaust Valve Seat Wear / Loss of Compression (2.5T Only): This is the most severe potential cause, specific to the 2.5L Turbo engine as detailed in TSB 01-001-23. A seized valve in the exhaust manifold leads to valve seat wear and compression loss. If basic fixes fail, a compression and leak-down test are mandatory to check for this.
• Cracked Cylinder Head: → Shop Engine Cylinder Head A less common but documented issue on the 2.5L Turbo engine family is a cracked cylinder head, often near the exhaust manifold, which can cause coolant to leak into a cylinder and lead to misfires. TSB 01-002-23a addresses this for other Mazda models with the same engine family.
• Leaking Valve Cover Gasket: → Shop Engine Valve Cover While the gasket itself doesn't cause a misfire, it can allow oil to leak into the spark plug tubes. This oil can foul the spark plug and damage the ignition coil boot, leading to a P0301 code. The fix requires replacing the gasket set, the fouled plug, and the damaged coil.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner and note if the Check Engine Light is solid or flashing.
2. Locate cylinder 1, which is the cylinder on the far passenger side, closest to the engine's drive belts.
3. Swap the ignition coil from cylinder 1 to cylinder 2. Clear the codes, start the engine, and drive to see if the code returns as P0302. If it does, the ignition coil is faulty and must be replaced.
4. If the code remains P0301, swap the spark plug from cylinder 1 to cylinder 3. Clear codes and drive. If the code changes to P0303, the spark plug is the issue. While the plug is out, inspect the spark plug tube for any signs of oil, which would indicate a leaking valve cover gasket.
5. If the misfire still persists on cylinder 1, the problem is more complex. Inspect the wiring and connector for the ignition coil and fuel injector for any visible damage.
6. FOR 2.5L TURBO MODELS: If the steps above do not solve the issue, professional diagnosis is strongly recommended. Check if your vehicle's VIN falls within the range of TSB 01-001-23. Insist that the shop performs a cylinder compression test and a leak-down test to check for the mechanical failure described in the TSB before replacing more parts.

Parts You'll Likely Need

• Ignition Coil (OEM #PE20-18-100A) — This is one of the most common failure items causing a single-cylinder misfire on modern engines.
  Trusted brands: Mazda (OEM), NGK, Bosch, Denso
  OEM price range: $90-$150
  Aftermarket price range: $50-$120
• Iridium Spark Plug (OEM #PY8V-18-110) — Spark plugs are a routine maintenance item and can become worn or fouled, leading to a weak or inconsistent spark. The 2.5L Turbo has a 40,000-mile replacement interval. 🎬 See how to install new spark plugs on your Mazda 3.
  Trusted brands: Mazda (OEM), NGK, Denso
  OEM price range: $25-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0300 — Indicates random/multiple cylinder misfires. If seen with P0301, it may suggest a problem common to all cylinders, like a vacuum leak or fuel pressure issue.
• P0302, P0303, P0304 — These codes indicate misfires in other cylinders. Seeing them together can point to the severe TSB issue on turbo models or a more widespread problem like bad fuel.
• U0401 — A code for 'Invalid Data Received from ECM/PCM'. Some owners have reported seeing this code appear alongside a P0301, especially after the car enters a limp mode. It can indicate a communication disruption between modules triggered by the primary engine fault.
• P0201 — Indicates a fault in the fuel injector circuit for cylinder 1. If this code is present with P0301, it strongly points to the fuel injector or its wiring as the root cause.
• P0506 — Idle Air Control (IAC) System RPM Lower Than Expected. NHTSA ODI #11675874 describes an instance where P0301 appeared simultaneously with P0506.

Technical Service Bulletins (TSBs) & Recalls

• 01-001-23: Details a severe failure on 2.5T engines where a seized exhaust manifold valve causes compression loss and misfires. Affects 2020-2021 Mazda3 Turbos with VINs lower than JM1BP**300247 (Japan) or 3MZBP**205173 (Mexico). The fix requires cylinder head and manifold replacement.
• Bulletin #01-007-19-3541: Notes that vehicles may exhibit a rough idle and/or the check engine light on with DTC P0301 stored in memory.
• Bulletin #01-012-19-3612: Specifically addresses Mazda3 vehicles that may experience the Check Engine Light ON with DTC P0301:00 stored in memory.

Platform-Specific Known Issues

• TSB 01-001-23: On 2021+ 2.5L Turbo models, misfire codes can be caused by a seized variable valve in the exhaust manifold, leading to loss of compression. The official fix involves replacing the cylinder head and exhaust manifold, a repair that can cost thousands of dollars. This TSB applies to specific VIN ranges: Japan-built (JM1BP) before July 8, 2020, and Mexico-built (3MZBP) before September 9, 2020.
• TSB 01-013/21 & 01-002/23: While documented for the CX-9, the same 2.5T engine family has known issues with coolant leaks at the cylinder head near the exhaust manifold due to cracks, which can cause misfires.
• TSB 01-003-23: Affecting 2021 models with the 2.5T engine, this TSB addresses excessive oil consumption due to faulty exhaust valve stem seals. While this typically triggers a P250F code, severe oil fouling from this issue could potentially lead to spark plug fouling and misfires.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Winding Resistance — expected: 0.49 - 0.57 ohms. Failure: A reading outside this range suggests a faulty coil.
• Ignition Coil Secondary Winding Resistance — expected: 9.5 - 11.1 kΩ (kilohms). Failure: A reading outside this range indicates a faulty coil.
• Fuel Injector Resistance — expected: 1.6 - 2.52 ohms. Failure: A reading outside this range, or one that is significantly different from other injectors, indicates a fault.
• High-Pressure Fuel System Pressure at Idle (2.5L Turbo) — expected: Approx. 3.0 MPa (435 psi). Failure: Significantly lower or fluctuating pressure can indicate a failing high-pressure fuel pump (HPFP) or a leak.
• Short-Term Fuel Trim (STFT) at Idle — expected: -5% to +5%. Failure: A large positive value (e.g., +20%) can indicate a vacuum leak, while a large negative value can indicate a leaking injector on the bank with the misfire.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $81: This is not a DTC, but a specific monitor within the OBD-II system's Mode $06 data. It tracks the misfire count for Cylinder 1 during the current and last ten driving cycles. Accessing this data can reveal intermittent misfires that haven't yet met the threshold to trigger a P0301 code. (see via An advanced OBD-II scanner or diagnostic software (like FORScan for Mazda) that can display Mode $06 test results.)

Scan Tool Commands That Help

• Mazda M-MDS / FORScan: Misfire Counter PID Monitoring — Use this to watch for misfires in real-time on cylinder 1 during a test drive. It helps confirm an intermittent misfire under specific load conditions (e.g., only during heavy acceleration) that may not set a code immediately.
• Mazda M-MDS / FORScan: Power Balance Test — This function allows the technician to sequentially disable the fuel injector for each cylinder. When the injector for a healthy cylinder is disabled, the engine RPM will drop noticeably. If disabling the injector for cylinder 1 causes little or no change in RPM, it confirms that cylinder 1 was not contributing power, narrowing the fault down from the ignition system.

Real Owner Repair Stories

• Mazdas247 Forum (2019 Mazda CX-5 Signature 2.5T (26,720 miles)) — Check Engine Light with code P0302 (Cylinder 2 Misfire).
  ❌ Tried (didn't work) Initial diagnosis by owner.
  ✅ What actually fixed it The owner took the vehicle to a Mazda dealership for service under TSB 01-001/23. The dealership performed the required repairs, which involve replacing the cylinder head and exhaust manifold to resolve the compression loss issue. The owner reported the experience was smooth and the fix was successful.
• NHTSA ODI #10695229 — An owner reported that the engine light stayed on with a P0301 code, indicating a cylinder 1 misfire issue. The vehicle was kept by the service department for almost three weeks before they were able to resolve the issue.

OEM Part Supersession History

• PE01-18-100, PE01-18-100A → PE20-18-100A — Standard part evolution and consolidation for multiple Skyactiv engine applications.
  Heads up: The listed part numbers are generally interchangeable for this engine family, but it is always best to use the latest revision (PE20-18-100A).

Model Year Variations Within This Range

• 2022+: For the 2022 model year, Mazda increased the horsepower of the 2.5L Turbo engine from 250 to 256 hp when using 93 octane fuel. This was likely achieved through software (PCM) updates and does not fundamentally change the diagnosis of a P0301 code, but indicates a different factory ECU calibration compared to 2021 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

• P0302 — Cylinder 2 Misfire. Often appears during diagnosis if a faulty ignition coil from cylinder 1 is swapped to cylinder 2, or as a sequential failure months later.
• P0303 — Cylinder 3 Misfire. Appears during diagnostic testing if a fouled or worn spark plug from cylinder 1 is swapped to cylinder 3.
• P250F — Engine Oil Level Too Low. Related via TSB 01-003-23 on 2021 2.5T models, where faulty exhaust valve stem seals cause oil consumption that can eventually foul spark plugs and cause misfires.
• P0300 — Random/Multiple Cylinder Misfire. Can appear alongside P0301 if carbon buildup on the intake valves or a vacuum leak is affecting multiple cylinders simultaneously.

Frequently Asked Questions